RraA

Initiation of mRNA decay in bacteria

The instability of messenger RNA is fundamental to the control of gene expression. In bacteria, mRNA degradation generally follows an "all-or-none" pattern. This implies that if control is to be efficient, it must occur at the initiating (and presumably rate-limiting) step of the degradation process. Studies of E. coli and B. subtilis, species separated by 3 billion years of evolution, have revealed the principal and very disparate enzymes involved in this process in the two organisms. The early view that mRNA decay in these two model organisms is radically different has given way to new models that can be resumed by "different enzymes-similar strategies". The recent characterization of key ribonucleases sheds light on an impressive case of convergent evolution that illustrates that the surprisingly similar functions of these totally unrelated enzymes are of general importance to RNA metabolism in bacteria. We now know that the major mRNA decay pathways initiate with an endonucleolytic cleavage in E. coli and B. subtilis and probably in many of the currently known bacteria for which these organisms are considered representative. We will discuss here the different pathways of eubacterial mRNA decay, describe the major players and summarize the events that can precede and/or favor nucleolytic inactivation of a mRNA, notably the role of the 5' end and translation initiation. Finally, we will discuss the role of subcellular compartmentalization of transcription, translation, and the RNA degradation machinery.

Potential regulatory interactions of Escherichia coli RraA protein with DEAD-box helicases

Members of the DEAD-box family of RNA helicases contribute to virtually every aspect of RNA metabolism, in organisms from all domains of life. Many of these helicases are constituents of multicomponent assemblies, and their interactions with partner proteins within the complexes underpin their activities and biological function. In Escherichia coli the DEAD-box helicase RhlB is a component of the multienzyme RNA degradosome assembly, and its interaction with the core ribonuclease RNase E boosts the ATP-dependent activity of the helicase. Earlier studies have identified the regulator of ribonuclease activity A (RraA) as a potential interaction partner of both RNase E and RhlB. We present structural and biochemical evidence showing how RraA can bind to, and modulate the activity of RhlB and another E. coli DEAD-box enzyme, SrmB. Crystallographic structures are presented of RraA in complex with a portion of the natively unstructured C-terminal tail of RhlB at 2.8-Å resolution, and in complex with the C-terminal RecA-like domain of SrmB at 2.9 Å. The models suggest two distinct mechanisms by which RraA might modulate the activity of these and potentially other helicases.

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.

Identification of genes involved in serum tolerance in the clinical strain Cronobacter sakazakii ES5

BACKGROUND

Cronobacter spp. are opportunistic pathogens that can cause septicemia and infections of the central nervous system primarily in premature, low-birth weight and/or immune-compromised neonates. Serum resistance is a crucial virulence factor for the development of systemic infections, including bacteremia. It was the aim of the current study to identify genes involved in serum tolerance in a selected Cronobacter sakazakii strain of clinical origin.

RESULTS

Screening of 2749 random transposon knock out mutants of a C. sakazakii ES 5 library for modified serum tolerance (compared to wild type) revealed 10 mutants showing significantly increased/reduced resistance to serum killing. Identification of the affected sites in mutants displaying reduced serum resistance revealed genes encoding for surface and membrane proteins as well as regulatory elements or chaperones. By this approach, the involvement of the yet undescribed Wzy_C superfamily domain containing coding region in serum tolerance was observed and experimentally confirmed. Additionally, knock out mutants with enhanced serum tolerance were observed. Examination of respective transposon insertion loci revealed regulatory (repressor) elements, coding regions for chaperones and efflux systems as well as the coding region for the protein YbaJ. Real time expression analysis experiments revealed, that knock out of the gene for this protein negatively affects the expression of the fimA gene, which is a key structural component of the formation of fimbriae. Fimbriae are structures of high immunogenic potential and it is likely that absence/truncation of the ybaJ gene resulted in a non-fimbriated phenotype accounting for the enhanced survival of this mutant in human serum.

CONCLUSION

By using a transposon knock out approach we were able to identify genes involved in both increased and reduced serum tolerance in Cronobacter sakazakii ES5. This study reveals first insights in the complex nature of serum tolerance of Cronobacter spp.

Bacterial helicases in post-transcriptional control

Among the five superfamilies of helicases involved in RNA and DNA metabolism, superfamily 2 and superfamily 5 include bacterial RNA-helicases. These enzymes have been shown to be involved in ribosome biogenesis and post-transcriptional gene regulation. Here, we focus on bacterial regulatory mechanisms that are mediated by RNA helicases belonging to superfamily 2, which includes DEAD-box and DEAH-box helicases. Some of these helicases are part of bacterial degradosomes and were shown to unwind RNA duplexes. We will review examples where these enzymes have been implicated in translatability and metabolic stability of bacterial transcripts. This article is part of a Special Issue entitled: The Biology of RNA helicases - Modulation for life.

A novel fluorescent reporter system for monitoring and identifying RNase III activity and its target RNAs

Bacteriophage vectors for achieving single-copy gene expression linked to a colorigenic reporter assay have been used successfully for genetic screening applications. However, the limited number of cloning sites in these vectors, combined with the requirement for lac- strains and the time- and/or media-dependence of the chemical-based colorimetric reaction, have limited the range of applications for these vectors. An alternative approach using a fluorescent reporter gene such as green fluorescent protein (GFP) or GFP derivatives could overcome some of these technical issues and facilitate real-time monitoring of promoter and/or protein activity. Here, we report the development of a novel translational bacteriophage fusion vector encoding enhanced GFP (eGFP) that can be incorporated into the chromosome as a single-copy gene. We identified a Bacillus promoter (BP) that is stably expressed in Escherichia coli and drives ~6-fold more expression of eGFP than the T7 promoter in the absence of inducer. Incorporating this BP and RNase III target signals into a single system enabled clear detection of the absence or downregulation of RNase III activity in vivo, thereby establishing a system for screening and identifying novel RNase III targets in a matter of days. An RNase III target signal identified in this manner was confirmed by post-transcriptional analysis. We anticipate that this novel translational fusion vector will be used extensively to study activity of both interesting RNases and related complex or to identify or validate targets of RNases that are otherwise difficult to study due to their sensitivity to environmental stresses and/or autoregulatory processes.

