Homing endonucleases encoded by germ line-limited genes in Tetrahymena thermophila have APETELA2 DNA binding domains

Programmed chromosome fragmentation in ciliated protozoa: multiple means to chromosome ends

SUMMARYCiliated protozoa undergo large-scale developmental rearrangement of their somatic genomes when forming a new transcriptionally active macronucleus during conjugation. This process includes the fragmentation of chromosomes derived from the germline, coupled with the efficient healing of the broken ends by de novo telomere addition. Here, we review what is known of developmental chromosome fragmentation in ciliates that have been well-studied at the molecular level (Tetrahymena, Paramecium, Euplotes, Stylonychia, and Oxytricha). These organisms differ substantially in the fidelity and precision of their fragmentation systems, as well as in the presence or absence of well-defined sequence elements that direct excision, suggesting that chromosome fragmentation systems have evolved multiple times and/or have been significantly altered during ciliate evolution. We propose a two-stage model for the evolution of the current ciliate systems, with both stages involving repetitive or transposable elements in the genome. The ancestral form of chromosome fragmentation is proposed to have been derived from the ciliate small RNA/chromatin modification process that removes transposons and other repetitive elements from the macronuclear genome during development. The evolution of this ancestral system is suggested to have potentiated its replacement in some ciliate lineages by subsequent fragmentation systems derived from mobile genetic elements.

Genome-Wide Identification and Expression Analysis of AP2/ERF Transcription Factor Related to Drought Stress in Cultivated Peanut (Arachis hypogaea L.)

APETALA2/ethylene response element-binding factor (AP2/ERF) transcription factors (TFs) have been found to regulate plant growth and development and response to various abiotic stresses. However, detailed information of AP2/ERF genes in peanut against drought has not yet been performed. Herein, 185 AP2/ERF TF members were identified from the cultivated peanut (A. hypogaea cv. Tifrunner) genome, clustered into five subfamilies: AP2 (APETALA2), ERF (ethylene-responsive-element-binding), DREB (dehydration-responsive-element-binding), RAV (related to ABI3/VP), and Soloist (few unclassified factors)). Subsequently, the phylogenetic relationship, intron-exon structure, and chromosomal location of AhAP2/ERF were further characterized. All of these AhAP2/ERF genes were distributed unevenly across the 20 chromosomes, and 14 tandem and 85 segmental duplicated gene pairs were identified which originated from ancient duplication events. Gene evolution analysis showed that A. hypogaea cv. Tifrunner were separated 64.07 and 66.44 Mya from Medicago truncatula L. and Glycine max L., respectively. Promoter analysis discovered many cis-acting elements related to light, hormones, tissues, and stress responsiveness process. The protein interaction network predicted the exitance of functional interaction among families or subgroups. Expression profiles showed that genes from AP2, ERF, and dehydration-responsive-element-binding subfamilies were significantly upregulated under drought stress conditions. Our study laid a foundation and provided a panel of candidate AP2/ERF TFs for further functional validation to uplift breeding programs of drought-resistant peanut cultivars.

Identification and analysis of micro-exons in AP2/ERF and MADS gene families

Micro‐exons are a set of ultrashort exons with lengths ≤ 51 nucleotides. Our previous study revealed that micro‐exons were enriched in AP2 domains and K‐box domains, which are crucial components of AP2/ERF (APETALA2/ethylene‐responsive element‐binding protein) and MADS‐box (an acronym of MCM1, AGAMOUS, DEFICIENS and SRF) genes, respectively. In this study, we analyzed micro‐exons in the AP2/ERF family from 63 species and demonstrated that 76.8% of micro‐exons are concentrated in AP2 domains. Most micro‐exons appeared in the AP2 subfamily of all the terrestrial plants, but not algae. In addition, micro‐exons and AP2 domains are conserved and under negative selection. The MIKC gene is a typical MADS‐box gene family in terrestrial plants and includes one MADS‐box domain and one K‐box domain. A total of 92.3% of micro‐exons were observed in K‐box domains, and two micro‐exons usually encoded a region of K‐box domain, which is the key to MADS‐box protein polymerization. Furthermore, the micro‐exons of the K‐box domain had higher ratios of nonsynonymous mutations than those of the AP2 domains. Overall, here we explored the relationships and differences among micro‐exons in AP2/ERF and MADS families, and revealed potential functional roles of micro‐exons in these domains.

Genomic identification of AP2/ERF transcription factors and functional characterization of two cold resistance-related AP2/ERF genes in celery (Apium graveolens L.)

