The Puf family of RNA-binding proteins in plants: phylogeny, structural modeling, activity and subcellular localization

Analysis of the root mRNA interactome from canola and rice: crop species that span the eudicot-monocot boundary

The advent of RNA interactome capture (RIC) has been important in characterizing the mRNA-binding proteomes (mRBPomes) of several eukaryotic taxa. To date, published plant poly(A)+ RIC studies have been restricted to Arabidopsis thaliana and specific to seedlings, suspension cell cultures, mesophyll protoplasts, leaves and embryos. The focus of this study was to expand RIC to root tissue in two crop species, the oilseed eudicot Brassica napus (canola) and the cereal monocot Oryza sativa (rice). The optimization and application of root RIC in these species resulted in the identification of 499 proteins and 334 proteins comprising the root mRBPomes of canola and rice, respectively, with 182 shared orthologous proteins between these two species. In both mRBPomes, approximately 80% of captured proteins were linked to RNA biology, with RRM-containing proteins and ribosomal proteins among the most overrepresented protein groups. Consistent with trends observed in other RIC studies, novel RNA-binding proteins were captured that lacked known RNA-binding domains and included numerous metabolic enzymes. The root mRBPomes from canola and rice shared a high degree of similarity at the compositional level, as shown by a comparative analysis of orthologs predicted for captured proteins to the published Arabidopsis RIC-derived mRBPomes, as well as our Arabidopsis root mRBPome data presented here. This analysis also revealed that 46 proteins in the canola and rice root mRBPomes were unique when orthologs were compared to the published Arabidopsis RBPomes, including those identified recently using phase separation approach that identified proteins bound to all RNA types. The results from this research expands the plant mRBPome into root tissue using two crop species that span the eudicot-monocot clade boundary, and provides fundamental knowledge on RNA-binding protein function in post-transcriptional control of genes in crop species for possible future development of beneficial traits.

RNA binding proteins (RBPs) on genetic stability and diseases

RNA-binding proteins (RBPs) are integral components of cellular machinery, playing crucial roles in the regulation of gene expression and maintaining genetic stability. Their interactions with RNA molecules govern critical processes such as mRNA splicing, stability, localization, and translation, which are essential for proper cellular function. These proteins interact with RNA molecules and other proteins to form ribonucleoprotein complexes (RNPs), hence controlling the fate of target RNAs. The interaction occurs via RNA recognition motif, the zinc finger domain, the KH domain and the double stranded RNA binding motif (all known as RNA-binding domains (RBDs). These domains are found within the coding sequences (intron and exon domains), 5' untranslated regions (5'UTR) and 3' untranslated regions (3'UTR). Dysregulation of RBPs can lead to genomic instability, contributing to various pathologies, including cancer neurodegenerative diseases, and metabolic disorders. This study comprehensively explores the multifaceted roles of RBPs in genetic stability, highlighting their involvement in maintaining genomic integrity through modulation of RNA processing and their implications in cellular signalling pathways. Furthermore, it discusses how aberrant RBP function can precipitate genetic instability and disease progression, emphasizing the therapeutic potential of targeting RBPs in restoring cellular homeostasis. Through an analysis of current literature, this study aims to delineate the critical role of RBPs in ensuring genetic stability and their promise as targets for innovative therapeutic strategies.

Diversity and structural-functional insights of alpha-solenoid proteins

Alpha-solenoids are a significant and diverse subset of structured tandem repeat proteins (STRPs) that are important in various domains of life. This review examines their structural and functional diversity and highlights their role in critical cellular processes such as signaling, apoptosis, and transcriptional regulation. Alpha-solenoids can be classified into three geometric folds: low curvature, high curvature, and corkscrew, as well as eight subfolds: ankyrin repeats; Huntingtin, elongation factor 3, protein phosphatase 2A, and target of rapamycin; armadillo repeats; tetratricopeptide repeats; pentatricopeptide repeats; Pumilio repeats; transcription activator-like; and Sel-1 and Sel-1-like repeats. These subfolds represent distinct protein families with unique structural properties and functions, highlighting the versatility of alpha-solenoids. The review also discusses their association with disease, highlighting their potential as therapeutic targets and their role in protein design. Advances in state-of-the-art structure prediction methods provide new opportunities and challenges in the functional characterization and classification of this kind of fold, emphasizing the need for continued development of methods for their identification and proper data curation and deposition in the main databases.

PUF3 RNA binding protein of Trypanosoma cruzi regulates mitochondrial morphology and function

The RNA-binding PUF proteins are post-transcriptional regulators found throughout the eukaryotic domain. In Trypanosoma cruzi, ten Puf genes termed Puf1 to Puf10 have been identified. Considering that the control of gene expression in this parasite is mainly at the post-transcriptional level, we characterized the PUF3 protein by knocking out and overexpressing the gene in T. cruzi epimastigotes and studied different genetic and biological features. The RNA-seq analyses in both genotypes showed significant changes in the number of regulated transcripts compared with wild-type parasites. Thus, the number of differentially expressed genes in the knockout (ΔTcPuf3) and the overexpressor (pTEXTcPuf3) were 238 and 187, respectively. In the knockout, a more significant proportion of genes was negatively regulated (166 out of 238). In contrast, in the overexpressor, positively regulated genes were predominant (149 out of 170). Additionally, when we predicted the subcellular location of the differentially expressed genes, the results revealed an important representation of nuclear genes encoding mitochondrial proteins. Therefore, we determined whether overexpression or knockout of TcPuf3 could lead to changes in both mitochondrial structure and cellular respiration. When mitochondria from ΔTcPuf3 and pTEXTcPuf3 parasites were analyzed by transmission electron microscopy (TEM), it was observed that the overexpressor had an abnormal mitochondrial morphology, evidenced by swelling. The results associated with cellular respiration showed that both the ΔTcPuf3 and pTEXTcPuf3 had a lower efficiency in routine respiration and the electron transport system capacity. Likewise, the mitochondria from overexpressing parasites showed a slight hyperpolarization. Additionally, several biological features, depending on the function of the mitochondria, were altered, such as growth, cell division, metacyclogenesis, ROS production, and response to benznidazole. In conclusion, our results suggest that although PUF3 is not an essential protein in T. cruzi, it influences mitochondrial transcripts, affecting mitochondrial morphology and function.

Heterogeneous identity, stiffness and growth characterise the shoot apex of Arabidopsis stem cell mutants

Stem cell homeostasis in the shoot apical meristem involves a core regulatory feedback loop between the signalling peptide CLAVATA3 (CLV3), produced in stem cells, and the transcription factor WUSCHEL, expressed in the underlying organising centre. clv3 mutant meristems display massive overgrowth, which is thought to be caused by stem cell overproliferation, although it is unknown how uncontrolled stem cell divisions lead to this altered morphology. Here, we reveal local buckling defects in mutant meristems, and use analytical models to show how mechanical properties and growth rates may contribute to the phenotype. Indeed, clv3 mutant meristems are mechanically more heterogeneous than the wild type, and also display regional growth heterogeneities. Furthermore, stereotypical wild-type meristem organisation, in which cells simultaneously express distinct fate markers, is lost in mutants. Finally, cells in mutant meristems are auxin responsive, suggesting that they are functionally distinguishable from wild-type stem cells. Thus, all benchmarks show that clv3 mutant meristem cells are different from wild-type stem cells, suggesting that overgrowth is caused by the disruption of a more complex regulatory framework that maintains distinct genetic and functional domains in the meristem.

Current insight into the role of mRNA decay pathways in fungal pathogenesis

Pathogenic fungal species can cause superficial and mucosal infections, to potentially fatal systemic or invasive infections in humans. These infections are more common in immunocompromised or critically ill patients and have a significant morbidity and fatality rate. Fungal pathogens utilize several strategies to adapt the host environment resulting in efficient and comprehensive alterations in their cellular metabolism. Fungal virulence is regulated by several factors and post-transcriptional regulation mechanisms involving mRNA molecules are one of them. Post-transcriptional controls have emerged as critical regulatory mechanisms involved in the pathogenesis of fungal species. The untranslated upstream and downstream regions of the mRNA, as well as RNA-binding proteins, regulate morphogenesis and virulence by controlling mRNA degradation and stability. The limited number of available therapeutic drugs, the emergence of multidrug resistance, and high death rates associated with systemic fungal illnesses pose a serious risk to human health. Therefore, new antifungal treatments that specifically target mRNA pathway components can decrease fungal pathogenicity and when combined increase the effectiveness of currently available antifungal drugs. This review summarizes the mRNA degradation pathways and their role in fungal pathogenesis.

