Novel role for RNase PH in the degradation of structured RNA

Phosphoproteome modulation by nucleoside diphosphate kinase affects photosynthesis & stress tolerance of Nostoc PCC 7120

Nucleoside diphosphate kinase (Ndk/NDK/NDPK) is known to possess pleiotropic functions, one of which is that as a protein kinase, and has been shown to be involved in stress tolerance in plants. To assess its role in the cyanobacterium Nostoc PCC 7120, which is hitherto unreported, recombinant strain overexpressing Ndk, Anndk+ was generated. Phosphoproteomic analysis of Anndk+ and its comparison with that of the vector control, AnpAM, revealed differential phosphorylation at S/T/Y sites of proteins belonging to varied functional groups, with over 17 % phosphoproteins involved in photosynthesis. A total of 177 phosphopeptides and 117 phosphoproteins were identified, including newly identified phosphopeptides in any cyanobacteria. Compared to AnpAM, the Anndk+ cells exhibited (i) lower photosynthetic efficiency and electron transport rate at low PAR (photosynthetically active radiation), (ii) no change in photochemical quenching across PAR, (iii) but distinct non-photochemical quenching [zero Y(NPQ) and high Y(NO) in Anndk+ and high Y(NPQ) and low (NO) in AnpAM], and (iv) increased tolerance to γ-radiation, oxidative, salt and DCMU stresses. The observed modulation of phosphoproteome linked to physiological changes upon overexpression of Ndk in Nostoc could be a combination of direct protein kinase activity of Ndk and/or indirectly through other protein kinases and phosphatases whose phosphorylation status is mediated by Ndk. This is the first report on a direct correlation between Ndk levels, phosphorylation status of proteins and stress tolerance in any cyanobacteria.

A comparative analysis of mycobacterial ribonucleases: Towards a therapeutic novel drug target

Bacterial responses to continuously changing environments are addressed through modulation of gene expression at the level of transcription initiation, RNA processing and/or decay. Ribonucleases (RNases) are hydrolytic or phosphorolytic enzymes involved in a majority of RNA metabolism reactions. RNases play a crucial role in RNA degradation, either independently or in collaboration with various trans-acting regulatory factors. The genus Mycobacterium consists of five subgenera: Mycobacteroides, Mycolicibacterium, Mycobacterium, Mycolicibacter and Mycolicibacillus, which include 63 fully sequenced species (pathogenic/non-pathogenic) to date. These include 13 different RNases, among which 5 are exonucleases (RNase PH, PNPase, RNase D, nano-RNases and RNase AS) and 8 are endonucleases (RNase J, RNase H, RNase P, RNase III, RNase BN, RNase Z, RNase G and RNase E), although RNase J and RNase BN were later identified to have exoribonuclease functions also. Here, we provide a detailed comparative insight into the Escherichia coli and mycobacterial RNases with respect to their types, phylogeny, structure, function, regulation and mechanism of action, with the main emphasis on RNase E. Among these 13 different mycobacterial RNases, 10 are essential for cell survival and have diverse structures hence, they are promising drug targets. RNase E is also an essential endonuclease that is abundant in many bacteria, forms an RNA degradosome complex that controls central RNA processing/degradation and has a conserved 5' sensor domain/DNase-I like region in its RNase domain. The essential mycobacterial RNases especially RNase E provide a potential repertoire of drug targets that can be exploited for inhibitor/modulator screening against many deadly mycobacterial diseases.

Next-Generation Sequencing Application: A Systematic Approach for High-Quality RNA Isolation from Skeletal Muscles

RNA extraction and analyses from tissues using bulk RNA-Sequencing (RNA-Seq) provide a more accurate picture of the gene expression compared to other molecular biology techniques for RNA quantification. Challenges associated with high-quality RNA extraction from skeletal muscles require a modification of standard protocols. Here, we describe a procedure for high-quality RNA isolation from intrinsic laryngeal muscles transferable to skeletal muscles with comparable technical and biological difficulties. Standard protocols for RNA isolation were optimized by maximizing the pooling strategy, determining the sample weight, applying cryogenic muscle disruption, and incorporating RNase-inhibiting reagents during the tissue preparation steps.