This study analyzed the AP2/ERF transcription factors in celery and showed that two dehydration-responsive-element-binding (DREB) transcription factors, AgDREB1 and AgDREB2, contribute to the enhanced resistance to abiotic stress in transgenic Arabidopsis. The AP2/ERF family is a large family of transcription factors (TFs) in higher plants that plays a central role in plant growth, development, and response to environmental stress. Here, 209 AP2/ERF family members were identified in celery based on genomic and transcriptomic data. The TFs were classified into four subfamilies (i.e., DREB, ERF, RAV, and AP2) and Soloist. Evolution analysis indicated that the AP2/ERF TFs are ancient molecules and have expanded in the long-term evolution process of plants and whole-genome duplication events. AgAP2/ERF proteins may be associated with multiple biological processes as predicted by the interaction network. The expression profiles and sequence alignment analysis of the TFs in the DREB-A1 group showed that eight genes could be divided into four branches. Two genes, AgDREB1 and AgDREB2, from the DREB-A1 group were selected for further analysis. Subcellular localization assay suggested that the two proteins are nuclear proteins. Yeast one hybrid assay demonstrated that the two proteins could bind to the dehydration-responsive element (DRE). The overexpression of AgDREB1 and AgDREB2 in Arabidopsis induced the increased tolerance to cold treatment and the up-regulation of the COR genes expression. AgDREB1 and AgDREB2 might function as transcriptional activators in regulating the downstream genes by binding to corresponding DRE to enhance stress tolerance in celery.

Regulation of Apetala2/Ethylene Response Factors in Plants

Multiple environmental stresses affect growth and development of plants. Plants try to adapt under these unfavorable condition through various evolutionary mechanisms like physiological and biochemical alterations connecting various network of regulatory processes. Transcription factors (TFs) like APETALA2/ETHYLENE RESPONSE FACTORS (AP2/ERFs) are an integral component of these signaling cascades because they regulate expression of a wide variety of down stream target genes related to stress response and development through different mechanism. This downstream regulation of transcript does not always positively or beneficially affect the plant but also they display some developmental defects like senescence and reduced growth under normal condition or sensitivity to stress condition. Therefore, tight auto/cross regulation of these TFs at transcriptional, translational and domain level is crucial to understand. The present manuscript discuss the multiple regulation and advantage of plasticity and specificity of these family of TFs to a wide or single downstream target(s) respectively. We have also discussed the concern which comes with the unwanted associated traits, which could only be averted by further study and exploration of these AP2/ERFs.

Fusion of a superfamily 1 helicase and an inactivated DNA polymerase is a signature of common evolutionary history of Polintons, polinton-like viruses, Tlr1 transposons and transpovirons

Polintons (polintoviruses), polinton-like viruses (PLVs) and virophages belong to a recently described major class of eukaryotic viruses that is characterized by a distinct virion morphogenetic protein module and, in many members, a protein-primed family B DNA polymerase (pDNAP). All Polintons, by definition, encode a pDNAP and a retrovirus-like integrase. Most of the PLV lack these genes and instead encode a large protein containing a superfamily 1 (SF1) helicase domain. We show here that the SF1 helicase domain-containing proteins of the PLV also contain an inactivated pDNAP domain. This unique helicase-pDNAP fusion is also encoded by transpovirons, enigmatic plasmid-like genetic elements that are associated with giant viruses of the family Mimiviridae. These findings indicate the directionality of evolution of different groups of viruses and mobile elements in the Polinton-centered class. We propose that the PLV evolved from a polinton via fusion of the pDNAP gene with a helicase gene that was accompanied by mutations in the pDNAP active site, likely resulting in inactivation of the polymerase activity. The transpovirons could have evolved from PLV via the loss of several genes including those encoding the morphogenetic module proteins. These findings reaffirm the central evolutionary position of the Polintons in the evolution of eukaryotic viruses and other mobile genetic elements.