Analysis of the Genetic Diversity in Tea Plant Germplasm in Fujian Province Based on Restriction Site-Associated DNA Sequencing

Fujian province, an important tea-producing area in China, has abundant tea cultivars. To investigate the genetic relationships of tea plant cultivars in Fujian province and the characteristics of the tea plant varieties, a total of 70 tea cultivars from Fujian and other 12 provinces in China were subjected to restriction site-associated DNA sequencing (RAD-seq). A total of 60,258,975 single nucleotide polymorphism (SNP) sites were obtained. These 70 tea plant cultivars were divided into three groups based on analyzing the phylogenetic tree, principal component, and population structure. Selection pressure analysis indicated that nucleotide diversity was high in Southern China and genetically distinct from cultivars of Fujian tea plant cultivars, according to selection pressure analysis. The selected genes have significant enrichment in pathways associated with metabolism, photosynthesis, and respiration. There were ten characteristic volatiles screened by gas chromatography-mass spectrometry (GC-MS) coupled with multivariate statistical methods, among which the differences in the contents of methyl salicylate, 3-carene, cis-3-hexen-1-ol, (E)-4-hexen-1-ol, and 3-methylbutyraldehyde can be used as reference indicators of the geographical distribution of tea plants. Furthermore, a metabolome genome-wide association study (mGWAS) revealed that 438 candidate genes were related to the aroma metabolic pathway. Further analysis showed that 31 genes of all the selected genes were screened and revealed the reasons for the genetic differences in aroma among tea plant cultivars in Fujian and Southern China. These results reveal the genetic diversity in the Fujian tea plants as well as a theoretical basis for the conservation, development, and utilization of the Fujian highly aromatic tea plant cultivars.

Comprehensive Identification of the Pum Gene Family and Its Involvement in Kernel Development in Maize

The Pumilio (Pum) RNA-binding protein family regulates post-transcription and plays crucial roles in stress response and growth. However, little is known about Pum in plants. In this study, a total of 19 ZmPum genes were identified and classified into two groups in maize. Although each ZmPum contains the conserved Pum domain, the ZmPum members show diversity in the gene and protein architectures, physicochemical properties, chromosomal location, collinearity, cis-elements, and expression patterns. The typical ZmPum proteins have eight α-helices repeats, except for ZmPum2, 3, 5, 7, and 14, which have fewer α-helices. Moreover, we examined the expression profiles of ZmPum genes and found their involvement in kernel development. Except for ZmPum2, ZmPum genes are expressed in maize embryos, endosperms, or whole seeds. Notably, ZmPum4, 7, and 13 exhibited dramatically high expression levels during seed development. The study not only contributes valuable information for further validating the functions of ZmPum genes but also provides insights for improvement and enhancing maize yield.

Characterisation of the trans-membrane nucleoporins GP210 and NDC1 in Arabidopsis thaliana

The nuclear pore is structurally conserved across eukaryotes as are many of the pore's constituent proteins. The transmembrane nuclear pore proteins GP210 and NDC1 span the nuclear envelope holding the nuclear pore in place. Orthologues of GP210 and NDC1 in Arabidopsis were investigated through characterisation of T-DNA insertional mutants. While the T-DNA insert into GP210 reduced expression of the gene, the insert in the NDC1 gene resulted in increased expression in both the ndc1 mutant as well as the ndc1/gp210 double mutant. The ndc1 and gp210 individual mutants showed little phenotypic difference from wild-type plants, but the ndc1/gp210 mutant showed a range of phenotypic effects. As with many plant nuclear pore protein mutants, these effects included non-nuclear phenotypes such as reduced pollen viability, reduced growth and glabrous leaves in mature plants. Importantly, however, ndc1/gp210 exhibited nuclear-specific effects including modifications to nuclear shape in different cell types. We also observed functional changes to nuclear transport in ndc1/gp210 plants, with low levels of cytoplasmic fluorescence observed in cells expressing nuclear-targeted GFP. The lack of phenotypes in individual insertional lines, and the relatively mild phenotype suggests that additional transmembrane nucleoporins, such as the recently-discovered CPR5, likely compensate for their loss.

Understanding the possible influence of Pumilio RNA binding proteins on terpenoid indole alkaloid biosynthesis in Catharanthus roseus

Catharanthus roseus is a clinically significant medicinal plant; the sole source of chemotherapy agents, vincristine and vinblastine (specialized metabolites, terpenoid indole alkaloids/TIAs). Owing to large clinical demand and low bioavailability, several studies have focused on biosynthesis and regulation of TIA biosynthesis in C. roseus. However, transcription factor mediated regulation has been a major research focus, and the impact of post-transcriptional regulation remains under-explored. RNA binding proteins (RBPs) are an emerging class of post-transcriptional regulators having a profound influence on transcript stability. Pumilio (Pum) RBPs are evolutionarily conserved post-transcriptional regulators, involved in RNA degradation across eukaryotes. However, their potential influence on TIA biosynthesis has not been studied till date in any medicinal plants including C. roseus. Thus, the present study aimed at identification and computational characterization of Pum in C. roseus, followed by expression and functional analyses. The genome-wide identification and characterization revealed twelve CrPum isoforms. The effect of CrPum2, 3, and 5 knockdown on TIA biosynthesis (specifically vindoline and catharanthine) was analyzed via high performance liquid chromatography. CrPum5 knockdown was associated with increased TIA levels and upregulation of key TIA pathway genes. Thus, the present study is the first to report the potential influence of Pum on TIA biosynthesis in C. roseus. Further studies to elucidate the mechanism of Pum activity could provide new insights into the molecular regulation of TIA biosynthesis. A holistic understanding of regulatory mechanisms could benefit the metabolic engineering programs aimed at higher productivity of plant specialized metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01193-5.

RNA-Binding Proteins: The Key Modulator in Stress Granule Formation and Abiotic Stress Response

To cope with abiotic environmental stress, plants rapidly change their gene expression transcriptionally and post-transcriptionally, the latter by translational suppression of selected proteins and the assembly of cytoplasmic stress granules (SGs) that sequester mRNA transcripts. RNA-binding proteins (RBPs) are the major players in these post-transcriptional processes, which control RNA processing in the nucleus, their export from the nucleus, and overall RNA metabolism in the cytoplasm. Because of their diverse modular domain structures, various RBP types dynamically co-assemble with their targeted RNAs and interacting proteins to form SGs, a process that finely regulates stress-responsive gene expression. This review summarizes recent findings on the involvement of RBPs in adapting plants to various abiotic stresses via modulation of specific gene expression events and SG formation. The relationship of these processes with the stress hormone abscisic acid (ABA) is discussed.

The Role of Pumilio RNA Binding Protein in Plants

Eukaryotic organisms have a posttranscriptional/translational regulation system for the control of translational efficiency. RNA binding proteins (RBPs) have been known to control target genes. One type of protein, Pumilio (Pum)/Puf family RNA binding proteins, show a specific binding of 3′ untranslational region (3′ UTR) of target mRNA and function as a post-transcriptional/translational regulator in eukaryotic cells. Plant Pum protein is involved in development and biotic/abiotic stresses. Interestingly, Arabidopsis Pum can control target genes in a sequence-specific manner and rRNA processing in a sequence-nonspecific manner. As shown in in silico Pum gene expression analysis, Arabidopsis and rice Pum genes are responsive to biotic/abiotic stresses. Plant Pum can commonly contribute to host gene regulation at the post-transcriptional/translational step, as can mammalian Pum. However, the function of plant Pum proteins is not yet fully known. In this review, we briefly summarize the function of plant Pum in defense, development, and environmental responses via recent research and bioinformatics data.

