A Novel RNA Phosphorylation State Enables 5' End-Dependent Degradation in Escherichia coli

5' and 3' modifications controlling RNA degradation: from safeguards to executioners

RNA degradation is a key process in the regulation of gene expression. In all organisms, RNA degradation participates in controlling coding and non-coding RNA levels in response to developmental and environmental cues. RNA degradation is also crucial for the elimination of defective RNAs. Those defective RNAs are mostly produced by 'mistakes' made by the RNA processing machinery during the maturation of functional transcripts from their precursors. The constant control of RNA quality prevents potential deleterious effects caused by the accumulation of aberrant non-coding transcripts or by the translation of defective messenger RNAs (mRNAs). Prokaryotic and eukaryotic organisms are also under the constant threat of attacks from pathogens, mostly viruses, and one common line of defence involves the ribonucleolytic digestion of the invader's RNA. Finally, mutations in components involved in RNA degradation are associated with numerous diseases in humans, and this together with the multiplicity of its roles illustrates the biological importance of RNA degradation. RNA degradation is mostly viewed as a default pathway: any functional RNA (including a successful pathogenic RNA) must be protected from the scavenging RNA degradation machinery. Yet, this protection must be temporary, and it will be overcome at one point because the ultimate fate of any cellular RNA is to be eliminated. This special issue focuses on modifications deposited at the 5' or the 3' extremities of RNA, and how these modifications control RNA stability or degradation.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

RNA polyadenylation and its consequences in prokaryotes

Post-transcriptional addition of poly(A) tails to the 3' end of RNA is one of the fundamental events controlling the functionality and fate of RNA in all kingdoms of life. Although an enzyme with poly(A)-adding activity was discovered in Escherichia coli more than 50 years ago, its existence and role in prokaryotic RNA metabolism were neglected for many years. As a result, it was not until 1992 that E. coli poly(A) polymerase I was purified to homogeneity and its gene was finally identified. Further work revealed that, similar to its role in surveillance of aberrant nuclear RNAs of eukaryotes, the addition of poly(A) tails often destabilizes prokaryotic RNAs and their decay intermediates, thus facilitating RNA turnover. Moreover, numerous studies carried out over the last three decades have shown that polyadenylation greatly contributes to the control of prokaryotic gene expression by affecting the steady-state level of diverse protein-coding and non-coding transcripts including antisense RNAs involved in plasmid copy number control, expression of toxin-antitoxin systems and bacteriophage development. Here, we review the main findings related to the discovery of polyadenylation in prokaryotes, isolation, and characterization and regulation of bacterial poly(A)-adding activities, and discuss the impact of polyadenylation on prokaryotic mRNA metabolism and gene expression.This article is part of the theme issue '5' and 3' modifications controlling RNA degradation'.

Analysis of differentially upregulated proteins in ptsHIcrr- and rppH- mutants in Escherichia coli during an adaptive laboratory evolution experiment