Genome-wide analysis of AP2/ERF transcription factors in carrot (Daucus carota L.) reveals evolution and expression profiles under abiotic stress

AP2/ERF is a large transcription factor family that regulates plant physiological processes, such as plant development and stress response. Carrot (Daucus carota L.) is an important economical crop with a genome size of 480 Mb; the draft genome sequencing of this crop has been completed by our group. However, little is known about the AP2/ERF factors in carrot. In this study, a total of 267 putative AP2/ERF factors were identified from the whole-genome sequence of carrot. These AP2/ERF proteins were phylogenetically clustered into five subfamilies based on their similarity to the amino acid sequences from Arabidopsis. The distribution and comparative genome analysis of the AP2/ERF factors among plants showed the AP2/ERF factors had expansion during the evolutionary process, and the AP2 domain was highly conserved during evolution. The number of AP2/ERF factors in land plants expanded during their evolution. A total of 60 orthologous and 145 coorthologous AP2/ERF gene pairs between carrot and Arabidopsis were identified, and the interaction network of orthologous genes was constructed. The expression patterns of eight AP2/ERF family genes from each subfamily (DREB, ERF, AP2, and RAV) were related to abiotic stresses. Yeast one-hybrid and β-galactosidase activity assays confirmed the DRE and GCC box-binding activities of DREB subfamily genes. This study is the first to identify and characterize the AP2/ERF transcription factors in carrot using whole-genome analysis, and the findings may serve as references for future functional research on the transcription factors in carrot.

The Cryptosporidium parvum ApiAP2 gene family: insights into the evolution of apicomplexan AP2 regulatory systems

We provide the first comprehensive analysis of any transcription factor family in Cryptosporidium, a basal-branching apicomplexan that is the second leading cause of infant diarrhea globally. AP2 domain-containing proteins have evolved to be the major regulatory family in the phylum to the exclusion of canonical regulators. We show that apicomplexan and perkinsid AP2 domains cluster distinctly from other chromalveolate AP2s. Protein-binding specificity assays of C. parvum AP2 domains combined with motif conservation upstream of co-regulated gene clusters allowed the construction of putative AP2 regulons across the in vitro life cycle. Orthologous Apicomplexan AP2 (ApiAP2) expression has been rearranged relative to the malaria parasite P. falciparum, suggesting ApiAP2 network rewiring during evolution. C. hominis orthologs of putative C. parvum ApiAP2 proteins and target genes show greater than average variation. C. parvum AP2 domains display reduced binding diversity relative to P. falciparum, with multiple domains binding the 5'-TGCAT-3', 5'-CACACA-3' and G-box motifs (5'-G[T/C]GGGG-3'). Many overrepresented motifs in C. parvum upstream regions are not AP2 binding motifs. We propose that C. parvum is less reliant on ApiAP2 regulators in part because it utilizes E2F/DP1 transcription factors. C. parvum may provide clues to the ancestral state of apicomplexan transcriptional regulation, pre-AP2 domination.

AINTEGUMENTA-LIKE proteins: hubs in a plethora of networks

Members of the AINTEGUMENTA-LIKE (AIL) family of APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain transcription factors are expressed in all dividing tissues in the plant, where they have central roles in developmental processes such as embryogenesis, stem cell niche specification, meristem maintenance, organ positioning, and growth. When overexpressed, AIL proteins induce adventitious growth, including somatic embryogenesis and ectopic organ formation. The Arabidopsis (Arabidopsis thaliana) genome contains eight AIL genes, including AINTEGUMENTA, BABY BOOM, and the PLETHORA genes. Studies on these transcription factors have revealed their intricate relationship with auxin as well as their involvement in an increasing number of gene regulatory networks, in which extensive crosstalk and feedback loops have a major role.

DNA-binding domains of plant-specific transcription factors: structure, function, and evolution

The families of the plant-specific transcription factors (TFs) are defined by their characteristic DNA-binding domains (DBDs), such as AP2/ERF, B3, NAC, SBP, and WRKY. Recently, three-dimensional structures of the DBDs, including those in complexes with DNA, were determined by NMR spectroscopy and X-ray crystallography. In this review we summarize the functional and evolutionary implications arising from structure analyses. The unexpected structural similarity between B3 and the noncatalytic DBD of the restriction endonuclease EcoRII allowed us to build structural models of the B3/DNA complex. Most of the DBDs of plant-specific TFs are likely to have originated from endonucleases associated with transposable elements. After the DBDs have been established in unicellular eukaryotes, they experienced extensive plant-specific expansion, by acquiring new functions.