The Pumilio RNA-binding protein APUM24 regulates seed maturation by fine-tuning the BPM-WRI1 module in Arabidopsis

Pumilio RNA-binding proteins participate in messenger RNA (mRNA) degradation and translational repression, but their roles in plant development are largely unclear. Here, we show that Arabidopsis PUMILIO PROTEIN24 (APUM24), an atypical Pumilio-homology domain-containing protein, plays an important part in regulating seed maturation, a major stage of plant development. APUM24 is strongly expressed in maturing seeds. Reducing APUM24 expression resulted in abnormal seed maturation, wrinkled seeds, and lower seed oil contents, and APUM24 knockdown resulted in lower levels of WRINKLED 1 (WRI1), a key transcription factor controlling seed oil accumulation, and lower expression of WRI1 target genes. APUM24 reduces the mRNA stability of BTB/POZMATH (BPM) family genes, thus decreasing BPM protein levels. BPM is responsible for the 26S proteasome-mediated degradation of WRI1 and has important functions in plant growth and development. The 3' untranslated regions of BPM family genes contain putative Pumilio response elements (PREs), which are bound by APUM24. Reduced BPM or increased WRI1 expression rescued the deficient seed maturation of apum24-2 knockdown mutants, and APUM24 overexpression resulted in increased seed size and weight. Therefore, APUM24 is crucial to seed maturation through its action as a positive regulator fine-tuning the BPM-WRI1 module, making APUM24 a promising target for breeding strategies to increase crop yields.

A diagnostic marker kit for Fusarium wilt and sterility mosaic diseases resistance in pigeonpea

Fusarium wilt (FW) and sterility mosaic diseases (SMD) are key biotic constraints to pigeonpea production. Occurrence of these two diseases in congenial conditions is reported to cause complete yield loss in susceptible pigeonpea cultivars. Various studies to elucidate genomic architecture of the two traits have revealed significant marker-trait associations for use in breeding programs. However, these DNA markers could not be used effectively in genomics-assisted breeding for developing FW and SMD resistant varieties primarily due to pathogen variability, location or background specificity, lesser phenotypic variance explained by the reported QTL and cost-inefficiency of the genotyping assays. Therefore, in the present study, a novel approach has been used to develop a diagnostic kit for identification of suitable FW and SMD resistant lines. This kit was developed with 10 markers each for FW and SMD resistance. Investigation of the diversity of these loci has shown the role of different alleles in different resistant genotypes. Two genes (C.cajan_03691 and C.cajan_18888) for FW resistance and four genes (C.cajan_07858, C.cajan_20995, C.cajan_21801 and C.cajan_17341) for SMD resistance have been identified. More importantly, we developed a customized and cost-effective Kompetitive allele-specific PCR genotyping assay for the identified genes in order to encourage their downstream applications in pigeonpea breeding programs. The diagnostic marker kit developed here will offer great strength to pigeonpea varietal development program, since the resistance against these two diseases is essentially required for nominating an improved line in varietal release pipeline.

Emerging Roles of RNA-Binding Proteins in Seed Development and Performance

Seed development, dormancy, and germination are key physiological events that are not only important for seed generation, survival, and dispersal, but also contribute to agricultural production. RNA-binding proteins (RBPs) directly interact with target mRNAs and fine-tune mRNA metabolism by governing post-transcriptional regulation, including RNA processing, intron splicing, nuclear export, trafficking, stability/decay, and translational control. Recent studies have functionally characterized increasing numbers of diverse RBPs and shown that they participate in seed development and performance, providing significant insight into the role of RBP-mRNA interactions in seed processes. In this review, we discuss recent research progress on newly defined RBPs that have crucial roles in RNA metabolism and affect seed development, dormancy, and germination.

Plant PUF RNA-binding proteins: A wealth of diversity for post-transcriptional gene regulation

PUF proteins are a conserved group of sequence-specific RNA-binding proteins that typically function to negatively regulate mRNA stability and translation. PUFs are well characterized at the molecular, structural and functional levels in Drosophila, Caenorhabditis elegans, budding yeast and human systems. Although usually encoded by small gene families, PUFs are over-represented in the plant genome, with up to 36 genes identified in a single species. PUF gene expansion in plants has resulted in extensive variability in gene expression patterns, diversity in predicted RNA-binding domain structure, and novel combinations of key amino acids involved in modular nucleotide binding. Reports on the characterization of plant PUF structure and function continue to expand, and include RNA target identification, subcellular distribution, crystal structure, and molecular mechanisms. Arabidopsis PUF mutant analysis has provided insight into biological function, and has identified roles related to development and environmental stress tolerance. The diversity of plant PUFs implies an extensive role for this family of proteins in post-transcriptional gene regulation. This diversity also holds the potential for providing novel RNA-binding domains that could be engineered to produce designer PUFs to alter the metabolism of target RNAs in the cell.

Core spliceosomal Sm proteins as constituents of cytoplasmic mRNPs in plants

In recent years, research has increasingly focused on the key role of post-transcriptional regulation of messenger ribonucleoprotein (mRNP) function and turnover. As a result of the complexity and dynamic nature of mRNPs, the full composition of a single mRNP complex remains unrevealed and mRNPs are poorly described in plants. Here we identify canonical Sm proteins as part of the cytoplasmic mRNP complex, indicating their function in the post-transcriptional regulation of gene expression in plants. Sm proteins comprise an evolutionarily ancient family of small RNA-binding proteins involved in pre-mRNA splicing. The latest research indicates that Sm could also impact on mRNA at subsequent stages of its life cycle. In this work we show that in the microsporocyte cytoplasm of Larix decidua, the European larch, Sm proteins accumulate within distinct cytoplasmic bodies, also containing polyadenylated RNA. To date, several types of cytoplasmic bodies involved in the post-transcriptional regulation of gene expression have been described, mainly in animal cells. Their role and molecular composition in plants remain less well established, however. A total of 222 mRNA transcripts have been identified as cytoplasmic partners for Sm proteins. The specific colocalization of these mRNAs with Sm proteins within cytoplasmic bodies has been confirmed via microscopic analysis. The results from this work support the hypothesis, that evolutionarily conserved Sm proteins have been adapted to perform a whole repertoire of functions related to the post-transcriptional regulation of gene expression in Eukaryota. This adaptation presumably enabled them to coordinate the interdependent processes of splicing element assembly, mRNA maturation and processing, and mRNA translation regulation, and its degradation.

Molecular Basis for a Cell Fate Switch in Response to Impaired Ribosome Biogenesis in the Arabidopsis Root Epidermis

The Arabidopsis (Arabidopsis thaliana) root epidermis consists of a position-dependent pattern of root hair cells and non-hair cells. Underlying this cell type patterning is a network of transcription factors including a central MYB-basic helix-loop-helix-WD40 complex containing WEREWOLF (WER), GLABRA3 (GL3)/ENHANCER OF GLABRA3, and TRANSPARENT TESTA GLABRA1. In this study, we used a genetic enhancer screen to identify apum23-4, a mutant allele of the ribosome biogenesis factor (RBF) gene ARABIDOPSIS PUMILIO23 (APUM23), which caused prospective root hair cells to instead adopt the non-hair cell fate. We discovered that this cell fate switch relied on MYB23, a MYB protein encoded by a WER target gene and acting redundantly with WER. In the apum23-4 mutant, MYB23 exhibited ectopic expression that was WER independent and instead required ANAC082, a recently identified ribosomal stress response mediator. We examined additional RBF mutants that produced ectopic non-hair cells and determined that this cell fate switch is generally linked to defects in ribosome biogenesis. Furthermore, the flagellin peptide flg22 triggers the ANAC082-MYB23-GL2 pathway. Taken together, our study provides a molecular explanation for root epidermal cell fate switch in response to ribosomal defects and, more generally, it demonstrates a novel regulatory connection between stress conditions and cell fate control in plants.

The evolution of the Puf superfamily of proteins across the tree of eukaryotes

BACKGROUND

Eukaryotic gene expression is controlled by a number of RNA-binding proteins (RBP), such as the proteins from the Puf (Pumilio and FBF) superfamily (PufSF). These proteins bind to RNA via multiple Puf repeat domains, each of which specifically recognizes a single RNA base. Recently, three diversified PufSF proteins have been described in model organisms, each of which is responsible for the maturation of ribosomal RNA or the translational regulation of mRNAs; however, less is known about the role of these proteins across eukaryotic diversity.

RESULTS

Here, we investigated the distribution and function of PufSF RBPs in the tree of eukaryotes. We determined that the following PufSF proteins are universally conserved across eukaryotes and can be broadly classified into three groups: (i) Nop9 orthologues, which participate in the nucleolar processing of immature 18S rRNA; (ii) 'classical' Pufs, which control the translation of mRNA; and (iii) PUM3 orthologues, which are involved in the maturation of 7S rRNA. In nearly all eukaryotes, the rRNA maturation proteins, Nop9 and PUM3, are retained as a single copy, while mRNA effectors ('classical' Pufs) underwent multiple lineage-specific expansions. We propose that the variation in number of 'classical' Pufs relates to the size of the transcriptome and thus the potential mRNA targets. We further distinguished full set of PufSF proteins in divergent metamonad Giardia intestinalis and initiated their cellular and biochemical characterization.

