Functional Analysis of Vibrio vulnificus Orthologs of Escherichia coli RraA and RNase E

Identification of the global regulatory roles of RraA via the integrative transcriptome and proteome in Vibrio alginolyticus

Bacterial ribonuclease E (RNase E) is vital for posttranscriptional regulation by degrading and processing RNA. The RraA protein inhibits RNase E activity through protein-protein interactions, exerting a global regulatory effect on gene expression. However, the specific role of RraA remains unclear. In this study, we investigated rraA expression in Vibrio alginolyticus ZJ-T and identified three promoters responsible for its expression, resulting in transcripts with varying 5'-UTR lengths. During the stationary phase, rraA was significantly posttranscriptionally inhibited. Deletion of rraA had no impact on bacterial growth in rich medium Luria-Bertani broth with salt (LBS) but resulted in decreased biofilm formation and increased resistance to polymyxin B. Transcriptome analysis revealed 350 differentially expressed genes (DEGs) between the wild type and the rraA mutant, while proteome analysis identified 267 differentially expressed proteins (DEPs). Integrative analysis identified 55 genes common to both DEGs and DEPs, suggesting that RraA primarily affects gene expression at the posttranscriptional level. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis demonstrated that RraA facilitates the conversion of fatty acids, propionic acid, and branched-chain amino acids to acetyl-CoA while enhancing amino acid and peptide uptake. Notably, RraA positively regulates the expression of virulence-associated genes, including those involved in biofilm formation and the type VI secretion system. This study expands the understanding of the regulatory network of RraA through transcriptome analysis, emphasizing the importance of proteomic analysis in investigating posttranscriptional regulation.IMPORTANCERraA is an inhibitor protein of ribonuclease E that interacts with and suppresses its endonucleolytic activity, thereby playing a widespread regulatory role in the degradation and maturation of diverse mRNAs and noncoding small RNAs. However, the physiological functions and associated regulon of RraA in Vibrio alginolyticus have not been fully elucidated. Here, we report that RraA impacts virulence-associated physiological processes, namely, antibiotic resistance and biofilm formation, in V. alginolyticus. By conducting an integrative analysis of both the transcriptome and proteome, we revealed the involvement of RraA in carbon metabolism, amino acid catabolism, and transport, as well as in the type VI secretion system. Collectively, these findings elucidate the regulatory influence of RraA on multiple pathways associated with metabolism and pathogenesis in V. alginolyticus.

Regulator of ribonuclease activity modulates the pathogenicity of Vibrio vulnificus

RraA, a protein regulator of RNase E activity, plays a unique role in modulating the mRNA abundance in Escherichia coli. The marine pathogenic bacterium Vibrio vulnificus also possesses homologs of RNase E (VvRNase E) and RraA (VvRraA1 and VvRraA2). However, their physiological roles have not yet been investigated. In this study, we demonstrated that VvRraA1 expression levels affect the pathogenicity of V. vulnificus. Compared to the wild-type strain, the VvrraA1-deleted strain (ΔVvrraA1) showed decreased motility, invasiveness, biofilm formation ability as well as virulence in mice; these phenotypic changes of ΔVvrraA1 were restored by the exogenous expression of VvrraA1. Transcriptomic analysis indicated that VvRraA1 expression levels affect the abundance of a large number of mRNA species. Among them, the half-lives of mRNA species encoding virulence factors (e.g., smcR and htpG) that have been previously shown to affect VvrraA1 expression-dependent phenotypes were positively correlated with VvrraA1 expression levels. These findings suggest that VvRraA1 modulates the pathogenicity of V. vulnificus by regulating the abundance of a subset of mRNA species.

Substrate-dependent effects of quaternary structure on RNase E activity

RNase E is an essential, multifunctional ribonuclease encoded in E. coli by the rne gene. Structural analysis indicates that the ribonucleolytic activity of this enzyme is conferred by rne-encoded polypeptide chains that (1) dimerize to form a catalytic site at the protein-protein interface, and (2) multimerize further to generate a tetrameric quaternary structure consisting of two dimerized Rne-peptide chains. We identify here a mutation in the Rne protein's catalytic region (E429G), as well as a bacterial cell wall peptidoglycan hydrolase (Amidase C [AmiC]), that selectively affect the specific activity of the RNase E enzyme on long RNA substrates, but not on short synthetic oligonucleotides, by enhancing enzyme multimerization. Unlike the increase in specific activity that accompanies concentration-induced multimerization, enhanced multimerization associated with either the E429G mutation or interaction of the Rne protein with AmiC is independent of the substrate's 5' terminus phosphorylation state. Our findings reveal a previously unsuspected substrate length-dependent regulatory role for RNase E quaternary structure and identify cis-acting and trans-acting factors that mediate such regulation.