AP2/ERF Transcription Factor in Rice: Genome-Wide Canvas and Syntenic Relationships between Monocots and Eudicots

The transcription factor family intimately regulates gene expression in response to hormones, biotic and abiotic factors, symbiotic interactions, cell differentiation, and stress signalling pathways in plants. In this study, 170 AP2/ERF family genes are identified by phylogenetic analysis of the rice genome (Oryza sativa l. japonica) and they are divided into a total of 11 groups, including four major groups (AP2, ERF, DREB, and RAV), 10 subgroups, and two soloists. Gene structure analysis revealed that, at position-6, the amino acid threonine (Thr-6) is conserved in the double domain AP2 proteins compared to the amino acid arginine (Arg-6), which is preserved in the single domain of ERF proteins. In addition, the histidine (His) amino acid is found in both domains of the double domain AP2 protein, which is missing in single domain ERF proteins. Motif analysis indicates that most of the conserved motifs, apart from the AP2/ERF domain, are exclusively distributed among the specific clades in the phylogenetic tree and regulate plausible functions. Expression analysis reveals a widespread distribution of the rice AP2/ERF family genes within plant tissues. In the vegetative organs, the transcripts of these genes are found most abundant in the roots followed by the leaf and stem; whereas, in reproductive tissues, the gene expression of this family is observed high in the embryo and lemma. From chromosomal localization, it appears that repetition and tandem-duplication may contribute to the evolution of new genes in the rice genome. In this study, interspecies comparisons between rice and wheat reveal 34 rice loci and unveil the extent of collinearity between the two genomes. It was subsequently ascertained that chromosome-9 has more orthologous loci for CRT/DRE genes whereas chromosome-2 exhibits orthologs for ERF subfamily members. Maximum conserved synteny is found in chromosome-3 for AP2 double domain subfamily genes. Macrosynteny between rice and Arabidopsis, a distant, related genome, uncovered 11 homologs/orthologs loci in both genomes. The distribution of AP2/ERF family gene paralogs in Arabidopsis was most frequent in chromosome-1 followed by chromosome-5. In Arabidopsis, ERF subfamily gene orthologs are found on chromosome-1, chromosome-3, and chromosome-5, whereas DRE subfamily genes are found on chromosome-2 and chromosome-5. Orthologs for RAV and AP2 with double domains in Arabidopsis are located on chromosome-1 and chromosome-3, respectively. In conclusion, the data generated in this survey will be useful for conducting genomic research to determine the precise role of the AP2/ERF gene during stress responses with the ultimate goal of improving crops.

The floral homeotic protein APETALA2 recognizes and acts through an AT-rich sequence element

Cell fate specification in development requires transcription factors for proper regulation of gene expression. In Arabidopsis, transcription factors encoded by four classes of homeotic genes, A, B, C and E, act in a combinatorial manner to control proper floral organ identity. The A-class gene APETALA2 (AP2) promotes sepal and petal identities in whorls 1 and 2 and restricts the expression of the C-class gene AGAMOUS (AG) from whorls 1 and 2. However, it is unknown how AP2 performs these functions. Unlike the other highly characterized floral homeotic proteins containing MADS domains, AP2 has two DNA-binding domains referred to as the AP2 domains and its DNA recognition sequence is still unknown. Here, we show that the second AP2 domain in AP2 binds a non-canonical AT-rich target sequence, and, using a GUS reporter system, we demonstrate that the presence of this sequence in the AG second intron is important for the restriction of AG expression in vivo. Furthermore, we show that AP2 binds the AG second intron and directly regulates AG expression through this sequence element. Computational analysis reveals that the binding site is highly conserved in the second intron of AG orthologs throughout Brassicaceae. By uncovering a biologically relevant AT-rich target sequence, this work shows that AP2 domains have wide-ranging target specificities and provides a missing link in the mechanisms that underlie flower development. It also sets the foundation for understanding the basis of the broad biological functions of AP2 in Arabidopsis, as well as the divergent biological functions of AP2 orthologs in dicotyledonous plants.

Nuclear dualism

Nuclear dualism is a characteristic feature of the ciliated protozoa. Tetrahymena have two different nuclei in each cell. The larger, polyploid, somatic macronucleus (MAC) is the site of transcriptional activity in the vegetatively growing cell. The smaller, diploid micronucleus (MIC) is transcriptionally inactive in vegetative cells, but is transcriptionally active in mating cells and responsible for the genetic continuity during sexual reproduction. Although the MICs and MACs develop from mitotic products of a common progenitor and reside in a common cytoplasm, they are different from one another in almost every respect.