CONCLUSIONS

Our data suggest that the last eukaryotic common ancestor (LECA) already contained all three types of PufSF proteins and that 'classical' Pufs then underwent lineage-specific expansions.

Mitochondrial Biogenesis Is Positively Regulated by Casein Kinase I Hrr25 Through Phosphorylation of Puf3 in Saccharomyces cerevisiae

Mitochondrial biogenesis requires coordinated expression of genes encoding mitochondrial proteins, which in Saccharomyces cerevisiae is achieved in part via post-transcriptional control by the Pumilio RNA-binding domain protein Puf3 Puf3 binds to the 3'-UTR of many messenger RNAs (mRNAs) that encode mitochondrial proteins, regulating their turnover, translation, and/or mitochondrial targeting. Puf3 hyperphosphorylation correlates with increased mitochondrial biogenesis; however, the kinase responsible for Puf3 phosphorylation is unclear. Here, we show that the casein kinase I protein Hrr25 negatively regulates Puf3 by mediating its phosphorylation. An hrr25 mutation results in reduced phosphorylation of Puf3 in vivo and a puf3 deletion mutation reverses growth defects of hrr25 mutant cells grown on medium with a nonfermentable carbon source. We show that Hrr25 directly phosphorylates Puf3, and that the interaction between Puf3 and Hrr25 is mediated through the N-terminal domain of Puf3 and the kinase domain of Hrr25 We further found that an hrr25 mutation reduces GFP expression from GFP reporter constructs carrying the 3'-UTR of Puf3 targets. Downregulation of GFP expression due to an hrr25 mutation can be reversed either by puf3Δ or by mutations to the Puf3-binding sites in the 3'-UTR of the GFP reporter constructs. Together, our data indicate that Hrr25 is a positive regulator of mitochondrial biogenesis by phosphorylating Puf3 and inhibiting its function in downregulating target mRNAs encoding mitochondrial proteins.

Genomic survey of RNA recognition motif (RRM) containing RNA binding proteins from barley (Hordeum vulgare ssp. vulgare)

One of the major mechanisms of post-transcriptional gene regulation is achieved by proteins bearing well-defined sequence motifs involved in 'RNA binding'. In eukaryotes, RNA binding proteins (RBPs) are key players of RNA metabolism that includes synthesis, processing, editing, modifying, transport, storage and stability of RNA. In plants, the family of RBPs is vastly expanded compared to other eukaryotes including humans. In this study we identified 363 RBPs in the barley genome. Gene ontology enrichment analysis of barley RBPs indicated these proteins were in all the major cellular compartments and associated with key biological processes including translation, splicing, seed development and stress signaling. Members with the classical RNA binding motifs such as the RNA recognition motif (RRM), KH domain, Helicase, CRM, dsRNA and Pumilio were identified in the repertoire of barley RBPs. Similar to Arabidopsis, the RRM containing RBPs were the most abundant in barley genome. In-depth analysis of the RRM containing proteins - polyA binding proteins, Ser/Arg rich proteins and Glycine-rich RBPs were undertaken. Reanalysis of the proteome dataset of various stages during barley malting identified 38 RBPs suggesting an important role for these proteins during the malting process. This survey provides a systematic analysis of barley RBPs and serves as the basis for the further functional characterization of this important family of proteins.

Functions, mechanisms and regulation of Pumilio/Puf family RNA binding proteins: a comprehensive review

The Pumilio (Pum)/Puf family proteins are ubiquitously present across eukaryotes, including yeast, plants and humans. They generally bind to the 3' untranslated regions of single stranded RNA targets in a sequence specific manner and destabilize them, although a few reports suggest their role in stabilizing the target transcripts. The Pum isoforms are implicated in a wide array of biological processes including stem cell maintenance, development, ribosome biogenesis as well as human diseases. Further studies on Pum would be interesting and important to understand their evolutionarily conserved and divergent features across species, which can have potential implications in medicine, plant sciences as well as basic molecular and cell biological studies. A large number of research reports exists, pertaining to various aspects of Pum, in individual experimental systems. This review is a comprehensive summary of the functions, types, mechanism of action as well as the regulation of Pum in various species. Also, the research questions to be addressed in future are discussed.

Searching for a Match: Structure, Function and Application of Sequence-Specific RNA-Binding Proteins

Plants encode over 1800 RNA-binding proteins (RBPs) that modulate a myriad of steps in gene regulation from chromatin organization to translation, yet only a small number of these proteins and their target transcripts have been functionally characterized. Two classes of eukaryotic RBPs, pentatricopeptide repeat (PPR) and pumilio/fem-3 binding factors (PUF), recognize and bind to specific sequential RNA sequences through protein-RNA interactions. These modular proteins possess helical structural units containing key residues with high affinity for specific nucleotides, whose sequential order determines binding to a specific target RNA sequence. PPR proteins are nucleus-encoded, but largely regulate post-transcriptional gene regulation within plastids and mitochondria, including splicing, translation and RNA editing. Plant PUFs are involved in gene regulatory processes within the cell nucleus and cytoplasm. The modular structures of PPRs and PUFs that determine sequence specificity has facilitated identification of their RNA targets and biological functions. The protein-based RNA-targeting of PPRs and PUFs contrasts to the prokaryotic cluster regularly interspaced short palindromic repeats (CRISPR)-associated proteins (Cas) that target RNAs in prokaryotes. Together the PPR, PUF and CRISPR-Cas systems provide varied opportunities for RNA-targeted engineering applications.

Identification of Arabidopsis CCR4-NOT Complexes with Pumilio RNA-Binding Proteins, APUM5 and APUM2

CCR4/CAF1 are widely conserved deadenylases in eukaryotes. They form a large complex that includes NOT1 as a scaffold protein and various NOT proteins that are core components of multiple levels of gene expression control. The CCR4-NOT complex also contains several RNA-binding proteins as accessory proteins, which are required for target recognition by CCR4/CAF1 deadenylases. AtCCR4a/b, orthologs of human CCR4 in Arabidopsis, have various physiological effects. AtCCR4 isoforms are likely to have specific target mRNAs related to each physiological effect; however, AtCCR4 does not have RNA-binding capability. Therefore, identifying factors that interact with AtCCR4a/b is indispensable to understand its function as a regulator of gene expression, as well as the target mRNA recognition mechanism. Here, we identified putative components of the AtCCR4-NOT complex using co-immunoprecipitation in combination with mass spectrometry using FLAG-tagged AtCCR4b and subsequent verification with a yeast two-hybrid assay. Interestingly, four of 11 AtCAF1 isoforms interacted with both AtCCR4b and AtNOT1, whereas two isoforms interacted only with AtNOT1 in yeast two-hybrid assays. These results imply that Arabidopsis has multiple CCR4-NOT complexes with various combinations of deadenylases. We also revealed that the RNA-binding protein Arabidopsis Pumilio 5 and 2 interacted with AtCCR4a/b in the cytoplasm with a few foci.

Posttranscriptional Regulation of RhBRC1 (Rosa hybrida BRANCHED1) in Response to Sugars is Mediated via its Own 3' Untranslated Region, with a Potential Role of RhPUF4 (Pumilio RNA-Binding Protein Family)

The shoot branching pattern is a determining phenotypic trait throughout plant development. During shoot branching, BRANCHED1 (BRC1) plays a master regulator role in bud outgrowth, and its transcript levels are regulated by various exogenous and endogenous factors. RhBRC1 (the homologous gene of BRC1 in Rosa hybrida) is a main branching regulator whose posttranscriptional regulation in response to sugar was investigated through its 3'UTR. Transformed Rosa calluses containing a construction composed of the CaMV35S promoter, the green fluorescent protein (GFP) reporter gene, and the 3'UTR of RhBRC1 (P35S:GFP::3'UTR_RhBRC1) were obtained and treated with various combinations of sugars and with sugar metabolism effectors. The results showed a major role of the 3'UTR of RhBRC1 in response to sugars, involving glycolysis/the tricarboxylic acid cycle (TCA) and the oxidative pentose phosphate pathway (OPPP). In Rosa vegetative buds, sequence analysis of the RhBRC1 3'UTR identified six binding motifs specific to the Pumilio/FBF RNA-binding protein family (PUF) and probably involved in posttranscriptional regulation. RhPUF4 was highly expressed in the buds of decapitated plants and in response to sugar availability in in-vitro-cultured buds. RhPUF4 was found to be close to AtPUM2, which encodes an Arabidopsis PUF protein. In addition, sugar-dependent upregulation of RhPUF4 was also found in Rosa calluses. RhPUF4 expression was especially dependent on the OPPP, supporting its role in OPPP-dependent posttranscriptional regulation of RhBRC1. These findings indicate that the 3'UTR sequence could be an important target in the molecular regulatory network of RhBRC1 and pave the way for investigating new aspects of RhBRC1 regulation.