From conformational chaos to robust regulation: the structure and function of the multi-enzyme RNA degradosome

The RNA degradosome is a massive multi-enzyme assembly that occupies a nexus in RNA metabolism and post-transcriptional control of gene expression inEscherichia coliand many other bacteria. Powering RNA turnover and quality control, the degradosome serves also as a machine for processing structured RNA precursors during their maturation. The capacity to switch between destructive and processing modes involves cooperation between degradosome components and is analogous to the process of RNA surveillance in other domains of life. Recruitment of components and cellular compartmentalisation of the degradosome are mediated through small recognition domains that punctuate a natively unstructured segment within a scaffolding core. Dynamic in conformation, variable in composition and non-essential under certain laboratory conditions, the degradosome has nonetheless been maintained throughout the evolution of many bacterial species, due most likely to its diverse contributions in global cellular regulation. We describe the role of the degradosome and its components in RNA decay pathways inE. coli, and we broadly compare these pathways in other bacteria as well as archaea and eukaryotes. We discuss the modular architecture and molecular evolution of the degradosome, its roles in RNA degradation, processing and quality control surveillance, and how its activity is regulated by non-coding RNA. Parallels are drawn with analogous machinery in organisms from all life domains. Finally, we conjecture on roles of the degradosome as a regulatory hub for complex cellular processes.

Transcriptomic and proteomic profiling of maize embryos exposed to camptothecin

BACKGROUND

Camptothecin is a plant alkaloid that specifically binds topoisomerase I, inhibiting its activity and inducing double stranded breaks in DNA, activating the cell responses to DNA damage and, in response to severe treatments, triggering cell death.

RESULTS

Comparative transcriptomic and proteomic analyses of maize embryos that had been exposed to camptothecin were conducted. Under the conditions used in this study, camptothecin did not induce extensive degradation in the genomic DNA but induced the transcription of genes involved in DNA repair and repressed genes involved in cell division. Camptothecin also affected the accumulation of several proteins involved in the stress response and induced the activity of certain calcium-dependent nucleases. We also detected changes in the expression and accumulation of different genes and proteins involved in post-translational regulatory processes.

CONCLUSIONS

This study identified several genes and proteins that participate in DNA damage responses in plants. Some of them may be involved in general responses to stress, but others are candidate genes for specific involvement in DNA repair. Our results open a number of new avenues for researching and improving plant resistance to DNA injury.

The crystal structure of hexamer RraA from Pseudomonas aeruginosa reveals six conserved protein-protein interaction sites

RNase E functions as the rate-limiting enzyme in the global mRNA metabolism as well as in the maturation of functional RNAs. The endoribonuclease, binding to the PNPase trimer, the RhlB monomer, and the enolase dimer, assembles into an RNA degradosome necessary for effective RNA metabolism. The RNase E processing is found to be negatively regulated by the protein modulator RraA which appears to work by interacting with the non-catalytic region of the endoribonuclease and significantly reduce the interaction between RNase E and PNPase, RhlB and enolase of the RNA degradosome. Here we report the crystal structure of RraA from P. aeruginosa to a resolution of 2.0 Å. The overall architecture of RraA is very similar to other known RraAs, which are highly structurally conserved. Gel filtration and dynamic light scattering experiments suggest that the protein regulator is arranged as a hexamer, consistent with the crystal packing of "a dimer of trimer" arrangement. Structure and sequence conservation analysis suggests that the hexamer RraA contains six putative charged protein-protein interaction sites which may serve as binding sites for RNase E.

Structural and kinetic characterization of 4-hydroxy-4-methyl-2-oxoglutarate/4-carboxy-4-hydroxy-2-oxoadipate aldolase, a protocatechuate degradation enzyme evolutionarily convergent with the HpaI and DmpG pyruvate aldolases

4-Hydroxy-4-methyl-2-oxoglutarate/4-carboxy-4-hydroxy-2-oxoadipate (HMG/CHA) aldolase from Pseudomonas putida F1 catalyzes the last step of the bacterial protocatechuate 4,5-cleavage pathway. The preferred substrates of the enzyme are 2-keto-4-hydroxy acids with a 4-carboxylate substitution. The enzyme also exhibits oxaloacetate decarboxylation and pyruvate α-proton exchange activity. Sodium oxalate is a competitive inhibitor of the aldolase reaction. The pH dependence of k(cat)/K(m) and k(cat) for the enzyme is consistent with a single deprotonation with pK(a) values of 8.0 ± 0.1 and 7.0 ± 0.1 for free enzyme and enzyme substrate complex, respectively. The 1.8 Å x-ray structure shows a four-layered α-β-β-α sandwich structure with the active site at the interface of two adjacent subunits of a hexamer; this fold resembles the RNase E inhibitor, RraA, but is novel for an aldolase. The catalytic site contains a magnesium ion ligated by Asp-124 as well as three water molecules bound by Asp-102 and Glu-199'. A pyruvate molecule binds the magnesium ion through both carboxylate and keto oxygen atoms, completing the octahedral geometry. The carbonyl oxygen also forms hydrogen bonds with the guanadinium group of Arg-123, which site-directed mutagenesis confirms is essential for catalysis. A mechanism for HMG/CHA aldolase is proposed on the basis of the structure, kinetics, and previously established features of other aldolase mechanisms.

Studies on a Vibrio vulnificus functional ortholog of Escherichia coli RNase E imply a conserved function of RNase E-like enzymes in bacteria

RNase E (Rne) plays a key role in the processing and degradation of RNA in Escherichia coli. In the genome of Vibrio vulnificus, one open reading frame potentially encodes a protein homologous to E. coli RNase E, designated RNase EV, which N-terminal (1-500 amino acids) has 86.4% amino acid identity to the N-terminal catalytic part of RNase E (N-Rne). Here, we report that both the full-length and the N-terminal part of RNase EV (N-RneV) functionally complement E. coli RNase E and their expression consequently supports normal growth of RNase E-depleted E. coli cells. E. coli cells expressing N-RneV showed copy numbers of ColE1-type plasmid similar to that of E. coli cells expressing N-Rne, indicating in vivo ribonucleolytic activity of N-RneV on RNA I, an antisense regulator of ColE1-type plasmid replication. In vitro cleavage assays further showed that N-RneV has cleavage activity and specificity of RNase E on RNase E-targeted sequence of RNA I (BR13). Our findings suggest that RNase E-like proteins have conserved enzymatic properties that determine substrate specificity across species.