Genomes of the T4-related bacteriophages as windows on microbial genome evolution

The T4-related bacteriophages are a group of bacterial viruses that share morphological similarities and genetic homologies with the well-studied Escherichia coli phage T4, but that diverge from T4 and each other by a number of genetically determined characteristics including the bacterial hosts they infect, the sizes of their linear double-stranded (ds) DNA genomes and the predicted compositions of their proteomes. The genomes of about 40 of these phages have been sequenced and annotated over the last several years and are compared here in the context of the factors that have determined their diversity and the diversity of other microbial genomes in evolution. The genomes of the T4 relatives analyzed so far range in size between ~160,000 and ~250,000 base pairs (bp) and are mosaics of one another, consisting of clusters of homology between them that are interspersed with segments that vary considerably in genetic composition between the different phage lineages. Based on the known biological and biochemical properties of phage T4 and the proteins encoded by the T4 genome, the T4 relatives reviewed here are predicted to share a genetic core, or "Core Genome" that determines the structural design of their dsDNA chromosomes, their distinctive morphology and the process of their assembly into infectious agents (phage morphogenesis). The Core Genome appears to be the most ancient genetic component of this phage group and constitutes a mere 12-15% of the total protein encoding potential of the typical T4-related phage genome. The high degree of genetic heterogeneity that exists outside of this shared core suggests that horizontal DNA transfer involving many genetic sources has played a major role in diversification of the T4-related phages and their spread to a wide spectrum of bacterial species domains in evolution. We discuss some of the factors and pathways that might have shaped the evolution of these phages and point out several parallels between their diversity and the diversity generally observed within all groups of interrelated dsDNA microbial genomes in nature.

The construction and use of log-odds substitution scores for multiple sequence alignment

Most pairwise and multiple sequence alignment programs seek alignments with optimal scores. Central to defining such scores is selecting a set of substitution scores for aligned amino acids or nucleotides. For local pairwise alignment, substitution scores are implicitly of log-odds form. We now extend the log-odds formalism to multiple alignments, using Bayesian methods to construct "BILD" ("Bayesian Integral Log-odds") substitution scores from prior distributions describing columns of related letters. This approach has been used previously only to define scores for aligning individual sequences to sequence profiles, but it has much broader applicability. We describe how to calculate BILD scores efficiently, and illustrate their uses in Gibbs sampling optimization procedures, gapped alignment, and the construction of hidden Markov model profiles. BILD scores enable automated selection of optimal motif and domain model widths, and can inform the decision of whether to include a sequence in a multiple alignment, and the selection of insertion and deletion locations. Other applications include the classification of related sequences into subfamilies, and the definition of profile-profile alignment scores. Although a fully realized multiple alignment program must rely upon more than substitution scores, many existing multiple alignment programs can be modified to employ BILD scores. We illustrate how simple BILD score based strategies can enhance the recognition of DNA binding domains, including the Api-AP2 domain in Toxoplasma gondii and Plasmodium falciparum.

The CRF domain defines cytokinin response factor proteins in plants

BACKGROUND

Cytokinin Response Factors (CRFs) are a small subset of AP2/ERF transcription factor genes shown in Arabidopsis to regulate leaf development as part of the cytokinin signal transduction pathway. This study examines the phylogenetic distribution of CRF genes in other plant species, and attempts to identify the extent of sequence conservation and potential gene function among all CRF genes.

RESULTS

We identified CRF genes in representatives of all major land plant lineages, including numerous flowering plant taxa in addition to the model systems in which ERF genes have been catalogued. Comparative analysis across this broader sampling has identified strongly conserved amino acid motifs other than the AP2/ERF domain for all CRF proteins as well as signature sequences unique to specific clades of CRF genes. One of these motifs, here designated as the CRF domain, is conserved in and unique to CRF proteins distinguishing them from related genes. We show that this novel domain of approximately 65 amino acids is found in CRF proteins from all groups of land plants and only in CRF genes. Phylogenetic analyses suggest that the evolution of CRF genes has included numerous duplication events. In this phylogenetic context we examine protein evolution including the gain and loss of accessory domains, correlate these molecular evolutionary events with experimental data on cytokinin regulation and speculate on the function and evolution of the CRF domain within AP2/ERF transcription factor proteins. We also tested a prediction drawn from the phylogenetic analyses that four CRF domain containing genes from Tomato, previously unexamined for cytokinin response, are transcriptionally inducible by cytokinin, supporting the link between CRF genes, CRF-specific domains and cytokinin regulation.

CONCLUSION

CRF genes can be identified in all lineages of land plants, as a distinct subset of AP2/ERF proteins containing a specific and unique CRF domain. The CRF domain can be used to identify previously unclassified predicted genes or genes identified only as members of the AP2/ERF protein family. CRF domain presence and phylogenetic relatedness to known Arabidopsis CRF genes predicts gene function to some extent.