The previous deletion of the cytoplasmic components of the phosphotransferase system (PTS) in Escherichia coli JM101 resulted in the PTS^- derivative strain PB11 with severely impaired growth capability in glucose as the sole carbon source. Previous adaptive laboratory evolution (ALE) experiment led to select a fast-growing strain named PB12 from PB11. Comparative genome analysis of PB12 showed a chromosomal deletion, which result in the loss of several genes including rppH which codes for the RNA pyrophosphohydrolase RppH, involved in the preparation of hundreds of mRNAs for further degradation by RNase E. Previous inactivation of rppH in PB11 (PB11rppH^-) improved significantly its growing capabilities and increased several mRNAs respect its parental strain PB11. These previous results led to propose to the PB11rppH^- mutant as an intermediate between PB11 and PB12 strains merged during the early ALE experiment. In this contribution, we report the metabolic response to the PTS^- and rppH^- mutations in the deep of a proteomic approach to understanding the relevance of rppH^- phenotype during an ALE experiment. Differentially upregulated proteins between the wild-type JM101/PB11, PB11/PB11rppH^-, and PB11/PB12 comparisons led to identifying 45 proteins between strain comparisons. Downregulated or upregulated proteins in PB11rppH^- were found expressed at an intermediate level with respect to PB11 and PB12. Many of these proteins were found involved in non-previously metabolic traits reported in the study of the PTS^- strains, including glucose, amino acids, ribose transport; amino acid biosynthesis; NAD biosynthesis/salvage pathway, biosynthesis of Ac-CoA precursors; detoxification and degradation pathways; stress response; protein synthesis; and possible mutator activities between comparisons. No changes were found in the expression of galactose permease GalP, previously proposed as the primary glucose transporter in the absence of PTS selected by the PTS^- derivatives during the ALE experiment. This result suggests that the evolving PTS^- population selected other transporters such as LamB, MglB, and ManX instead of GalP for glucose uptake during the early ALE experiment. Analysis of the biological relevance of the metabolic traits developed by the studied strains provided valuable information to understand the relevance of the rppH^- mutation in the PTS^- background during an ALE experiment as a strategy for the selection of valuable phenotypes for metabolic engineering purposes.

After the Fact(or): Posttranscriptional Gene Regulation in Enterohemorrhagic Escherichia coli O157:H7

To adapt to ever-changing environments, pathogens quickly alter gene expression. This can occur through transcriptional, posttranscriptional, or posttranslational regulation. Historically, transcriptional regulation has been thoroughly studied to understand pathogen niche adaptation, whereas posttranscriptional and posttranslational gene regulation has only relatively recently been appreciated to play a central role in bacterial pathogenesis. Posttranscriptional regulation may involve chaperones, nucleases, and/or noncoding small RNAs (sRNAs) and typically controls gene expression by altering the stability and/or translation of the target mRNA. In this review, we highlight the global importance of posttranscriptional regulation to enterohemorrhagic Escherichia coli (EHEC) gene expression and discuss specific mechanisms of how EHEC regulates expression of virulence factors critical to host colonization and disease progression. The low infectious dose of this intestinal pathogen suggests that EHEC is particularly well adapted to respond to the host environment.

SMRT-Cappable-seq reveals complex operon variants in bacteria

Current methods for genome-wide analysis of gene expression require fragmentation of original transcripts into small fragments for short-read sequencing. In bacteria, the resulting fragmented information hides operon complexity. Additionally, in vivo processing of transcripts confounds the accurate identification of the 5' and 3' ends of operons. Here we develop a methodology called SMRT-Cappable-seq that combines the isolation of un-fragmented primary transcripts with single-molecule long read sequencing. Applied to E. coli, this technology results in an accurate definition of the transcriptome with 34% of known operons from RegulonDB being extended by at least one gene. Furthermore, 40% of transcription termination sites have read-through that alters the gene content of the operons. As a result, most of the bacterial genes are present in multiple operon variants reminiscent of eukaryotic splicing. By providing such granularity in the operon structure, this study represents an important resource for the study of prokaryotic gene network and regulation.

RNase E/G-dependent degradation of metE mRNA, encoding methionine synthase, in Corynebacterium glutamicum

Corynebacterium glutamicum is used for the industrial production of various metabolites, including L-glutamic acid and L-lysine. With the aim of understanding the post-transcriptional regulation of amino acid biosynthesis in this bacterium, we investigated the role of RNase E/G in the degradation of mRNAs encoding metabolic enzymes. In this study, we found that the cobalamin-independent methionine synthase MetE was overexpressed in ΔrneG mutant cells grown on various carbon sources. The level of metE mRNA was also approximately 6- to 10-fold higher in the ΔrneG mutant strain than in the wild-type strain. A rifampicin chase experiment showed that the half-life of metE mRNA was approximately 4.2 times longer in the ΔrneG mutant than in the wild-type strain. These results showed that RNase E/G is involved in the degradation of metE mRNA in C. glutamicum.