Characterization of the RNA degradosome of Pseudoalteromonas haloplanktis: conservation of the RNase E-RhlB interaction in the gammaproteobacteria

The degradosome is a multienzyme complex involved in mRNA degradation in Escherichia coli. The essential endoribonuclease RNase E contains a large noncatalytic region necessary for protein-protein interactions with other components of the RNA degradosome. Interacting proteins include the DEAD-box RNA helicase RhlB, the glycolytic enzyme enolase, and the exoribonuclease PNPase. Pseudoalteromonas haloplanktis, a psychrotolerant gammaproteobacterium distantly related to E. coli, encodes homologs of each component of the RNA degradosome. In P. haloplanktis, RNase E associates with RhlB and PNPase but not enolase. Plasmids expressing P. haloplanktis RNase E (Ph-RNase E) can complement E. coli strains lacking E. coli RNase E (Ec-RNase E). Ph-RNase E, however, does not confer a growth advantage to E. coli at low temperature. Ph-RNase E has a heterologous protein-protein interaction with Ec-RhlB but not with Ec-enolase or Ec-PNPase. The Ph-RNase E binding sites for RhlB and PNPase were mapped by deletion analysis. The PNPase binding site is located at the C-terminal end of Ph-RNase E at the same position as that in Ec-RNase E, but the sequence of the site is not conserved. The sequence of the RhlB binding site in Ph-RNase E is related to the sequence in Ec-RNase E. Together with the heterologous interaction between Ph-RNase E and Ec-RhlB, our results suggest that the underlying structural motif for the RNase E-RhlB interaction is conserved. Since the activity of Ec-RhlB requires its physical interaction with Ec-RNase E, conservation of the underlying structural motif over a large evolutionary distance could be due to constraints involved in the control of RhlB activity.

The critical role of RNA processing and degradation in the control of gene expression

The continuous degradation and synthesis of prokaryotic mRNAs not only give rise to the metabolic changes that are required as cells grow and divide but also rapid adaptation to new environmental conditions. In bacteria, RNAs can be degraded by mechanisms that act independently, but in parallel, and that target different sites with different efficiencies. The accessibility of sites for degradation depends on several factors, including RNA higher-order structure, protection by translating ribosomes and polyadenylation status. Furthermore, RNA degradation mechanisms have shown to be determinant for the post-transcriptional control of gene expression. RNases mediate the processing, decay and quality control of RNA. RNases can be divided into endonucleases that cleave the RNA internally or exonucleases that cleave the RNA from one of the extremities. Just in Escherichia coli there are >20 different RNases. RNase E is a single-strand-specific endonuclease critical for mRNA decay in E. coli. The enzyme interacts with the exonuclease polynucleotide phosphorylase (PNPase), enolase and RNA helicase B (RhlB) to form the degradosome. However, in Bacillus subtilis, this enzyme is absent, but it has other main endonucleases such as RNase J1 and RNase III. RNase III cleaves double-stranded RNA and family members are involved in RNA interference in eukaryotes. RNase II family members are ubiquitous exonucleases, and in eukaryotes, they can act as the catalytic subunit of the exosome. RNases act in different pathways to execute the maturation of rRNAs and tRNAs, and intervene in the decay of many different mRNAs and small noncoding RNAs. In general, RNases act as a global regulatory network extremely important for the regulation of RNA levels.

The regulatory protein RraA modulates RNA-binding and helicase activities of the E. coli RNA degradosome

The Escherichia coli endoribonuclease RNase E is an essential enzyme having key roles in mRNA turnover and the processing of several structured RNA precursors, and it provides the scaffold to assemble the multienzyme RNA degradosome. The activity of RNase E is inhibited by the protein RraA, which can interact with the ribonuclease's degradosome-scaffolding domain. Here, we report that RraA can bind to the RNA helicase component of the degradosome (RhlB) and the two RNA-binding sites in the degradosome-scaffolding domain of RNase E. In the presence of ATP, the helicase can facilitate the exchange of RraA for RNA stably bound to the degradosome. Our data suggest that RraA can affect multiple components of the RNA degradosome in a dynamic, energy-dependent equilibrium. The multidentate interactions of RraA impede the RNA-binding and ribonuclease activities of the degradosome and may result in complex modulation and rerouting of degradosome activity.

The ribonucleases J1 and J2 are essential for growth and have independent roles in mRNA decay in Streptococcus pyogenes

The paralogous ribonucleases J1 and J2, recently identified in Bacillus subtilis, have both endoribonucleolytic and 5'-to-3' exoribonucleolytic activities and participate in degradation and regulatory processing of mRNA. RNases J1 and J2 have partially overlapping target specificities, but only RNase J1 is essential for B. subtilis growth. Because mRNA decay is important in regulation of virulence factors of Streptococcus pyogenes (the group A streptococcus, GAS), we investigated the role of these newly described RNases in GAS. We found that conditional mutants for both RNases J1 and J2 require induction for growth, so we conclude that, unlike the case in B. subtilis, both of these RNases are essential for GAS growth, and therefore their functions are not redundant. We compared decay of representatives of the two classes of messages we had previously identified: Class I, which decay rapidly in exponential and stationary phase of growth (hasA and gyrA), and Class II, which are stable in stationary phase and exhibit a biphasic decay curve in exponential phase (sagA and sda). We report that RNases J1 and J2 affect the rate of decay of Class I messages and the length of the first phase in decay of Class II messages.