Suppression of multiple phenotypic and RNA processing defects by over-expression of Oligoribonuclease in Escherichia coli

Oligoribonuclease (Orn) is an essential Ribonuclease (RNase) from Escherichia coli (E. coli), which plays a critical role in the conversion of short RNA molecules (NanoRNAs) to mononucleotides. Although no additional functions have been ascribed to Orn since its discovery nearly 50 years ago, it was observed in this study that the growth defects caused by a lack of two other RNases that do not digest NanoRNAs, PNPase and RNase PH, could be suppressed by Orn over-expression. Further analyses showed that Orn over-expression can alleviate the growth defects caused by an absence of other RNases even when its expression was increased by a small degree, and it can carry out molecular reactions that are normally performed by RNase T and RNase PH. In addition, biochemical assays revealed that Orn can fully digest single stranded RNAs within a variety of structural contexts. These studies provide new insights into Orn function and its ability to participate in multiple aspects of E. coli RNA metabolism.

Unraveling the Role of Acetic Acid Bacteria Comparing Two Acetification Profiles From Natural Raw Materials: A Quantitative Approach in Komagataeibacter europaeus

The industrial production of vinegar is carried out by the activity of a complex microbiota of acetic acid bacteria (AAB) working, mainly, within bioreactors providing a quite specific and hard environment. The “omics” sciences can facilitate the identification and characterization analyses of these microbial communities, most of which are difficult to cultivate by traditional methods, outside their natural medium. In this work, two acetification profiles coming from the same AAB starter culture but using two natural raw materials of different alcoholic origins (fine wine and craft beer), were characterized and compared and the emphasis of this study is the effect of these raw materials. For this purpose, the composition and natural behavior of the microbiota present throughout these profiles were analyzed by metaproteomics focusing, mainly, on the quantitative protein profile of Komagataeibacter europaeus. This species provided a protein fraction significantly higher (73.5%) than the others. A submerged culture system and semi-continuous operating mode were employed for the acetification profiles and liquid chromatography with tandem mass spectrometry (LC-MS/MS) for the protein analyses. The results showed that neither of two raw materials barely modified the microbiota composition of the profiles, however, they had an effect on the protein expression changes in different biological process. A molecular strategy in which K. europaeus would prevail over other species by taking advantage of the different features offered by each raw material has been suggested. First, by assimilating the excess of inner acetic acid through the TCA cycle and supplying biosynthetic precursors to replenish the cellular material losses; second, by a previous assimilation of the excess of available glucose, mainly in the beer medium, through the glycolysis and the pentose phosphate pathway (PPP); and third, by triggering membrane mechanisms dependent on proton motive force to detoxify the cell at the final moments of acetification. This study could complement the current knowledge of these bacteria as well as to expand the use of diverse raw materials and optimize operating conditions to obtain quality vinegars.

Small RNAs Asserting Big Roles in Mycobacteria

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (Mtb), with 10.4 million new cases per year reported in the human population. Recent studies on the Mtb transcriptome have revealed the abundance of noncoding RNAs expressed at various phases of mycobacteria growth, in culture, in infected mammalian cells, and in patients. Among these noncoding RNAs are both small RNAs (sRNAs) between 50 and 350 nts in length and smaller RNAs (sncRNA) < 50 nts. In this review, we provide an up-to-date synopsis of the identification, designation, and function of these Mtb-encoded sRNAs and sncRNAs. The methodological advances including RNA sequencing strategies, small RNA antagonists, and locked nucleic acid sequence-specific RNA probes advancing the studies on these small RNA are described. Initial insights into the regulation of the small RNA expression and putative processing enzymes required for their synthesis and function are discussed. There are many open questions remaining about the biological and pathogenic roles of these small non-coding RNAs, and potential research directions needed to define the role of these mycobacterial noncoding RNAs are summarized.