CAP2 enhances germination of transgenic tobacco seeds at high temperature and promotes heat stress tolerance in yeast

We reported earlier that ectopic expression of CAP2, a single AP2 domain containing transcription activator from chickpea (Cicer arietinum) in tobacco improves growth and development, and tolerance to dehydration and salt stress, of the transgenic plants. Here, we report that, in addition, the CAP2-transgenic tobacco seeds also exhibit higher germination efficiency at high temperature and show higher expression levels of genes for tobacco heat shock proteins and a heat shock factor. CAP2 was able to activate the 5'-upstream activating sequence of tobacco heat shock factor. Surprisingly, expression of CAP2 cDNA in Saccharomyces cerevisiae also enhanced heat tolerance, with increased expression of the gene for yeast heat shock factor 1 (Hsf1) and its target, the gene for yeast heat shock protein 104 (Hsp104). Sequence analysis of the Hsf1 promoter revealed the presence of a dehydration-responsive element/C-repeat-like element (DRE/CRE). Recombinant CAP2 protein bound to the DRE/CRE in the Hsf1 promoter in a gel shift assay and transactivated the Hsf1 promoter-His reporter construct. The full-length CAP2 protein was required to provide thermotolerance in yeast. If these findings are taken together, our results suggest that CAP2 is involved in the heat stress response and provides an example of functioning of a plant transcription factor in yeast, highlighting the strong evolutionary conservation of the stress response mechanism.

The highly conserved family of Tetrahymena thermophila chromosome breakage elements contains an invariant 10-base-pair core

As a typical ciliate, Tetrahymena thermophila is a unicellular eukaryote that exhibits nuclear dimorphism: each cell contains a diploid, germ line micronucleus (MICN) and a polyploid, somatic macronucleus (MACN). During conjugation, when a new MACN differentiates from a mitotic descendant of the diploid fertilization nucleus, the five MICN chromosomes are site-specifically fragmented into 250 to 300 MACN chromosomes. The classic chromosome breakage sequence (CBS) is a 15-bp element (TAAACCAACCTCTTT) reported to be necessary and sufficient for chromosome breakage. To determine whether a CBS is present at every site of chromosome fragmentation and to assess the range of sequence variation tolerated, 31 CBSs were isolated without preconception as to the sequence of the chromosome breakage element. Additional CBS-related sequences were identified in the whole-genome sequence by their similarities to the classic CBS. Forty CBS elements behaved as authentic chromosome breakage sites. The CBS nucleotide sequence is more diverse than previously thought: nearly half of the CBS elements identified by unbiased methods have a variant of the classic CBS. Only an internal 10-bp core is completely conserved, but the entire 15-bp chromosome breakage sequence shows significant sequence conservation. Our results suggest that any one member of the CBS family provides a necessary and sufficient cis element for chromosome breakage. No chromosome breakage element totally unrelated to the classic CBS element was found; such elements, if they exist at all, must be rare.

Two AP2 domain containing genes isolated from the cold-hardy Citrus relative Poncirus trifoliata are induced in response to cold

Poncirus trifoliata (L.) Raf. is a cold-hardy, interfertile Citrus relative able to tolerate temperatures as low as -26°C when cold acclimated. Therefore, it has been used for improving cold tolerance in cold-sensitive commercial citrus varieties. A cold-induced cDNA library was constructed by subtractive hybridisation of non-acclimated and 2-d cold-acclimated P. trifoliata seedlings and many genes induced in response to cold were identified. Two of these cDNAs, PI-B05 and PI-C10, were selected from this library for further characterisation. Full-length cDNA sequences of these genes were obtained by 5' and 3' rapid amplification of cDNA ends (RACE). Sequence analysis revealed that PI-B05 contained an apetala2 / ethylene response factor (AP2 / ERF) domain and showed homology with ERF proteins from other plants, some of which are involved in environmental stress-induced gene expression. PI-C10 contained both AP2 / ERF and B3 DNA binding domains and showed homology with other plant proteins in the RAV subfamily of the AP2 / ERF transcription factors, some of which are induced in response to cold and other environmental stresses. Expression patterns of these genes in cold-tolerant P. trifoliata and cold-sensitive pummelo [Citrus grandis (L.) Osb.] in response to cold and drought at different time points were analysed by northern blots. Expression analysis showed that both genes were induced in response to cold, but not under drought conditions in cold-hardy P. trifoliata. However, little or no expression of these genes was detected by northern analysis in cold-sensitive pummelo under cold or drought conditions. The sequence analysis and expression data indicated that these genes may play a role in cold-responsive gene expression in cold-hardy P. trifoliata and could possibly be used for improving cold tolerance in cold-sensitive citrus cultivars.