Effect of wheat roots infected with the pathogenic fungus Gaeumannomyces graminis var. tritici on gene expression of the biocontrol bacterium Pseudomonas fluorescens Pf29Arp

Traits contributing to the competence of biocontrol bacteria to colonize plant roots are often induced in the rhizosphere in response to plant components. These interactions have been studied using the two partners in gnotobiotic systems. However, in nature, beneficial or pathogenic fungi often colonize roots. Influence of these plant-fungus interactions on bacterial behavior remains to be investigated. Here, we have examined the influence of colonization of wheat roots by the take-all fungus Gaeumannomyces graminis var. tritici on gene expression of the biocontrol bacterium Pseudomonas fluorescens Pf29Arp. Bacteria were inoculated onto healthy, early G. graminis var. tritici-colonized and necrotic roots and transcriptomes were compared by shotgun DNA microarray. Pf29Arp decreased disease severity when inoculated before the onset of necrosis. Necrotic roots exerted a broader effect on gene expression compared with early G. graminis var. tritici-colonized and healthy roots. A gene encoding a putative type VI secretion system effector was only induced in necrotic conditions. A common pool of Pf29Arp genes differentially expressed on G. graminis var. tritici-colonized roots was related to carbon metabolism and oxidative stress, with a highest fold-change with necrosis. Overall, the data showed that the association of the pathogenic fungus with the roots strongly altered Pf29Arp adaptation with differences between early and late G. graminis var. tritici infection steps.

Escherichia coli ribonuclease III activity is downregulated by osmotic stress: consequences for the degradation of bdm mRNA in biofilm formation

During the course of experiments aimed at identifying genes with ribonuclease III (RNase III)-dependent expression in Escherichia coli, we found that steady state levels of bdm mRNA were dependent on cellular concentrations of RNase III. The half-lives of adventitiously overexpressed bdm mRNA and the activities of a transcriptional bdm'-'cat fusion were observed to be dependent on cellular concentrations of RNase III, indicating the existence of cis-acting elements in bdm mRNA responsive to RNase III. In vitro and in vivo cleavage analyses of bdm mRNA identified two RNase III cleavage motifs, one in the 5'-untranslated region and the other in the coding region of bdm mRNA, and indicated that RNase III cleavages in the coding region constitute a rate-determining step for bdm mRNA degradation. We also discovered that downregulation of the ribonucleolytic activity of RNase III is required for the sustained elevation of RcsB-induced bdm mRNA levels during osmotic stress and that cells overexpressing bdm form biofilms more efficiently. These findings indicate that the Rcs signalling system has an additional regulatory pathway that functions to modulate bdm expression and consequently, adapt E. coli cells to osmotic stress.

Transcriptional regulation of the Escherichia coli gene rraB, encoding a protein inhibitor of RNase E

The Escherichia coli RNA degradosome is a protein complex that plays a critical role in the turnover of numerous RNAs. The key component of the degradosome complex is the endoribonuclease RNase E, a multidomain protein composed of an N-terminal catalytic region and a C-terminal region that organizes the other protein components of the degradosome. Previously, the RNase E inhibitors RraA and RraB were identified genetically and shown to bind to the C-terminal region of RNase E, thus affecting both the protein composition of the degradosome and the endonucleolytic activity of RNase E. In the present work, we investigated the transcriptional regulation of rraB. rraB was shown to be transcribed constitutively from its own promoter, PrraB. Transposon mutagenesis and screening for increased beta-galactosidase activity from a chromosomal PrraB-lacZ transcriptional fusion resulted in the isolation of a transposon insertion in glmS, encoding the essential enzyme glucosamine-6-phosphate synthase that catalyzes the first committed step of the uridine 5'-diphospho-N-acetyl-glucosamine (UDP-GlcNAc) pathway, which provides intermediates for peptidoglycan biogenesis. The glmS852::Tn5 allele resulted in an approximately 50% lower intracellular concentration of UDP-GlcNAc and conferred a fivefold increase in the level of rraB mRNA. This allele also mediated a twofold increase in beta-galactosidase activity from a chromosomal fusion of the 5' untranslated region of the rne gene to lacZ, suggesting that a reduction in cellular concentration of UDP-GlcNAc and the resulting increased expression of RraB might modulate the action of RNase E.

Structural genomics approach to drug discovery for Mycobacterium tuberculosis

Structural genomics has become a powerful tool for studying microorganisms at the molecular level. Advances in technology have enabled the assembly of high-throughput pipelines that can be used to automate X-ray crystal structure determination for many proteins in the genome of a target organism. In this paper, we describe the methods used in the Tuberculosis Structural Genomics Consortium (TBSGC), ranging from protein production and crystallization to diffraction data collection and processing. The TBSGC is unique in that it uses biological importance as a primary criterion for target selection. The over-riding goal is to solve structures of proteins that may be potential drug targets, in order to support drug discovery efforts. We describe the crystal structures of several significant proteins in the M. tuberculosis genome that have been solved by the TBSGC over the past few years. We conclude by describing the high-throughput screening facilities and virtual screening facilities we have implemented for identifying small-molecule inhibitors of proteins whose structures have been solved.

Messenger RNA decay and maturation in Bacillus subtilis

Our understanding of the ribonucleases that act to process and turn over RNA in Bacillus subtilis, a model Gram-positive organism, has increased greatly in recent years. This chapter discusses characteristics of B. subtilis ribonucleases that have been shown to participate in messenger RNA maturation and decay. Distinct features of a recently discovered ribonuclease, RNase J1, are reviewed, and are put in the context of a mechanism for the mRNA decay process in B. subtilis that differs greatly from the classical model developed for E. coli. This chapter is divided according to three parts of an mRNA-5' end, body, and 3' end-that could theoretically serve as sites for initiation of decay. How 5'-proximal elements affect mRNA half-life, and especially how these elements interface with RNase J1, forms the basis for a set of "rules" that may be useful in predicting mRNA stability.

Co-immunopurification of multiprotein complexes containing RNA-degrading enzymes

Co-immunopurification is a classical technique in which antiserum raised against a specific protein is used to purify a multiprotein complex. We describe work from our laboratory in which co-immunopurification was used to characterize the RNA degradosome of Escherichia coli, a multiprotein complex involved in RNA processing and mRNA degradation. Polyclonal rabbit antibodies raised against either RNase E or PNPase, two RNA degrading enzymes in the RNA degradosome, were used in co-immunopurification experiments aimed at studying the assembly of the RNA degradosome and mapping protein-protein interactions within the complex. In E. coli, this method has been largely supplanted by approaches in which proteins are engineered to contain tags that interact with commercially available antibodies. Nevertheless, we believe that the method described here is valid for the study of bacteria in which the genetic engineering needed to introduce tagged proteins is difficult or nonexistent. As an example, we briefly discuss ongoing work in our laboratory on the characterization of RNase E in the psychrotolerant bacterium Pseudoalteromonas haloplanktis.