RNase AM, a 5' to 3' exonuclease, matures the 5' end of all three ribosomal RNAs in E. coli

Bacterial ribosomal RNAs (rRNAs) are transcribed as precursors and require processing by Ribonucleases (RNases) to generate mature and functional rRNAs. Although the initial steps of rRNA processing in Escherichia coli (E. coli) were described several decades ago, the enzymes responsible for the final steps of 5S and 23S rRNA 5'-end maturation have remained unknown. Here, I show that RNase AM, a recently identified 5' to 3' exonuclease, performs the last step of 5S rRNA 5'-end maturation. RNase AM was also found to generate the mature 5' end of 23S rRNA, subsequent to a newly identified prior processing step. Additionally, RNase AM was found to mature the 5' end of 16S rRNA, a reaction previously attributed to RNase G. These findings indicate a major role for RNase AM in cellular RNA metabolism and establish a biological role for the first 5' to 3' RNA exonuclease identified in E. coli.

Bacterial ribonucleases and their roles in RNA metabolism

Ribonucleases (RNases) are mediators in most reactions of RNA metabolism. In recent years, there has been a surge of new information about RNases and the roles they play in cell physiology. In this review, a detailed description of bacterial RNases is presented, focusing primarily on those from Escherichia coli and Bacillus subtilis, the model Gram-negative and Gram-positive organisms, from which most of our current knowledge has been derived. Information from other organisms is also included, where relevant. In an extensive catalog of the known bacterial RNases, their structure, mechanism of action, physiological roles, genetics, and possible regulation are described. The RNase complement of E. coli and B. subtilis is compared, emphasizing the similarities, but especially the differences, between the two. Included are figures showing the three major RNA metabolic pathways in E. coli and B. subtilis and highlighting specific steps in each of the pathways catalyzed by the different RNases. This compilation of the currently available knowledge about bacterial RNases will be a useful tool for workers in the RNA field and for others interested in learning about this area.

Toxin YafQ Reduces Escherichia coli Growth at Low Temperatures

Toxin/antitoxin (TA) systems reduce metabolism under stress; for example, toxin YafQ of the YafQ/DinJ Escherichia coli TA system reduces growth by cleaving transcripts with in-frame 5'-AAA-G/A-3' sites, and antitoxin DinJ is a global regulator that represses its locus as well as controls levels of the stationary sigma factor RpoS. Here we investigated the influence on cell growth at various temperatures and found that deletion of the antitoxin gene, dinJ, resulted in both reduced metabolism and slower growth at 18°C but not at 37°C. The reduction in growth could be complemented by producing DinJ from a plasmid. Using a transposon screen to reverse the effect of the absence of DinJ, two mutations were found that inactivated the toxin YafQ; hence, the toxin caused the slower growth only at low temperatures rather than DinJ acting as a global regulator. Corroborating this result, a clean deletion of yafQ in the ΔdinJ ΔKmR strain restored both metabolism and growth at 18°C. In addition, production of YafQ was more toxic at 18°C compared to 37°C. Furthermore, by overproducing all the E. coli proteins, the global transcription repressor Mlc was found that counteracts YafQ toxicity only at 18°C. Therefore, YafQ is more effective at reducing metabolism at low temperatures, and Mlc is its putative target.