Functional and molecular characterization of the conserved Arabidopsis PUMILIO protein, APUM9

Here we demonstrate that the APUM9 RNA-binding protein and its co-factors play a role in mRNA destabilization and how this activity might regulate early plant development. APUM9 is a conserved PUF RNA-binding protein (RBP) under complex transcriptional control mediated by a transposable element (TE) that restricts its expression in Arabidopsis. Currently, little is known about the functional and mechanistic details of the plant PUF regulatory system and the biological relevance of the TE-mediated repression of APUM9 in plant development and stress responses. By combining a range of transient assays, we show here, that APUM9 binding to target transcripts can trigger their rapid decay via its conserved C-terminal RNA-binding domain. APUM9 directly interacts with DCP2, the catalytic subunit of the decapping complex and DCP2 overexpression induces rapid decay of APUM9 targeted mRNAs. We show that APUM9 negatively regulates the expression of ABA signaling genes during seed imbibition, and thereby might contribute to the switch from dormant stage to seed germination. By contrast, strong TE-mediated repression of APUM9 is important for normal plant growth in the later developmental stages. Finally, APUM9 overexpression plants show slightly enhanced heat tolerance suggesting that TE-mediated control of APUM9, might have a role not only in embryonic development, but also in plant adaptation to heat stress conditions.

Puf3 participates in ribosomal biogenesis in malaria parasites

In this study, we characterized the Puf family gene member Puf3 in the malaria parasites Plasmodium falciparum and Plasmodium yoelii Secondary structure prediction suggested that the RNA-binding domains of the Puf3 proteins consisted of 11 pumilio repeats that were similar to those in the human Puf-A (also known as PUM3) and Saccharomyces cerevisiae Puf6 proteins, which are involved in ribosome biogenesis. Neither P. falciparum (Pf)Puf3 nor P. yoelii (Py)Puf3 could be genetically disrupted, suggesting they may be essential for the intraerythrocytic developmental cycle. Cellular fractionation of PfPuf3 in the asexual stages revealed preferential partitioning to the nuclear fraction, consistent with nuclear localization of PfPuf3::GFP and PyPuf3::GFP as detected by immunofluorescence. Furthermore, PfPuf3 colocalized with the nucleolar marker PfNop1, demonstrating that PfPuf3 is a nucleolar protein in the asexual stages. We found, however, that PyPuf3 changed its localization from being nucleolar to being present in cytosolic puncta in the mosquito and liver stages, which may reflect alternative functions in these stages. Affinity purification of molecules that associated with a PTP-tagged variant of PfPuf3 revealed 31 proteins associated with the 60S ribosome, and an enrichment of 28S rRNA and internal transcribed spacer 2 sequences. Taken together, these results suggest an essential function for PfPuf3 in ribosomal biogenesis.

Nucleolar stress and sugar response in plants

The processes involved in ribosome biogenesis, including synthesis of ribosomal proteins, ribosome biogenesis-related factors, and ribosomal RNAs (rRNAs), must be coordinately orchestrated in response to changes in energy supply. In animal cells, defects in ribosome biogenesis induce a nucleolar stress response through the p53-mediated pathway. Our recent finding that an essential, sugar-inducible Arabidopsis gene, APUM24, encoded a pre-rRNA processing factor allowed the relationships between rRNA biogenesis, nucleolar stress, sugar response, and growth regulation to be understood in plants. A knockdown mutant of APUM24 developed sugar-dependent phenotypes including pre-rRNA processing defects, reductions in nucleolar size, and limited promotion of leaf and root growth. Alongside the absence of plant p53 homologs and the synchronous sugar-induced expression of ribosome biogenesis-related genes, these findings suggest the following hypothesis. Sugar supply may enhance ribosome biogenesis defects, leading to p53-independent induction of nucleolar stress responses that include negative regulation of growth and development in plants.

The PUF Protein Family: Overview on PUF RNA Targets, Biological Functions, and Post Transcriptional Regulation

Post-transcriptional regulation of gene expression plays a crucial role in many processes. In cells, it is mediated by diverse RNA-binding proteins. These proteins can influence mRNA stability, translation, and localization. The PUF protein family (Pumilio and FBF) is composed of RNA-binding proteins highly conserved among most eukaryotic organisms. Previous investigations indicated that they could be involved in many processes by binding corresponding motifs in the 3′UTR or by interacting with other proteins. To date, most of the investigations on PUF proteins have been focused on Caenorhabditis elegans, Drosophila melanogaster, and Saccharomyces cerevisiae, while only a few have been conducted on Arabidopsis thaliana. The present article provides an overview of the PUF protein family. It addresses their RNA-binding motifs, biological functions, and post-transcriptional control mechanisms in Caenorhabditis elegans, Drosophila melanogaster, Saccharomyces cerevisiae, and Arabidopsis thaliana. These items of knowledge open onto new investigations into the relevance of PUF proteins in specific plant developmental processes.

Reduced Expression of APUM24, Encoding a Novel rRNA Processing Factor, Induces Sugar-Dependent Nucleolar Stress and Altered Sugar Responses in Arabidopsis thaliana

Ribosome biogenesis is one of the most energy-consuming events in the cell and must therefore be coordinated with changes in cellular energy status. Here, we show that the sugar-inducible gene ARABIDOPSIS PUMILIO PROTEIN24 (APUM24) encodes a Pumilio homology domain-containing protein involved in pre-rRNA processing in Arabidopsis thaliana Null mutation of APUM24 resulted in aborted embryos due to abnormal gametogenesis and embryogenesis, whereas reduced expression of APUM24 caused several phenotypes characteristic of ribosome biogenesis or function-related mutants. APUM24 interacted with other pre-rRNA processing factors and a putative endonuclease for the removal of the internal transcribed spacer 2 (ITS2) of pre-rRNA in the nucleolus. The APUM24-containing complex also interacted with ITS2, and reduced APUM24 expression caused the overaccumulation of processing intermediates containing ITS2. Thus, APUM24 likely functions as an ITS2 removal-associated factor. Most importantly, the apum24 knockdown mutant was hypersensitive to highly concentrated sugar, and the mutant showed sugar-dependent overaccumulation of processing intermediates and nucleolar stress (changes in nucleolar size). Furthermore, reduced APUM24 expression diminished sugar-induced promotion of leaf and root growth. Hence, a breakdown in the coordinated expression of ribosome biogenesis-related genes with energy status may induce nucleolar stress and disturb proper sugar responses in Arabidopsis.

Structural basis for the specific recognition of 18S rRNA by APUM23

PUF (Pumilio/fem-3 mRNA binding factor) proteins, a conserved family of RNA-binding proteins, recognize specific single-strand RNA targets in a specific modular way. Although plants have a greater number of PUF protein members than do animal and fungal systems, they have been the subject of fewer structural and functional investigations. The aim of this study was to elucidate the involvement of APUM23, a nucleolar PUF protein in the plant Arabidopsis, in pre-rRNA processing. APUM23 is distinct from classical PUF family proteins, which are located in the cytoplasm and bind to 3'UTRs of mRNA to modulate mRNA expression and localization. We found that the complete RNA target sequence of APUM23 comprises 11 nt in 18S rRNA at positions 1141-1151. The complex structure shows that APUM23 has 10 PUF repeats; it assembles into a C-shape, with an insertion located within the inner concave surface. We found several different RNA recognition features. A notable structural feature of APUM23 is an insertion in the third PUF repeat that participates in nucleotide recognition and maintains the correct conformation of the target RNA. Our findings elucidate the mechanism for APUM23's-specific recognition of 18S rRNA.