Effects of Escherichia coli RraA orthologs of Vibrio vulnificus on the ribonucleolytic activity of RNase E in vivo

RraA is a recently discovered protein inhibitor of RNase E that catalyzes the initial step in the decay and processing of numerous RNAs in Escherichia coli. In the genome of Vibrio vulnificus, two open reading frames that potentially encode proteins homologous to E. coli, RraA-designated RraAV1 and RraAV2, have respectively 80.1% and 59.0% amino acid identity to RraA. The authors report that coexpression of RraAV1 protein in E. coli cells overproducing RNase E rescued these cells from growth arrest and restored their normal growth, whereas coexpression of RraAV2 protein further inhibited the growth of E. coli cells, whose growth was already impaired by overproduction of RNase E. Analyses of the steady-state level of various RNase E substrates indicated that the coexpression of RraAV1 more efficiently inhibited RNase E action than coexpression of RraA, and consequently resulted in the more increased abundance of each RNA species tested in vivo. The inhibitory effect by RraAV2 coexpression on RNase E was observed only in the case of trpA mRNA, indicating the possibility of RNA substrate-dependent inhibition of RraAV2 on RNase E. The findings suggest that these regulators of ribonuclease activity have both a conserved inhibitory function and a differential inhibitory activity on RNase E-like enzymes across the species.

The take and give between retrotransposable elements and their hosts

Retrotransposons mobilize via RNA intermediates and usually carry with them the agent of their mobility, reverse transcriptase. Retrotransposons are streamlined, and therefore rely on host factors to proliferate. However, retrotransposons are exposed to cellular forces that block their paths. For this review, we have selected for our focus elements from among target-primed (TP) retrotransposons, also called non-LTR retrotransposons, and extrachromosomally-primed (EP) retrotransposons, also called LTR retrotransposons. The TP retrotransposons considered here are group II introns, LINEs and SINEs, whereas the EP elements considered are the Ty and Tf retrotransposons, with a brief comparison to retroviruses. Recurring themes for these elements, in hosts ranging from bacteria to humans, are tie-ins of the retrotransposons to RNA metabolism, DNA replication and repair, and cellular stress. Likewise, there are parallels among host-cell defenses to combat rampant retrotransposon spread. The interactions between the retrotransposon and the host, and their coevolution to balance the tension between retrotransposon proliferation and host survival, form the basis of this review.

YmdB: a stress-responsive ribonuclease-binding regulator of E. coli RNase III activity

The broad cellular actions of RNase III family enzymes include ribosomal RNA (rRNA) processing, mRNA decay, and the generation of noncoding microRNAs in both prokaryotes and eukaryotes. Here we report that YmdB, an evolutionarily conserved 18.8-kDa protein of Escherichia coli of previously unknown function, is a regulator of RNase III cleavages. We show that YmdB functions by interacting with a site in the RNase III catalytic region, that expression of YmdB is transcriptionally activated by both cold-shock stress and the entry of cells into stationary phase, and that this activation requires the sigma-factor-encoding gene, rpoS. We discovered that down-regulation of RNase III activity occurs during both stresses and is dependent on YmdB production during cold shock; in contrast, stationary-phase regulation was unperturbed in YmdB-null mutant bacteria, indicating the existence of additional, YmdB-independent, factors that dynamically regulate RNase III actions during normal cell growth. Our results reveal the previously unsuspected role of ribonuclease-binding proteins in the regulation of RNase III activity.

Regulation of ribonuclease E activity by the L4 ribosomal protein of Escherichia coli

Whereas ribosomal proteins (r-proteins) are known primarily as components of the translational machinery, certain of these r-proteins have been found to also have extraribosomal functions. Here we report the novel ability of an r-protein, L4, to regulate RNA degradation in Escherichia coli. We show by affinity purification, immunoprecipitation analysis, and E. coli two-hybrid screening that L4 interacts with a site outside of the catalytic domain of RNase E to regulate the endoribonucleolytic functions of the enzyme, thus inhibiting RNase E-specific cleavage in vitro, stabilizing mRNAs targeted by RNase E in vivo, and controlling plasmid DNA replication by stabilizing an antisense regulatory RNA normally attacked by RNase E. Broader effects of the L4-RNase E interaction on E. coli transcripts were shown by DNA microarray analysis, which revealed changes in the abundance of 65 mRNAs encoding the stress response proteins HslO, Lon, CstA, YjiY, and YaeL, as well as proteins involved in carbohydrate and amino acid metabolism and transport, transcription/translation, and DNA/RNA synthesis. Analysis of mRNA stability showed that the half lives of stress-responsive transcripts were increased by ectopic expression of L4, which normally increases along with other r-proteins in E. coli under stress conditions, and also by inactivation of RNase E. Our finding that L4 can inhibit RNase E-dependent decay may account at least in part for the elevated production of stress-induced proteins during bacterial adaptation to adverse environments.

Poly(A)-assisted RNA decay and modulators of RNA stability

In Escherichia coli, RNA degradation is orchestrated by the degradosome with the assistance of complementary pathways and regulatory cofactors described in this chapter. They control the stability of each transcript and regulate the expression of many genes involved in environmental adaptation. The poly(A)-dependent degradation machinery has diverse functions such as the degradation of decay intermediates generated by endoribonucleases, the control of the stability of regulatory non coding RNAs (ncRNAs) and the quality control of stable RNA. The metabolism of poly(A) and mechanism of poly(A)-assisted degradation are beginning to be understood. Regulatory factors, exemplified by RraA and RraB, control the decay rates of subsets of transcripts by binding to RNase E, in contrast to regulatory ncRNAs which, assisted by Hfq, target RNase E to specific transcripts. Destabilization is often consecutive to the translational inactivation of mRNA. However, there are examples where RNA degradation is the primary regulatory step.