Elucidation of pathways of ribosomal RNA degradation: an essential role for RNase E

Although normally stable in growing cells, ribosomal RNAs are degraded under conditions of stress, such as starvation, and in response to misassembled or otherwise defective ribosomes in a process termed RNA quality control. Previously, our laboratory found that large fragments of 16S and 23S rRNA accumulate in strains lacking the processive exoribonucleases RNase II, RNase R, and PNPase, implicating these enzymes in the later steps of rRNA breakdown. Here, we define the pathways of rRNA degradation in the quality control process and during starvation, and show that the essential endoribonuclease, RNase E, is required to make the initial cleavages in both degradative processes. We also present evidence that explains why the exoribonuclease, RNase PH, is required to initiate the degradation of rRNA during starvation. The data presented here provide the first detailed description of rRNA degradation in bacterial cells.

Novel Two-Step Hierarchical Screening of Mutant Pools Reveals Mutants under Selection in Chicks

Contaminated chicken/egg products are major sources of human salmonellosis, yet the strategies used by Salmonella to colonize chickens are poorly understood. We applied a novel two-step hierarchical procedure to identify new genes important for colonization and persistence of Salmonella enterica serotype Typhimurium in chickens. A library of 182 S. Typhimurium mutants each containing a targeted deletion of a group of contiguous genes (for a total of 2,069 genes deleted) was used to identify regions under selection at 1, 3, and 9 days postinfection in chicks. Mutants in 11 regions were under selection at all assayed times (colonization mutants), and mutants in 15 regions were under selection only at day 9 (persistence mutants). We assembled a pool of 92 mutants, each deleted for a single gene, representing nearly all genes in nine regions under selection. Twelve single gene deletion mutants were under selection in this assay, and we confirmed 6 of 9 of these candidate mutants via competitive infections and complementation analysis in chicks. STM0580, STM1295, STM1297, STM3612, STM3615, and STM3734 are needed for Salmonella to colonize and persist in chicks and were not previously associated with this ability. One of these key genes, STM1297 (selD), is required for anaerobic growth and supports the ability to utilize formate under these conditions, suggesting that metabolism of formate is important during infection. We report a hierarchical screening strategy to interrogate large portions of the genome during infection of animals using pools of mutants of low complexity. Using this strategy, we identified six genes not previously known to be needed during infection in chicks, and one of these (STM1297) suggests an important role for formate metabolism during infection.

Discovery of a Previously Unrecognized Ribonuclease from Escherichia coli That Hydrolyzes 5'-Phosphorylated Fragments of RNA

TrpH or YciV (locus tag b1266) from Escherichia coli is annotated as a protein of unknown function that belongs to the polymerase and histidinol phosphatase (PHP) family of proteins in the UniProt and NCBI databases. Enzymes from the PHP family have been shown to hydrolyze organophosphoesters using divalent metal ion cofactors at the active site. We found that TrpH is capable of hydrolyzing the 3'-phosphate from 3',5'-bis-phosphonucleotides. The enzyme will also sequentially hydrolyze 5'-phosphomononucleotides from 5'-phosphorylated RNA and DNA oligonucleotides, with no specificity toward the identity of the nucleotide base. The enzyme will not hydrolyze RNA or DNA oligonucleotides that are unphosphorylated at the 5'-end of the substrate, but it makes no difference whether the 3'-end of the oligonucleotide is phosphorylated. These results are consistent with the sequential hydrolysis of 5'-phosphorylated mononucleotides from oligonucleotides in the 5' → 3' direction. The catalytic efficiencies for hydrolysis of 3',5'-pAp, p(Ap)A, p(Ap)4A, and p(dAp)4dA were determined to be 1.8 × 10(5), 9.0 × 10(4), 4.6 × 10(4), and 2.9 × 10(3) M(-1) s(-1), respectively. TrpH was found to be more efficient at hydrolyzing RNA oligonucleotides than DNA oligonucleotides. This enzyme can also hydrolyze annealed DNA duplexes, albeit at a catalytic efficiency approximately 10-fold lower than that of the corresponding single-stranded oligonucleotides. TrpH is the first enzyme from E. coli that has been found to possess 5' → 3' exoribonuclease activity. We propose to name this enzyme RNase AM.