An Arabidopsis divergent pumilio protein, APUM24, is essential for embryogenesis and required for faithful pre-rRNA processing

Pumilio RNA-binding proteins are largely involved in mRNA degradation and translation repression. However, a few evolutionarily divergent Pumilios are also responsible for proper pre-rRNA processing in human and yeast. Here, we describe an essential Arabidopsis nucleolar Pumilio, APUM24, that is expressed in tissues undergoing rapid proliferation and cell division. A T-DNA insertion for APUM24 did not affect the male and female gametogenesis, but instead resulted in a negative female gametophytic effect on zygotic cell division immediately after fertilization. Additionally, the mutant embryos displayed defects in cell patterning from pro-embryo through globular stages. The mutant embryos were marked by altered auxin maxima, which were substantiated by the mislocalization of PIN1 and PIN7 transporters in the defective embryos. Homozygous apum24 callus accumulates rRNA processing intermediates, including uridylated and adenylated 5.8S and 25S rRNA precursors. An RNA-protein interaction assay showed that the histidine-tagged recombinant APUM24 binds RNAin vitro with no apparent specificity. Overall, our results demonstrated that APUM24 is required for rRNA processing and early embryogenesis in Arabidopsis.

Which Plant Proteins Are Involved in Antiviral Defense? Review on In Vivo and In Vitro Activities of Selected Plant Proteins against Viruses

Plants have evolved a variety of defense mechanisms to tackle virus attack. Endogenous plant proteins can function as virus suppressors. Different types of proteins mediate defense responses against plant viruses. Pathogenesis-related (PR) proteins are activated upon pathogen infections or in different stress situations and their production is one of many components in plant defense. Ribosome-inactivating proteins (RIPs) suppress translation by enzymatically damaging ribosomes and they have been found to have antiviral activity. RNA-binding proteins (RBPs) bind to target RNAs via specialized RNA-binding domain and can directly or indirectly function in plant defense system against RNA viruses. Proteins involved in silencing machinery, namely Dicer-like (DCL) proteins, Argonaute (AGO) proteins, and RNA-dependent RNA polymerases (RDRs) confer innate antiviral defense in plants as they are able to degrade foreign RNA of viral origin. This review aims to provide a comprehensive and up-to-date picture of plant proteins participating in antiviral defense. As a result we discuss proteins conferring plant antiviral resistance and their potential future applications in different fields of life including agriculture and medicine.

A Novel RNA-Binding Protein Involves ABA Signaling by Post-transcriptionally Repressing ABI2

The Stress Associated RNA-binding protein 1 (SRP1) repressed by ABA, salt and cold encodes a C2C2-type zinc finger protein in Arabidopsis. The knock-out mutation in srp1 reduced the sensitivity of seed to ABA and salt stress during germination and post-germinative growth stages. In contrast, SRP1-overexpressing seedlings were more sensitive to ABA and salt compared to wild type plants. In the presence of ABA, the transcript levels of ABA signaling and germination-related genes including ABI3. ABI5. EM1 and EM6 were less induced in srp1 compared to WT. Interestingly, expression of ABI2 encoding a protein phosphatase 2C protein were significantly up-regulated in srp1 mutants. By in vitro analysis, SRP1 was identified as a novel RNA-binding protein directly binding to 3'UTR of ABI2 mRNA. Moreover, transient expression assay proved the function of SRP1 in reducing the activity of luciferase whose coding sequence was fused with the ABI2 3'UTR. Together, it is suggested that SRP1 is involved in the ABA signaling by post-transcriptionally repressing ABI2 expression in Arabidopsis.

A Puf RNA-binding protein encoding gene PlM90 regulates the sexual and asexual life stages of the litchi downy blight pathogen Peronophythora litchii

Sexual and asexual reproduction are two key processes in the pathogenic cycle of many filamentous pathogens. However in Peronophythora litchii, the causal pathogen for the litchi downy blight disease, critical regulator(s) of sexual or asexual differentiation has not been elucidated. In this study, we cloned a gene named PlM90 from P. litchii, which encodes a putative Puf RNA-binding protein. We found that PlM90 was highly expressed during asexual development, and much higher than that during sexual development, while relatively lower during cyst germination and plant infection. By polyethylene glycol (PEG)-mediated protoplast transformation, we generated three PlM90-silenced transformants and found a severely impaired ability in sexual spore production and a delay in stages of zoospore release and encystment. However, the pathogenicity of P. litchii was not affected by PlM90-silencing. Therefore we conclude that PlM90 specifically regulates the sexual and asexual differentiation of P. litchii.

A large-scale chromosome-specific SNP discovery guideline

Single-nucleotide polymorphisms (SNPs) are the most prevalent type of variation in genomes that are increasingly being used as molecular markers in diversity analyses, mapping and cloning of genes, and germplasm characterization. However, only a few studies reported large-scale SNP discovery in Aegilops tauschii, restricting their potential use as markers for the low-polymorphic D genome. Here, we report 68,592 SNPs found on the gene-related sequences of the 5D chromosome of Ae. tauschii genotype MvGB589 using genomic and transcriptomic sequences from seven Ae. tauschii accessions, including AL8/78, the only genotype for which a draft genome sequence is available at present. We also suggest a workflow to compare SNP positions in homologous regions on the 5D chromosome of Triticum aestivum, bread wheat, to mark single nucleotide variations between these closely related species. Overall, the identified SNPs define a density of 4.49 SNPs per kilobyte, among the highest reported for the genic regions of Ae. tauschii so far. To our knowledge, this study also presents the first chromosome-specific SNP catalog in Ae. tauschii that should facilitate the association of these SNPs with morphological traits on chromosome 5D to be ultimately targeted for wheat improvement.

WRKY6 restricts Piriformospora indica-stimulated and phosphate-induced root development in Arabidopsis

BACKGROUND

Arabidopsis root growth is stimulated by Piriformospora indica, phosphate limitation and inactivation of the WRKY6 transcription factor. Combinations of these factors induce unexpected alterations in root and shoot growth, root architecture and root gene expression profiles.

RESULTS

The results demonstrate that P. indica promotes phosphate uptake and root development under Pi limitation in wrky6 mutant. This is associated with the stimulation of PHOSPHATE1 expression and ethylene production. Expression profiles from the roots of wrky6 seedlings identified genes involved in hormone metabolism, transport, meristem, cell and plastid proliferation, and growth regulation. 25 miRNAs were also up-regulated in these roots. We generated and discuss here a list of common genes which are regulated in growing roots and which are common to all three growth stimuli investigated in this study.

CONCLUSION

Since root development of wrky6 plants exposed to P. indica under phosphate limitation is strongly promoted, we propose that common genes which respond to all three growth stimuli are central for the control of root growth and architecture. They can be tested for optimizing root growth in model and agricultural plants.

Target selection by natural and redesigned PUF proteins

Pumilio/fem-3 mRNA binding factor (PUF) proteins bind RNA with sequence specificity and modularity, and have become exemplary scaffolds in the reengineering of new RNA specificities. Here, we report the in vivo RNA binding sites of wild-type (WT) and reengineered forms of the PUF protein Saccharomyces cerevisiae Puf2p across the transcriptome. Puf2p defines an ancient protein family present throughout fungi, with divergent and distinctive PUF RNA binding domains, RNA-recognition motifs (RRMs), and prion regions. We identify sites in RNA bound to Puf2p in vivo by using two forms of UV cross-linking followed by immunopurification. The protein specifically binds more than 1,000 mRNAs, which contain multiple iterations of UAAU-binding elements. Regions outside the PUF domain, including the RRM, enhance discrimination among targets. Compensatory mutants reveal that one Puf2p molecule binds one UAAU sequence, and align the protein with the RNA site. Based on this architecture, we redesign Puf2p to bind UAAG and identify the targets of this reengineered PUF in vivo. The mutant protein finds its target site in 1,800 RNAs and yields a novel RNA network with a dramatic redistribution of binding elements. The mutant protein exhibits even greater RNA specificity than wild type. The redesigned protein decreases the abundance of RNAs in its redesigned network. These results suggest that reengineering using the PUF scaffold redirects and can even enhance specificity in vivo.