Endonucleolytic initiation of mRNA decay in Escherichia coli

Instability is a fundamental property of mRNA that is necessary for the regulation of gene expression. In E. coli, the turnover of mRNA involves multiple, redundant pathways involving 3'-exoribonucleases, endoribonucleases, and a variety of other enzymes that modify RNA covalently or affect its conformation. Endoribonucleases are thought to initiate or accelerate the process of mRNA degradation. A major endoribonuclease in this process is RNase E, which is a key component of the degradative machinery amongst the Proteobacteria. RNase E is the central element in a multienzyme complex known as the RNA degradosome. Structural and functional data are converging on models for the mechanism of activation and regulation of RNase E and its paralog, RNase G. Here, we discuss current models for mRNA degradation in E. coli and we present current thinking on the structure and function of RNase E based on recent crystal structures of its catalytic core.

Transcript stability in the protein interaction network of Escherichia coli

Gene expression is a dynamic process which can be controlled by a number of mechanisms as genetic information flows from nucleic acids to proteins. The study of gene expression in the steady state, while informative, overlooks the underlying dynamics of the processes. Steady-state transcript levels are a result of both RNA synthesis and degradation, and as such, measurements of degradation rates can be used to determine their rates of synthesis as well as reveal regulation that occurs via changes in RNA stability. Messenger RNA degradation plays a central role in diverse cellular processes and is controlled primarily by the activity of the degradosome in prokaryotes. In this study, we use the currently available network of protein-protein interactions (PPIs) and mRNA half-lives in Escherichia coli to demonstrate that centrality of a protein in the PPI network is strongly correlated with its mRNA half-life. We find that interacting proteins tend to show similar half-lives, commonly referred to as assortative behavior in networks, which is frequently found in biological and social networks. While a major fraction of the interacting proteins show significantly lower differences in mRNA stabilities, a smaller but significant number of protein pairs tend to show higher differences than expected by chance. Higher differences in transcript stabilities often involved those that encode for transcription factors and enzymes, suggesting a feedback link at the post-translational level. We also note that although essential genes, which act as a proxy for in vivo centrality in PPI networks, are highly expressed compared to non-essential ones, they do not encode for more stable transcripts than non-essential genes. Our results provide a direct link between mRNA stability and centrality of a protein in PPI network indicating the importance of post-transcriptional mechanisms on nascent RNAs in the cell.

Intragenic suppressors of temperature-sensitive rne mutations lead to the dissociation of RNase E activity on mRNA and tRNA substrates in Escherichia coli

RNase E of Escherichia coli is an essential endoribonuclease that is involved in many aspects of RNA metabolism. Point mutations in the S1 RNA-binding domain of RNase E (rne-1 and rne-3071) lead to temperature-sensitive growth along with defects in 5S rRNA processing, mRNA decay and tRNA maturation. However, it is not clear whether RNase E acts similarly on all kinds of RNA substrates. Here we report the isolation and characterization of three independent intragenic second-site suppressors of the rne-1 and rne-3071 alleles that demonstrate for the first time the dissociation of the in vivo activity of RNase E on mRNA versus tRNA and rRNA substrates. Specifically, tRNA maturation and 9S rRNA processing were restored to wild-type levels in each of the three suppressor mutants (rne-1/172, rne-1/186 and rne-1/187), while mRNA decay and autoregulation of RNase E protein levels remained as defective as in the rne-1 single mutant. Each single amino acid substitution (Gly→Ala at amino acid 172; Phe → Cys at amino acid 186 and Arg → Leu at amino acid 187) mapped within the 5′ sensor region of the RNase E protein. Molecular models of RNase E suggest how suppression may occur.

Identification of amino acid residues in the catalytic domain of RNase E essential for survival of Escherichia coli: functional analysis of DNase I subdomain

RNase E is an essential Escherichia coli endoribonuclease that plays a major role in the decay and processing of a large fraction of RNAs in the cell. To better understand the molecular mechanisms of RNase E action, we performed a genetic screen for amino acid substitutions in the catalytic domain of the protein (N-Rne) that knock down the ability of RNase E to support survival of E. coli. Comparative phylogenetic analysis of RNase E homologs shows that wild-type residues at these mutated positions are nearly invariably conserved. Cells conditionally expressing these N-Rne mutants in the absence of wild-type RNase E show a decrease in copy number of plasmids regulated by the RNase E substrate RNA I, and accumulation of 5S ribosomal RNA, M1 RNA, and tRNA(Asn) precursors, as has been found in Rne-depleted cells, suggesting that the inability of these mutants to support cellular growth results from loss of ribonucleolytic activity. Purified mutant proteins containing an amino acid substitution in the DNase I subdomain, which is spatially distant from the catalytic site posited from crystallographic studies, showed defective binding to an RNase E substrate, p23 RNA, but still retained RNA cleavage activity-implicating a previously unidentified structural motif in the DNase I subdomain in the binding of RNase E to targeted RNA molecules, demonstrating the role of the DNase I domain in RNase E activity.

The RNA degradosome of Escherichia coli: an mRNA-degrading machine assembled on RNase E

The RNA degradosome of Escherichia coli is a multiprotein complex involved in the degradation of mRNA. The principal components are RNase E, PNPase, RhlB, and enolase. RNase E is a large multidomain protein with an N-terminal catalytic region and a C-terminal noncatalytic region that is mostly natively unstructured protein. The noncatalytic region contains sites for binding RNA and for protein-protein interactions with other components of the RNA degradosome. Several recent studies suggest that there are alternative forms of the RNA degradosome depending on growth conditions or other factors. These alternative forms appear to modulate RNase E activity in the degradation of mRNA. RNA degradosome-like complexes appear to be conserved throughout the Proteobacteria, but there is a surprising variability in composition that might contribute to the adaptation of these bacteria to the enormously wide variety of niches in which they live.