Plasticity of the gene functions for DNA replication in the T4-like phages

We have completely sequenced and annotated the genomes of several relatives of the bacteriophage T4, including three coliphages (RB43, RB49 and RB69), three Aeromonas salmonicida phages (44RR2.8t, 25 and 31) and one Aeromonas hydrophila phage (Aeh1). In addition, we have partially sequenced and annotated the T4-like genomes of coliphage RB16 (a close relative of RB43), A. salmonicida phage 65, Acinetobacter johnsonii phage 133 and Vibrio natriegens phage nt-1. Each of these phage genomes exhibited a unique sequence that distinguished it from its relatives, although there were examples of genomes that are very similar to each other. As a group the phages compared here diverge from one another by several criteria, including (a) host range, (b) genome size in the range between approximately 160 kb and approximately 250 kb, (c) content and genetic organization of their T4-like genes for DNA metabolism, (d) mutational drift of the predicted T4-like gene products and their regulatory sites and (e) content of open-reading frames that have no counterparts in T4 or other known organisms (novel ORFs). We have observed a number of DNA rearrangements of the T4 genome type, some exhibiting proximity to putative homing endonuclease genes. Also, we cite and discuss examples of sequence divergence in the predicted sites for protein-protein and protein-nucleic acid interactions of homologues of the T4 DNA replication proteins, with emphasis on the diversity in sequence, molecular form and regulation of the phage-encoded DNA polymerase, gp43. Five of the sequenced phage genomes are predicted to encode split forms of this polymerase. Our studies suggest that the modular construction and plasticity of the T4 genome type and several of its replication proteins may offer resilience to mutation, including DNA rearrangements, and facilitate the adaptation of T4-like phages to different bacterial hosts in nature.

Phylogenomic analysis of the GIY-YIG nuclease superfamily

Background

The GIY-YIG domain was initially identified in homing endonucleases and later in other selfish mobile genetic elements (including restriction enzymes and non-LTR retrotransposons) and in enzymes involved in DNA repair and recombination. However, to date no systematic search for novel members of the GIY-YIG superfamily or comparative analysis of these enzymes has been reported.

Results

We carried out database searches to identify all members of known GIY-YIG nuclease families. Multiple sequence alignments together with predicted secondary structures of identified families were represented as Hidden Markov Models (HMM) and compared by the HHsearch method to the uncharacterized protein families gathered in the COG, KOG, and PFAM databases. This analysis allowed for extending the GIY-YIG superfamily to include members of COG3680 and a number of proteins not classified in COGs and to predict that these proteins may function as nucleases, potentially involved in DNA recombination and/or repair. Finally, all old and new members of the GIY-YIG superfamily were compared and analyzed to infer the phylogenetic tree.

Conclusion

An evolutionary classification of the GIY-YIG superfamily is presented for the very first time, along with the structural annotation of all (sub)families. It provides a comprehensive picture of sequence-structure-function relationships in this superfamily of nucleases, which will help to design experiments to study the mechanism of action of known members (especially the uncharacterized ones) and will facilitate the prediction of function for the newly discovered ones.

Molecular evolution of the AP2 subfamily

The AP2 (APETALA2)/EREBP (Ethylene Responsive Element Binding Protein) multigene family includes developmentally and physiologically important transcription factors. AP2/EREBP genes are divided into two subfamilies: AP2 genes with two AP2 domains and EREBP genes with a single AP2/ERF (Ethylene Responsive Element Binding Factor) domain. Based on previous phylogenetic analyses, AP2 genes can be divided into two clades, AP2 and ANT groups. To clarify the molecular evolution of the AP2 subfamily, we isolated and sequenced genes with two AP2 domains from three gymnosperms, Cycas revoluta, Ginkgo biloba, and Gnetum parvifolium,as well as from the moss Physcomitrella patens. Expressions of AP2-like genes, including AP2, in Arabidopsis thaliana are regulated by the microRNA miR172. We found that the target site of miR172 is significantly conserved in gymnosperm AP2 homologs, suggesting that regulatory mechanisms of gene expression using microRNA have been conserved over the three hundred million years since the divergence of gymnosperm and flowering plant lineages. We inferred a phylogenetic relationship of these genes with the green alga Chlamydomonas reinhardtii and seed-plant genes available in public DNA databases. The phylogenetic tree showed that the AP2 subfamily diverged into the AP2 and ANT groups before the last common ancestor of land plants and after C. reinhardtii diverged from the land-plant lineage. The tree also indicated that each AP2 and ANT group further diverged into several clades through gene duplications prior to the divergence of gymnosperms and angiosperms.