Evolutionary Conservation and Diversification of Puf RNA Binding Proteins and Their mRNA Targets

Reprogramming of a gene’s expression pattern by acquisition and loss of sequences recognized by specific regulatory RNA binding proteins may be a major mechanism in the evolution of biological regulatory programs. We identified that RNA targets of Puf3 orthologs have been conserved over 100–500 million years of evolution in five eukaryotic lineages. Focusing on Puf proteins and their targets across 80 fungi, we constructed a parsimonious model for their evolutionary history. This model entails extensive and coordinated changes in the Puf targets as well as changes in the number of Puf genes and alterations of RNA binding specificity including that: 1) Binding of Puf3 to more than 200 RNAs whose protein products are predominantly involved in the production and organization of mitochondrial complexes predates the origin of budding yeasts and filamentous fungi and was maintained for 500 million years, throughout the evolution of budding yeast. 2) In filamentous fungi, remarkably, more than 150 of the ancestral Puf3 targets were gained by Puf4, with one lineage maintaining both Puf3 and Puf4 as regulators and a sister lineage losing Puf3 as a regulator of these RNAs. The decrease in gene expression of these mRNAs upon deletion of Puf4 in filamentous fungi (N. crassa) in contrast to the increase upon Puf3 deletion in budding yeast (S. cerevisiae) suggests that the output of the RNA regulatory network is different with Puf4 in filamentous fungi than with Puf3 in budding yeast. 3) The coregulated Puf4 target set in filamentous fungi expanded to include mitochondrial genes involved in the tricarboxylic acid (TCA) cycle and other nuclear-encoded RNAs with mitochondrial function not bound by Puf3 in budding yeast, observations that provide additional evidence for substantial rewiring of post-transcriptional regulation. 4) Puf3 also expanded and diversified its targets in filamentous fungi, gaining interactions with the mRNAs encoding the mitochondrial electron transport chain (ETC) complex I as well as hundreds of other mRNAs with nonmitochondrial functions. The many concerted and conserved changes in the RNA targets of Puf proteins strongly support an extensive role of RNA binding proteins in coordinating gene expression, as originally proposed by Keene. Rewiring of Puf-coordinated mRNA targets and transcriptional control of the same genes occurred at different points in evolution, suggesting that there have been distinct adaptations via RNA binding proteins and transcription factors. The changes in Puf targets and in the Puf proteins indicate an integral involvement of RNA binding proteins and their RNA targets in the adaptation, reprogramming, and function of gene expression.

A map of the evolutionary history of Puf proteins and their RNA targets shows that reprogramming of global gene expression programs via adaptive mutations that affect protein-RNA interactions is an important source of biological diversity.

We set out to trace the evolutionary history of an RNA binding protein and how its interactions with targets change over evolution. Identifying this natural history is a step toward understanding the critical differences between organisms and how gene expression programs are rewired during evolution. Using bioinformatics and experimental approaches, we broadly surveyed the evolution of binding targets of a particular family of RNA binding proteins—the Puf proteins, whose protein sequences and target RNA sequences are relatively well-characterized—across 99 eukaryotic species. We found five groups of species in which targets have been conserved for at least 100 million years and then took advantage of genome sequences from a large number of fungal species to deeply investigate the conservation and changes in Puf proteins and their RNA targets. Our analyses identified multiple and extensive reconfigurations during the natural history of fungi and suggest that RNA binding proteins and their RNA targets are profoundly involved in evolutionary reprogramming of gene expression and help define distinct programs unique to each organism. Continuing to uncover the natural history of RNA binding proteins and their interactions will provide a unique window into the gene expression programs of present day species and point to new ways to engineer gene expression programs.

A Nucleolar PUF RNA-binding Protein with Specificity for a Unique RNA Sequence

PUF proteins are a conserved group of sequence specific RNA-binding proteins that bind to RNA in a modular fashion. The RNA-binding domain of PUF proteins typically consists of eight clustered Puf repeats. Plant genomes code for large families of PUF proteins that show significant variability in their predicted Puf repeat number, organization, and amino acid sequence. Here we sought to determine whether the observed variability in the RNA-binding domains of four plant PUFs results in a preference for nonclassical PUF RNA target sequences. We report the identification of a novel RNA binding sequence for a nucleolar Arabidopsis PUF protein that contains an atypical RNA-binding domain. The Arabidopsis PUM23 (APUM23) binding sequence was 10 nucleotides in length, contained a centrally located UUGA core element, and had a preferred cytosine at nucleotide position 8. These RNA sequence characteristics differ from those of other PUF proteins, because all natural PUFs studied to date bind to RNAs that contain a conserved UGU sequence at their 5' end and lack specificity for cytosine. Gel mobility shift assays validated the identity of the APUM23 binding sequence and supported the location of 3 of the 10 predicted Puf repeats in APUM23, including the cytosine-binding repeat. The preferred 10-nucleotide sequence bound by APUM23 is present within the 18S rRNA sequence, supporting the known role of APUM23 in 18S rRNA maturation. This work also reveals that APUM23, an ortholog of yeast Nop9, could provide an advanced structural backbone for Puf repeat engineering and target-specific regulation of cellular RNAs.

Association Mapping for Fruit, Plant and Leaf Morphology Traits in Eggplant

An eggplant (Solanum melongena) association panel of 191 accessions, comprising a mixture of breeding lines, old varieties and landrace selections was SNP genotyped and phenotyped for key breeding fruit and plant traits at two locations over two seasons. A genome-wide association (GWA) analysis was performed using the mixed linear model, which takes into account both a kinship matrix and the sub-population membership of the accessions. Overall, 194 phenotype/genotype associations were uncovered, relating to 30 of the 33 measured traits. These associations involved 79 SNP loci mapping to 39 distinct chromosomal regions distributed over all 12 eggplant chromosomes. A comparison of the map positions of these SNPs with those of loci derived from conventional linkage mapping showed that GWA analysis both validated many of the known controlling loci and detected a large number of new marker/trait associations. Exploiting established syntenic relationships between eggplant chromosomes and those of tomato and pepper recognized orthologous regions in ten eggplant chromosomes harbouring genes influencing breeders’ traits.

Post-Transcriptional Coordination of the Arabidopsis Iron Deficiency Response is Partially Dependent on the E3 Ligases RING DOMAIN LIGASE1 (RGLG1) and RING DOMAIN LIGASE2 (RGLG2)

Acclimation to changing environmental conditions is mediated by proteins, the abundance of which is carefully tuned by an elaborate interplay of DNA-templated and post-transcriptional processes. To dissect the mechanisms that control and mediate cellular iron homeostasis, we conducted quantitative high-resolution iTRAQ proteomics and microarray-based transcriptomic profiling of iron-deficient Arabidopsis thaliana plants. A total of 13,706 and 12,124 proteins was identified with a quadrupole-Orbitrap hybrid mass spectrometer in roots and leaves, respectively. This deep proteomic coverage allowed accurate estimates of post-transcriptional regulation in response to iron deficiency. Similarly regulated transcripts were detected in only 13% (roots) and 11% (leaves) of the 886 proteins that differentially accumulated between iron-sufficient and iron-deficient plants, indicating that the majority of the iron-responsive proteins was post-transcriptionally regulated. Mutants harboring defects in the RING DOMAIN LIGASE1 (RGLG1)(1) and RING DOMAIN LIGASE2 (RGLG2) showed a pleiotropic phenotype that resembled iron-deficient plants with reduced trichome density and the formation of branched root hairs. Proteomic and transcriptomic profiling of rglg1 rglg2 double mutants revealed that the functional RGLG protein is required for the regulation of a large set of iron-responsive proteins including the coordinated expression of ribosomal proteins. This integrative analysis provides a detailed catalog of post-transcriptionally regulated proteins and allows the concept of a chiefly transcriptionally regulated iron deficiency response to be revisited. Protein data are available via ProteomeXchange with identifier PXD002126.

Pumilio-based RNA in vivo imaging

Subcellular, sequence-specific detection of RNA in vivo is a powerful tool to study the macromolecular transport that occurs through plasmodesmata. The RNA-binding domain of Pumilio proteins can be engineered to bind RNA sequences of choice and fused to fluorescent proteins for RNA imaging. This chapter describes the construction of a Pumilio-based imaging system to track the RNA of Tobacco mosaic virus in vivo, and practical aspects of RNA live-cell imaging.

Association Mapping for Fruit, Plant and Leaf Morphology Traits in Eggplant

An eggplant (Solanum melongena) association panel of 191 accessions, comprising a mixture of breeding lines, old varieties and landrace selections was SNP genotyped and phenotyped for key breeding fruit and plant traits at two locations over two seasons. A genome-wide association (GWA) analysis was performed using the mixed linear model, which takes into account both a kinship matrix and the sub-population membership of the accessions. Overall, 194 phenotype/genotype associations were uncovered, relating to 30 of the 33 measured traits. These associations involved 79 SNP loci mapping to 39 distinct chromosomal regions distributed over all 12 eggplant chromosomes. A comparison of the map positions of these SNPs with those of loci derived from conventional linkage mapping showed that GWA analysis both validated many of the known controlling loci and detected a large number of new marker/trait associations. Exploiting established syntenic relationships between eggplant chromosomes and those of tomato and pepper recognized orthologous regions in ten eggplant chromosomes harbouring genes influencing breeders' traits.