A ribonuclease coordinates siRNA amplification and mRNA cleavage during RNAi

Effective silencing by RNA-interference (RNAi) depends on mechanisms that amplify and propagate the silencing signal. In some organisms, small-interfering RNAs (siRNAs) are amplified from target mRNAs by RNA-dependent RNA polymerase (RdRP). Both RdRP recruitment and mRNA silencing require Argonaute proteins, which are generally thought to degrade RNAi targets by directly cleaving them. However, in C. elegans, the enzymatic activity of the primary Argonaute, RDE-1, is not required for silencing activity. We show that RDE-1 can instead recruit an endoribonuclease, RDE-8, to target RNA. RDE-8 can cleave RNA in vitro and is needed for the production of 3' uridylated fragments of target mRNA in vivo. We also find that RDE-8 promotes RdRP activity, thereby ensuring amplification of siRNAs. Together, our findings suggest a model in which RDE-8 cleaves target mRNAs to mediate silencing, while generating 3' uridylated mRNA fragments to serve as templates for the RdRP-directed amplification of the silencing signal.

Messenger RNA degradation in bacterial cells

mRNA degradation is an important mechanism for controlling gene expression in bacterial cells. This process involves the orderly action of a battery of cellular endonucleases and exonucleases, some universal and others present only in certain species. These ribonucleases function with the assistance of ancillary enzymes that covalently modify the 5' or 3' end of RNA or unwind base-paired regions. Triggered by initiating events at either the 5' terminus or an internal site, mRNA decay occurs at diverse rates that are transcript specific and governed by RNA sequence and structure, translating ribosomes, and bound sRNAs or proteins. In response to environmental cues, bacteria are able to orchestrate widespread changes in mRNA lifetimes by modulating the concentration or specific activity of cellular ribonucleases or by unmasking the mRNA-degrading activity of cellular toxins.

RNase J is required for processing of a small number of RNAs in Rhodobacter sphaeroides

All bacteria contain multiple exoribonucleases to ensure a fast breakdown of different RNA molecules, either for maturation or for complete degradation to the level of mononucleotides. This efficient RNA degradation plays pivotal roles in the post-transcriptional gene regulation, in RNA processing and maturation as well as in RNA quality control mechanisms and global adaption to stress conditions. Besides different 3'-to-5' exoribonucleases mostly with overlapping functions in vivo many bacteria additionally possess the 5'-to-3' exoribonuclease, RNase J, to date the only known bacterial ribonuclease with this activity. An RNA-seq approach was applied to identify specific targets of RNase J in the α-proteobacterium Rhodobacter sphaeroides. Only few transcripts were strongly affected by the lack of RNase J implying that its function is mostly required for specific processing/degradation steps in this bacterium. The accumulation of diverse RNA fragments in the RNase J deletion mutant points to RNA features that apparently cannot be targeted by the conventional 3'-exoribonucleases in Gram-negative bacteria.

Ribonuclease PH interacts with an acidic ribonuclease E site through a basic 80-amino acid domain

In this work, we characterize the domains for the in vivo interaction between ribonuclease E (RNase E) and ribonuclease PH (RNase PH). We initially explored the interaction using pull-down assays with full wild-type proteins expressed from a chromosomal monocopy gene. Once the interaction was confirmed, we narrowed down the sites of interaction in each enzyme to an acidic 16-amino acid region in the carboxy-terminal domain of RNase E and a basic 80-amino acid region in RNase PH including an α3 helix. Our results suggest two novel functional domains of interaction between ribonucleases.