Importance of a diphosphorylated intermediate for RppH-dependent RNA degradation

Deprotection of the 5' end appears to be a universal mechanism for triggering the degradation of mRNA in bacteria and eukaryotes. In Escherichia coli, for example, converting the 5' triphosphate of primary transcripts to a monophosphate accelerates cleavage at internal sites by the endonuclease RNase E. Previous studies have shown that the RNA pyrophosphohydrolase RppH catalyzes this transformation in vitro and generates monophosphorylated decay intermediates in vivo. Recently, we reported that purified E. coli RppH unexpectedly reacts faster with diphosphorylated than with triphosphorylated substrates. By using a novel assay, it was also determined that diphosphorylated mRNA decay intermediates are abundant in wild-type E. coli and that their fractional level increases to almost 100% for representative mRNAs in mutant cells lacking RppH. These findings indicate that the conversion of triphosphorylated to monophosphorylated RNA in E. coli is a stepwise process involving sequential phosphate removal and the transient formation of a diphosphorylated intermediate. The latter RNA phosphorylation state, which was previously unknown in bacteria, now appears to define the preferred biological substrates of E. coli RppH. The enzyme responsible for generating it remains to be identified.

Epitranscriptomics: RNA Modifications in Bacteria and Archaea

The increasingly complex functionality of RNA is contrasted by its simple chemical composition. RNA is generally built from only four different nucleotides (adenine, guanine, cytosine, and uracil). To date, >160 chemical modifications are known to decorate RNA molecules and thereby alter their function or stability. Many RNA modifications are conserved throughout bacteria, archaea, and eukaryotes, while some are unique to each branch of life. Most known modifications occur at internal positions, while there is limited diversity at the termini. The dynamic nature of RNA modifications and newly discovered regulatory functions of some of these RNA modifications gave birth to a new field, now often referred to as "epitranscriptomics." This review highlights the major developments in this field and summarizes detection principles for internal as well as 5'-terminal mRNA modifications in prokaryotes and archaea to investigate their biological significance.

Enzymes Involved in Posttranscriptional RNA Metabolism in Gram-Negative Bacteria

Gene expression in Gram-negative bacteria is regulated at many levels, including transcription initiation, RNA processing, RNA/RNA interactions, mRNA decay, and translational controls involving enzymes that alter translational efficiency. In this review, we discuss the various enzymes that control transcription, translation, and RNA stability through RNA processing and degradation. RNA processing is essential to generate functional RNAs, while degradation helps control the steady-state level of each individual transcript. For example, all the pre-tRNAs are transcribed with extra nucleotides at both their 5' and 3' termini, which are subsequently processed to produce mature tRNAs that can be aminoacylated. Similarly, rRNAs that are transcribed as part of a 30S polycistronic transcript are matured to individual 16S, 23S, and 5S rRNAs. Decay of mRNAs plays a key role in gene regulation through controlling the steady-state level of each transcript, which is essential for maintaining appropriate protein levels. In addition, degradation of both translated and nontranslated RNAs recycles nucleotides to facilitate new RNA synthesis. To carry out all these reactions, Gram-negative bacteria employ a large number of endonucleases, exonucleases, RNA helicases, and poly(A) polymerase, as well as proteins that regulate the catalytic activity of particular RNases. Under certain stress conditions, an additional group of specialized endonucleases facilitate the cell's ability to adapt and survive. Many of the enzymes, such as RNase E, RNase III, polynucleotide phosphorylase, RNase R, and poly(A) polymerase I, participate in multiple RNA processing and decay pathways.

Nutrient depletion and TOR inhibition induce 18S and 25S ribosomal RNAs resistant to a 5'-phosphate-dependent exonuclease in Candida albicans and other yeasts

Background

Messenger RNA (mRNA) represents a small percentage of RNAs in a cell, with ribosomal RNA (rRNA) making up the bulk of it. To isolate mRNA from eukaryotes, typically poly-A selection is carried out. Recently, a 5´-phosphate-dependent, 5´→3´ processive exonuclease called Terminator has become available. It will digest only RNA that has a 5´-monophosphate end and therefore it is very useful to eliminate most of rRNAs in cell.