Regulation of protein function by interfering protein species

Abstract Most proteins do not function alone but act in protein complexes. For several transcriptional regulators, it is known that they have to homo- or heterodimerize prior to DNA binding. These protein interactions occur through defined protein-protein-interaction (PPI) domains. More than two decades ago, inhibitor of DNA binding (ID), a small protein containing a single helix-loop-helix (HLH) motif was identified. ID is able to interact with the larger DNA-binding basic helix-loop-helix (bHLH) transcription factors, but due to the lack of the basic domain required for DNA binding, ID traps bHLH proteins in non-functional complexes. Work in plants has, in the recent years, identified more small proteins acting in analogy to ID. A hallmark of these small negative acting proteins is the presence of a protein-interaction domain and the absence of other functional domains required for transcriptional activation or DNA binding. Because these proteins are often very small and function in analogy to microRNAs (meaning in a dominant-negative manner), we propose to refer to these protein species as 'microProteins' (miPs). miPs can be encoded in the genome as individual transcription units but can also be produced by alternative splicing. Other negatively acting proteins, consisting of more than one domain, have also been identified, and we propose to call these proteins 'interfering proteins' (iPs). The aim of this review is to state more precisely how to discriminate miPs from iPs. Therefore, we will highlight recent findings on both protein species and describe their mode of action. Furthermore, miPs have the ability to regulate proteins of diverse functions, emphasizing their value as biotechnological tools.

Reduced Dormancy5 encodes a protein phosphatase 2C that is required for seed dormancy in Arabidopsis

Seed dormancy determines germination timing and contributes to crop production and the adaptation of natural populations to their environment. Our knowledge about its regulation is limited. In a mutagenesis screen of a highly dormant Arabidopsis thaliana line, the reduced dormancy5 (rdo5) mutant was isolated based on its strongly reduced seed dormancy. Cloning of RDO5 showed that it encodes a PP2C phosphatase. Several PP2C phosphatases belonging to clade A are involved in abscisic acid signaling and control seed dormancy. However, RDO5 does not cluster with clade A phosphatases, and abscisic acid levels and sensitivity are unaltered in the rdo5 mutant. RDO5 transcript could only be detected in seeds and was most abundant in dry seeds. RDO5 was found in cells throughout the embryo and is located in the nucleus. A transcriptome analysis revealed that several genes belonging to the conserved PUF family of RNA binding proteins, in particular Arabidopsis PUMILIO9 (APUM9) and APUM11, showed strongly enhanced transcript levels in rdo5 during seed imbibition. Further transgenic analyses indicated that APUM9 reduces seed dormancy. Interestingly, reduction of APUM transcripts by RNA interference complemented the reduced dormancy phenotype of rdo5, indicating that RDO5 functions by suppressing APUM transcript levels.

The Arabidopsis thaliana tandem zinc finger 1 (AtTZF1) protein in RNA binding and decay

The Arabidopsis thaliana tandem zinc finger 1 (AtTZF1) protein is characterized by two tandem-arrayed CCCH-type zinc fingers. We have previously found that AtTZF1 affects hormone-mediated growth, stress and gene expression responses. While much has been learned at the genetic and physiological level, the molecular mechanisms underlying the effects of AtTZF1 on gene expression remain obscure. A human TZF protein, hTTP, is known to bind and trigger the degradation of mRNAs containing AU-rich elements (AREs) at the 3' untranslated regions. However, while the TZF motif of hTTP is characterized by C(X8)C(X5)C(X3)H-(X18)-C(X8)C(X5)C(X3)H, AtTZF1 contains an atypical motif of C(X7)C(X5)C(X3)H-(X16)-C(X5)C(X4)C(X3)H. Moreover, the TZF motif of AtTZF1 is preceded by an arginine-rich (RR) region that is unique to plants. Using fluorescence anisotropy and electrophoretic mobility shift binding assays, we have demonstrated that AtTZF1 binds to RNA molecules with specificity and the interaction is dependent on the presence of zinc. Compared with hTTP, in which TZF is solely responsible for RNA binding, both TZF and RR regions of AtTZF1 are required to achieve high-affinity RNA binding. Moreover, zinc finger integrity is vital for RNA binding. Using a plant protoplast transient expression analysis we have further revealed that AtTZF1 can trigger the decay of ARE-containing mRNAs in vivo. Taken together, our results support the notion that AtTZF1 is involved in RNA turnover.

Characterization of TgPuf1, a member of the Puf family RNA-binding proteins from Toxoplasma gondii

Background

Puf proteins act as translational regulators and affect many cellular processes in a wide range of eukaryotic organisms. Although Puf proteins have been well characterized in many model systems, little is known about the structural and functional characteristics of Puf proteins in the parasite Toxoplasma gondii.

Methods

Using a combination of conventional molecular approaches, we generated endogenous TgPuf1 tagged with hemagglutinin (HA) epitope and investigated the TgPuf1 expression levels and localization in the tachyzoites and bradyzoites. We used RNA Electrophoretic Mobility Shfit Assay (EMSA) to determine whether the recombination TgPuf1 has conserverd RNA binding activity and specificity.

Results

TgPuf1 was expressed at a significantly higher level in bradyzoites than in tachyzoites. TgPuf1 protein was predominantly localized within the cytoplasm and showed a much more granular cytoplasmic staining pattern in bradyzoites. The recombinant Puf domain of TgPuf1 showed strong binding affinity to two RNA fragments containing Puf-binding motifs from other organisms as artificial target sequences. However, two point mutations in the core Puf-binding motif resulted in a significant reduction in binding affinity, indicating that TgPuf1 also binds to conserved Puf-binding motif.

Conclusions

TgPuf1 appears to exhibit different expression levels in the tachyzoites and bradyzoites, suggesting that TgPuf1 may function in regulating the proliferation or/and differentiation that are important in providing parasites with the ability to respond rapidly to changes in environmental conditions. This study provides a starting point for elucidating the function of TgPuf1 during parasite development.

Depletion of the Trypanosome Pumilio domain protein PUF2 or of some other essential proteins causes transcriptome changes related to coding region length

Pumilio domain RNA-binding proteins are known mainly as posttranscriptional repressors of gene expression that reduce mRNA translation and stability. Trypanosoma brucei has 11 PUF proteins. We show here that PUF2 is in the cytosol, with roughly the same number of molecules per cell as there are mRNAs. Although PUF2 exhibits a low level of in vivo RNA binding, it is not associated with polysomes. PUF2 also decreased reporter mRNA levels in a tethering assay, consistent with a repressive role. Depletion of PUF2 inhibited growth of bloodstream-form trypanosomes, causing selective loss of mRNAs with long open reading frames and increases in mRNAs with shorter open reading frames. Reexamination of published RNASeq data revealed the same trend in cells depleted of some other proteins. We speculate that these length effects could be caused by inhibition of the elongation phase of transcription or by an influence of translation status or polysomal conformation on mRNA decay.

APUM5, encoding a Pumilio RNA binding protein, negatively regulates abiotic stress responsive gene expression

BACKGROUND

A mutant screening was carried out previously to look for new genes related to the Cucumber mosaic virus infection response in Arabidopsis. A Pumilio RNA binding protein-coding gene, Arabidopsis Pumilio RNA binding protein 5 (APUM5), was obtained from this screening.

RESULTS

APUM5 transcriptional profiling was carried out using a bioinformatics tool. We found that APUM5 was associated with both biotic and abiotic stress responses. However, bacterial and fungal pathogen infection susceptibility was not changed in APUM5 transgenic plants compared to that in wild type plants although APUM5 expression was induced upon pathogen infection. In contrast, APUM5 was involved in the abiotic stress response. 35S-APUM5 transgenic plants showed hypersensitive phenotypes under salt and drought stresses during germination, primary root elongation at the seedling stage, and at the vegetative stage in soil. We also showed that some abiotic stress-responsive genes were negatively regulated in 35S-APUM5 transgenic plants. The APUM5-Pumilio homology domain (PHD) protein bound to the 3' untranslated region (UTR) of the abiotic stress-responsive genes which contained putative Pumilio RNA binding motifs at the 3' UTR.

CONCLUSIONS

These results suggest that APUM5 may be a new post-transcriptional regulator of the abiotic stress response by direct binding of target genes 3' UTRs.