Phylogeny and domain evolution in the APETALA2-like gene family

The combined processes of gene duplication, nucleotide substitution, domain duplication, and intron/exon shuffling can generate a complex set of related genes that may differ substantially in their expression patterns and functions. The APETALA2-like (AP2-like) gene family exhibits patterns of both gene and domain duplication, coupled with changes in sequence, exon arrangement, and expression. In angiosperms, these genes perform an array of functions including the establishment of the floral meristem, the specification of floral organ identity, the regulation of floral homeotic gene expression, the regulation of ovule development, and the growth of floral organs. To determine patterns of gene diversification, we conducted a series of broad phylogenetic analyses of AP2-like sequences from green plants. These studies indicate that the AP2 domain was duplicated prior to the divergence of the two major lineages of AP2-like genes, euAP2 and AINTEGUMENTA (ANT). Structural features of the AP2-like genes as well as phylogenetic analyses of nucleotide and amino acid (aa) sequences of the AP2-like gene family support the presence of the two major lineages. The ANT lineage is supported by a 10-aa insertion in the AP2-R1 domain and a 1-aa insertion in the AP2-R2 domain, relative to all other members of the AP2-like family. MicroRNA172-binding sequences, the function of which has been studied in some of the AP2-like genes in Arabidopsis, are restricted to the euAP2 lineage. Within the ANT lineage, the euANT lineage is characterized by four conserved motifs: one in the 10-aa insertion in the AP2-R1 domain (euANT1) and three in the predomain region (euANT2, euANT3, and euANT4). Our expression studies show that the euAP2 homologue from Amborella trichopoda, the putative sister to all other angiosperms, is expressed in all floral organs as well as leaves.

Discovery of the principal specific transcription factors of Apicomplexa and their implication for the evolution of the AP2-integrase DNA binding domains

The comparative genomics of apicomplexans, such as the malarial parasite Plasmodium, the cattle parasite Theileria and the emerging human parasite Cryptosporidium, have suggested an unexpected paucity of specific transcription factors (TFs) with DNA binding domains that are closely related to those found in the major families of TFs from other eukaryotes. This apparent lack of specific TFs is paradoxical, given that the apicomplexans show a complex developmental cycle in one or more hosts and a reproducible pattern of differential gene expression in course of this cycle. Using sensitive sequence profile searches, we show that the apicomplexans possess a lineage-specific expansion of a novel family of proteins with a version of the AP2 (Apetala2)-integrase DNA binding domain, which is present in numerous plant TFs. About 20–27 members of this apicomplexan AP2 (ApiAP2) family are encoded in different apicomplexan genomes, with each protein containing one to four copies of the AP2 DNA binding domain. Using gene expression data from Plasmodium falciparum, we show that guilds of ApiAP2 genes are expressed in different stages of intraerythrocytic development. By analogy to the plant AP2 proteins and based on the expression patterns, we predict that the ApiAP2 proteins are likely to function as previously unknown specific TFs in the apicomplexans and regulate the progression of their developmental cycle. In addition to the ApiAP2 family, we also identified two other novel families of AP2 DNA binding domains in bacteria and transposons. Using structure similarity searches, we also identified divergent versions of the AP2-integrase DNA binding domain fold in the DNA binding region of the PI-SceI homing endonuclease and the C-terminal domain of the pleckstrin homology (PH) domain-like modules of eukaryotes. Integrating these findings, we present a reconstruction of the evolutionary scenario of the AP2-integrase DNA binding domain fold, which suggests that it underwent multiple independent combinations with different types of mobile endonucleases or recombinases. It appears that the eukaryotic versions have emerged from versions of the domain associated with mobile elements, followed by independent lineage-specific expansions, which accompanied their recruitment to transcription regulation functions.

Homing into the origin of the AP2 DNA binding domain

The AP2 DNA binding domain was thought to be plant specific because of its presence in plant, but not animal, transcriptional regulators, particularly members of the AP2/ERF family. Two recent studies have identified the AP2 domain in bacteria, bacteriophage and a ciliate as part of proteins that also encode site-specific endonucleases. The association of AP2 with an enzyme known to catalyze its own movement within populations and between species explains the unusual distribution of AP2 and, as such, adds to a growing list of phenomena where mobile DNA has promoted evolutionary novelty.