Evolutionary dynamics of Polynucelotide phosphorylases

Polynucleotide phosphorylase (PNPase) is an evolutionarily conserved 3'→5' phosphate-dependent exoribonucease belonging to the PDX family of proteins. It consists of two catalytic RNase PH domains (PNP1 and PNP2), an α-helical domain and two RNA-binding domains. The PNP1 and PNP2 domains share substantial sequence and structural homology with RNase PH (RPH), which is another PDX family member found in all the three major kingdoms of life, suggesting that these three domains originated from a common ancestor. Phylogenetic analysis (based on the PNPase/RNase PH sequence information for 43 vertebrate taxa) shows that PNP2 and RPH are sister taxa which arose through duplication of the ancestral PNP1 domain. Also, all three domains (PNP1, PNP2 and RPH), along with the KH and S1 domains have undergone significant and directional sequence change, as determined by branch and site-specific dN/dS analyses. In general, codons that show dN/dS ratios that are significantly greater than 1.0 are outside the ordered regions (α-helices and β-sheets) of these protein domains. In addition, sites that have been selected for mutagenesis in these proteins lie embedded in regions where there is a preponderance of codons with dN/dS values that are not significantly different from 0.0. Overall, this report is an attempt to further our understanding of the evolutionary history of these three protein domains, and define the evolutionary events that led to their refinement in the vertebrate lineage leading to mammals.

A conserved loop in polynucleotide phosphorylase (PNPase) essential for both RNA and ADP/phosphate binding

Polynucleotide phosphorylase (PNPase) reversibly catalyzes RNA phosphorolysis and polymerization of nucleoside diphosphates. Its homotrimeric structure forms a central channel where RNA is accommodated. Each protomer core is formed by two paralogous RNase PH domains: PNPase1, whose function is largely unknown, hosts a conserved FFRR loop interacting with RNA, whereas PNPase2 bears the putative catalytic site, ∼20 Å away from the FFRR loop. To date, little is known regarding PNPase catalytic mechanism. We analyzed the kinetic properties of two Escherichia coli PNPase mutants in the FFRR loop (R79A and R80A), which exhibited a dramatic increase in Km for ADP/Pi binding, but not for poly(A), suggesting that the two residues may be essential for binding ADP and Pi. However, both mutants were severely impaired in shifting RNA electrophoretic mobility, implying that the two arginines contribute also to RNA binding. Additional interactions between RNA and other PNPase domains (such as KH and S1) may preserve the enzymatic activity in R79A and R80A mutants. Inspection of enzyme structure showed that PNPase has evolved a long-range acting hydrogen bonding network that connects the FFRR loop with the catalytic site via the F380 residue. This hypothesis was supported by mutation analysis. Phylogenetic analysis of PNPase domains and RNase PH suggests that such network is a unique feature of PNPase1 domain, which coevolved with the paralogous PNPase2 domain.

Intracellular ribonucleases involved in transcript processing and decay: precision tools for RNA

In order to adapt to changing environmental conditions and regulate intracellular events such as division, cells are constantly producing new RNAs while discarding old or defective transcripts. These functions require the coordination of numerous ribonucleases that precisely cleave and trim newly made transcripts to produce functional molecules, and rapidly destroy unnecessary cellular RNAs. In recent years our knowledge of the nature, functions and structures of these enzymes in bacteria, archaea and eukaryotes has dramatically expanded. We present here a synthetic overview of the recent development in this dynamic area which has seen the identification of many new endoribonucleases and exoribonucleases. Moreover, the increasing pace at which the structures of these enzymes, or of their catalytic domains, have been solved has provided atomic level detail into their mechanisms of action. Based on sequence conservation and structural data, these proteins have been grouped into families, some of which contain only ribonuclease members, others including a variety of nucleolytic enzymes that act upon DNA and/or RNA. At the other extreme some ribonucleases belong to families of proteins involved in a wide variety of enzymatic reactions. Functional characterization of these fascinating enzymes has provided evidence for the extreme diversity of their biological functions that include, for example, removal of poly(A) tails (deadenylation) or poly(U) tails from eukaryotic RNAs, processing of tRNA and mRNA 3' ends, maturation of rRNAs and destruction of unnecessary mRNAs. This article is part of a Special Issue entitled: RNA Decay mechanisms.