Results

We have found that in the pathogenic yeast Candida albicans, while 18S and 25S components isolated from yeast in robust growth phase are easily eliminated by Terminator, those isolated from cells in the nutritionally diminished stationary phase, become resistant to digestion by this enzyme. Additional digestions with alkaline phosphatase, tobacco pyrophosphatase combined with Terminator point toward the 5′-prime end of 18S and 25S as the source of this resistance. Inhibition of TOR by rapamycin also induces resistance by these molecules. We also find that these molecules are incorporated into the ribosome and are not just produced incidentally. Finally, we show that three other yeasts show the same behavior.

Conclusions

Digestion of RNA by Terminator has revealed 18S and 25S rRNA molecules different from the accepted processed ones seen in ribosome generation. The reason for these molecules and the underlying mechanism for their formation is unknown. The preservation of this behavior across these yeasts suggests a useful biological role for it, worthy of further inquiry.

Crystal structure of an RNA-cleaving DNAzyme

In addition to storage of genetic information, DNA can also catalyze various reactions. RNA-cleaving DNAzymes are the catalytic DNAs discovered the earliest, and they can cleave RNAs in a sequence-specific manner. Owing to their great potential in medical therapeutics, virus control, and gene silencing for disease treatments, RNA-cleaving DNAzymes have been extensively studied; however, the mechanistic understandings of their substrate recognition and catalysis remain elusive. Here, we report three catalytic form 8-17 DNAzyme crystal structures. 8-17 DNAzyme adopts a V-shape fold, and the Pb²⁺ cofactor is bound at the pre-organized pocket. The structures with Pb²⁺ and the modification at the cleavage site captured the pre-catalytic state of the RNA cleavage reaction, illustrating the unexpected Pb²⁺-accelerated catalysis, intrinsic tertiary interactions, and molecular kink at the active site. Our studies reveal that DNA is capable of forming a compacted structure and that the functionality-limited bio-polymer can have a novel solution for a functional need in catalysis.

The rph-1-Encoded Truncated RNase PH Protein Inhibits RNase P Maturation of Pre-tRNAs with Short Leader Sequences in the Absence of RppH

RNase PH, encoded by the rph gene, is a 3'→5' exoribonuclease that in E. coli participates primarily in the 3' maturation of pre-tRNAs and the degradation of rRNA in stationary-phase cells. Interestingly, the routinely used laboratory strains of MG1655 and W3110 have naturally acquired the rph-1 allele, encoding a truncated catalytically inactive RNase PH protein which is widely assumed to be benign. Contrary to this assumption, we show that the rph-1-encoded Rph-1 protein inhibits RNase P-mediated 5'-end maturation of primary pre-tRNAs with leaders of <5 nucleotides in the absence of RppH, an RNA pyrophosphohydrolase. In contrast, RppH is not required for 5'-end maturation of endonucleolytically generated pre-tRNAs in the rph-1 strain and for any tRNAs in Δrph mutant or rph⁺ strains. We propose that the Rph-1 protein bound to the 3' end of the substrate creates a steric hindrance that in the presence of a triphosphate at the 5' end reduces the ability of RNase P to bind to the pre-tRNA.IMPORTANCE In this paper, we demonstrate that the rph-1 mutation found in commonly used E. coli strains leads to the synthesis of a truncated functionally inactive RNase PH protein that interferes with the 5'-end maturation of specific tRNAs with short 5' leaders by RNase P in the absence of RppH, an RNA pyrophosphohydrolase that converts primary 5' triphosphates into 5' monophosphates. The data presented indicate that the presence of the triphosphate interferes with RNase P binding to the pre-tRNA.