Dark-induced mRNA instability involves RNase E/G-type endoribonuclease cleavage at the AU-box and SD sequences in cyanobacteria

Light-responsive gene expression is crucial to photosynthesizing organisms. Here, we studied functions of cis-elements (AU-box and SD sequences) and a trans-acting factor (ribonuclease, RNase) in light-responsive expression in cyanobacteria. The results indicated that AU-rich nucleotides with an AU-box, UAAAUAAA, just upstream from an SD confer instability on the mRNA under darkness. An RNase E/G homologue, Slr1129, of the cyanobacterium Synechocystis sp. strain PCC 6803 was purified and confirmed capable of endoribonucleolytic cleavage at the AU- (or AG)-rich sequences in vitro. The cleavage depends on the primary target sequence and secondary structure of the mRNA. Complementation tests using Escherichia coli rne/rng mutants showed that Slr1129 fulfilled the functions of both the RNase E and RNase G. An analysis of systematic mutations in the AU-box and SD sequences showed that the cis-elements also affect significantly mRNA stability in light-responsive genes. These results strongly suggested that dark-induced mRNA instability involves RNase E/G-type cleavage at the AU-box and SD sequences in cyanobacteria. The mechanical impact and a possible common mechanism with RNases for light-responsive gene expression are discussed.

Messenger RNA Decay

This chapter discusses several topics relating to the mechanisms of mRNA decay. These topics include the following: important physical properties of mRNA molecules that can alter their stability; methods for determining mRNA half-lives; the genetics and biochemistry of proteins and enzymes involved in mRNA decay; posttranscriptional modification of mRNAs; the cellular location of the mRNA decay apparatus; regulation of mRNA decay; the relationships among mRNA decay, tRNA maturation, and ribosomal RNA processing; and biochemical models for mRNA decay. Escherichia coli has multiple pathways for ensuring the effective decay of mRNAs and mRNA decay is closely linked to the cell's overall RNA metabolism. Finally, the chapter highlights important unanswered questions regarding both the mechanism and importance of mRNA decay.

Protection of psbAII transcript from ribonuclease degradation in vitro by DnaK2 and DnaJ2 chaperones of the cyanobacterium Synechococcus elongatus PCC 7942

Three dnaK and four dnaJ genes have been identified in the genome of cyanobacterium Synechococcus elongatus PCC 7942. Our comprehensive analysis of yeast two-hybrid screening revealed a specific interaction among DnaK2, DnaJ2, and RNase E, an essential endoribonuclease. We examined the effects of DnaK2 and DnaJ2 on RNase E activity by monitoring the digestion of psbAII transcript in vitro. The addition of DnaK2 and DnaJ2 obviously inhibited RNase E activity in an ATP-dependent manner. These results suggest that DnaK2 and DnaJ2 are involved in RNA degradation through interaction with RNase E.

Role of mRNA stability in growth phase regulation of gene expression in the group A streptococcus

The impressive disease spectrum of Streptococcus pyogenes (the group A streptococcus [GAS]) is believed to be determined by its ability to modify gene expression in response to environmental stimuli. Virulence gene expression is controlled tightly by several different transcriptional regulators in this organism. In addition, expression of most, if not all, GAS genes is determined by a global mechanism dependent on growth phase. To begin an analysis of growth-phase regulation, we compared the transcriptome 2 h into stationary phase to that in late exponential phase of a serotype M3 GAS strain. We identified the arc transcript as more abundant in stationary phase in addition to the sag and sda transcripts that had been previously identified. We found that in stationary phase, the stability of sagA, sda, and arcT transcripts increased dramatically. We found that polynucleotide phosphorylase (PNPase [encoded by pnpA]) is rate limiting for decay of sagA and sda transcripts in late exponential phase, since the stability of these mRNAs was greater in a pnpA mutant, while stability of control mRNAs was unaffected by this mutation. Complementation restored the wild-type decay rate. Furthermore, in a pnpA mutant, the sagA mRNA appeared to be full length, as determined by Northern hybridization. It seems likely that mRNAs abundant in stationary phase are insensitive to the normal decay enzyme(s) and instead require PNPase for this process. It is possible that PNPase activity is limited in stationary phase, allowing persistence of these important virulence factor transcripts at this phase of growth.

Identification of a novel regulatory protein (CsrD) that targets the global regulatory RNAs CsrB and CsrC for degradation by RNase E

In Escherichia coli, the global regulatory protein CsrA (carbon store regulator A) binds to leader segments of target mRNAs, affecting their translation and stability. CsrA activity is regulated by two noncoding RNAs, CsrB and CsrC, which act by sequestering multiple CsrA dimers. Here, we describe a protein (CsrD) that controls the degradation of CsrB/C RNAs. The dramatic stabilization of CsrB/C RNAs in a csrD mutant altered the expression of CsrA-controlled genes in a manner predicted from the previously described Csr regulatory circuitry. A deficiency in RNase E, the primary endonuclease involved in mRNA decay, also stabilized CsrB/C, although the half-lives of other RNAs that are substrates for RNase E (rpsO, rpsT, and RyhB) were unaffected by csrD. Analysis of the decay of CsrB RNA, both in vitro and in vivo, suggested that CsrD is not a ribonuclease. Interestingly, the CsrD protein contains GGDEF and EAL domains, yet unlike typical proteins in this large superfamily, its activity in the regulation of CsrB/C decay does not involve cyclic di-GMP metabolism. The two predicted membrane-spanning regions are dispensable for CsrD activity, while HAMP-like, GGDEF, and EAL domains are required. Thus, these studies demonstrate a novel process for the selective targeting of RNA molecules for degradation by RNase E and a novel function for a GGDEF-EAL protein.

Retention of Core Catalytic Functions by a Conserved Minimal Ribonuclease E Peptide That Lacks the Domain Required for Tetramer Formation

Ribonuclease E (RNase E) is a multifunctional endoribonuclease that has been evolutionarily conserved in both Gram-positive and Gram-negative bacteria. X-ray crystallography and biochemical studies have concluded that the Escherichia coli RNase E protein functions as a homotetramer formed by Zn linkage of dimers within a region extending from amino acid residues 416 through 529 of the 116-kDa protein. Using fragments of RNase E proteins from E. coli and Haemophilus influenzae, we show here that RNase E derivatives that are as short as 395 amino acid residues and that lack the Zn-link region shown previously to be essential for tetramer formation (i.e. amino acid residues 400-415) are catalytically active enzymes that retain the 5' to 3' scanning ability and cleavage site specificity characteristic of full-length RNase E and that also confer colony forming ability on rne null mutant bacteria. Further truncation leads to loss of these properties. Our results, which identify a minimal catalytically active RNase E sequence, indicate that contrary to current models, a tetrameric quaternary structure is not required for RNase E to carry out its core enzymatic functions.

RNase E maintenance of proper FtsZ/FtsA ratio required for nonfilamentous growth of Escherichia coli cells but not for colony-forming ability

Inactivation or deletion of the RNase E-encoding rne gene of Escherichia coli results in the growth of bacterial cells as filamentous chains in liquid culture (K. Goldblum and D. Apirion, J. Bacteriol. 146:128-132, 1981) and the loss of colony-forming ability (CFA) on solid media. RNase E dysfunction is also associated with abnormal processing of ftsQAZ transcripts (K. Cam, G. Rome, H. M. Krisch, and J.-P. Bouché, Nucleic Acids Res. 24:3065-3070, 1996), which encode proteins having a central role in septum formation during cell division. We show here that RNase E regulates the relative abundances of FtsZ and FtsA proteins and that RNase E depletion results in decreased FtsZ, increased FtsA, and consequently an altered FtsZ/FtsA ratio. However, while restoration of the level of FtsZ to normal in rne null mutant bacteria reverses the filamentation phenotype, it does not restore CFA. Conversely, overexpression of a related RNase, RNase G, in rne-deleted bacteria restores CFA, as previously reported, without affecting FtsZ abundance. Our results demonstrate that RNase E activity is required to maintain a proper cellular ratio of the FtsZ and FtsA proteins in E. coli but that FtsZ deficiency does not account for the nonviability of cells lacking RNase E.

RraA rescues Escherichia coli cells over-producing RNase E from growth arrest by modulating the ribonucleolytic activity

RraA is an evolutionary conserved protein inhibitor of RNase E, which catalyzes the initial step in the decay and processing of numerous RNAs in Escherichia coli and forms the core component of the degradosome, a large protein complex involved in RNA metabolism. Here, we report that co-expression of RraA reduces the ribonucleolytic activity in cells over-producing RNase E and consequently rescues these cells from growth arrest. These findings suggest that inability of cells over-producing RNase E to normally grow results from increased cellular ribonucleolytic activity and RraA is able to effectively modulate the catalytic activity of RNase E in vivo.

Differential modulation of E. coli mRNA abundance by inhibitory proteins that alter the composition of the degradosome

In Escherichia coli the initial step in the processing or decay of many messenger and structural RNAs is mediated by the endonuclease RNase E, which forms the core of a large RNA-catalysis machine termed the degradosome. Previous experiments have identified a protein that globally modulates RNA abundance by binding to RNase E and regulating its endonucleolytic activity. Here we report the discovery of RraB, which interacts with a different site on RNase E and interferes with cleavage of a different set of transcripts. We show that expression of RraA or RraB in vivo is accompanied by dramatic, distinct, and inhibitor-specific changes in degradosome composition--and that these are in turn associated with alterations in RNA decay and global transcript abundance profiles that are dissimilar to the profile observed during simple RNase E deficiency. Our results reveal the existence of endonuclease binding proteins that modulate the remodelling of degradosome composition in bacteria and argue that such degradosome remodelling is a mechanism for the differential regulation of RNA cleavages in E. coli.

The RNA degradosome: life in the fast lane of adaptive molecular evolution

In Escherichia coli, the multi-enzyme RNA degradosome contributes to the global, posttranscriptional regulation of gene expression. The degradosome components are recognized through natively unstructured "microdomains" comprising as few as 15-40 amino acids. Consequently, the degradosome might experience a comparatively smaller number of evolutionary constraints, because there is little requirement to maintain a folded state for the interaction sites. New regulatory properties of the degradosome could arise with relative rapidity, because partners that modify its function could be recruited by quickly evolving microdomains. The unusual combination of the centrality of RNA degradation in gene expression and the generality of natively unstructured microdomains in recognition can fortuitously confer a capacity for efficacious adaptive change to degradosome-like assemblies in eubacteria.

Regulation of RraA, a protein inhibitor of RNase E-mediated RNA decay

The recently discovered RraA protein acts as an inhibitor of the essential endoribonuclease RNase E, and we demonstrated that ectopic expression of RraA affects the abundance of more than 700 transcripts in Escherichia coli (K. Lee, X. Zhan, J. Gao, J. Qiu, Y. Feng, R. Meganathan, S. N. Cohen, and G. Georgiou, Cell 114:623-634, 2003). We show that rraA is expressed from its own promoter, P(rraA), located in the menA-rraA intergenic region. Primer extension and lacZ fusion analysis revealed that transcription from P(rraA) is elevated upon entry into stationary phase in a sigma(s)-dependent manner. In addition, the stability of the rraA transcript is dependent on RNase E activity, suggesting the involvement of a feedback circuit in the regulation of the RraA level in E. coli.

Nuclear suppressors define three factors that participate in both 5' and 3' end processing of mRNAs in Chlamydomonas chloroplasts

Chloroplast RNA processing and degradation are orchestrated by nucleus-encoded factors. Although several transcript-specific factors have been identified, those involved in global RNA metabolism have mostly remained elusive. Using Chlamydomonas reinhardtii, we have identified three pleiotropic nuclear mutations, mcd3, mcd4 and mcd5, which cause quantitative variation between polycistronic transcripts and accumulation of transcripts with novel 3' ends. The mcd3, mcd4 and mcd5 mutants were initially isolated as photoautotrophic suppressors of the petD 5' mutants LS2 and LS6, which harbour four nucleotide linker-scanning mutations near the 5' end of the mature transcript. The LS mutants accumulate 1-3% of the wild-type (WT) petD mRNA level and no cytochrome b6/f complex subunit IV, which is the petD gene product and required for photosynthesis. Each suppressor restores approximately 15% of the WT petD mRNA and subunit IV levels. Genetic analysis showed mcd4 to be recessive, and suggested that MCD4 interacts with the petD mRNA stability factor MCD1. To assess the specificity of mcd3, mcd4 and mcd5, transcripts from 32 chloroplast genes were analysed by RNA filter hybridizations. mcd3 and mcd4 displayed aberrant transcript patterns for 17 genes, whereas only three were altered in mcd5. Since the mutations affect multiple RNAs in a variety of ways, our data suggest that MCD3, MCD4 and MCD5 may participate in a series of multiprotein complexes responsible for RNA maturation and degradation in Chlamydomonas chloroplasts.