Bacterial RNA Degradosomes: Molecular Machines under Tight Control

Bacterial RNA degradosomes are multienzyme molecular machines that act as hubs for post-transcriptional regulation of gene expression. The ribonuclease activities of these complexes require tight regulation, as they are usually essential for cell survival while potentially destructive. Recent studies have unveiled a wide variety of regulatory mechanisms including autoregulation, post-translational modifications, and protein compartmentalization. Recently, the subcellular organization of bacterial RNA degradosomes was found to present similarities with eukaryotic messenger ribonucleoprotein (mRNP) granules, membraneless compartments that are also involved in mRNA and protein storage and/or mRNA degradation. In this review, we present the current knowledge on the composition and targets of RNA degradosomes, the most recent developments regarding the regulation of these machineries, and their similarities with the eukaryotic mRNP granules.

Functional implications of hexameric assembly of RraA proteins from Vibrio vulnificus

RNase E has a pivotal role in the degradation and processing of RNAs in Escherichia coli, and protein inhibitors RraA and RraB control its enzymatic activity. The halophilic pathogenic bacterium Vibrio vulnificus also expresses orthologs of RNase E and RraA—RNase EV, RraAV1, and RraAV2 (herein renamed as VvRNase E, VvRraA1, and VvRraA2). A previous study showed that VvRraA1 actively inhibits the ribonucleolytic activity of VvRNase E by interacting with the C-terminal region of VvRNase E. However, the molecular mechanism underlying the effect of VvRraA1 on the ribonucleolytic activity of VvRNase E has not yet been elucidated. In this study, we report that the oligomer formation of VvRraA proteins affects binding efficiency to VvRNase E as well as inhibitory activity on VvRNase E action. The hexameric structure of VvRraA1 was converted to lower oligomeric forms when the Cys 9 residue was substituted with an Asp residue (VvRraA1-C9D), showing decreased inhibitory activity of VvRraA1 on VvRNase E in vivo. These results indicated that the intermolecular disulfide linkage contributed critically to the hexamerization of VvRraA1 for its proper function. On the contrary, the VvRraA2 that existed in a trimeric state did not bind to or inhibit VvRNase E. An in vitro cleavage assay further showed the reduced inhibitory effect of VvRraA-C9D on VvRNase E activity compared to wild-type VvRraA1. These findings provide insight into how VvRraA proteins can regulate VvRNase E action on its substrate RNA in V. vulnificus. In addition, based on structural and functional comparison of RraA homologs, we suggest that hexameric assembly of RraA homologs may well be required for their action on RNase E-like proteins.

Bdm-Mediated Regulation of Flagellar Biogenesis in Escherichia coli and Salmonella enterica Serovar Typhimurium

Synthesis of the flagellar apparatus in Escherichia coli is mediated via complex regulatory pathways. A previous study indicated that the protein encoded by the biofilm-dependent modulation (bdm) gene is linked closely with a regulatory pathway for flagellar assembly. However, the specific role of Bdm in flagellar biogenesis remains unknown. Herein, we showed that Bdm interacts with FlgM and inhibits its function as an anti-σ28 factor, which induces the transcription of flagellar late-class genes in E. coli. In addition, we observed that deletion of the yddX gene, a potential Salmonella enterica serovar Typhimurium homolog of bdm, also resulted in downregulation of flagellar late-class genes and in the formation of short flagella, leading to decreased virulence in mice. The expression levels of late-class flagellar genes in yddX-deleted S. Typhimurium cells were restored to those of the wild type when either E. coli bdm or S. Typhimurium yddX was expressed exogenously. These results suggest that Bdm-mediated regulation of flagellar assembly is a conserved regulatory pathway in those members of the Enterobacteriaceae family whose genomes show the existence of homologs of bdm.

Crystal structure of Streptomyces coelicolor RraAS2, an unusual member of the RNase E inhibitor RraA protein family

Bacterial ribonuclease E (RNase E) plays a crucial role in the processing and decay of RNAs. A small protein named RraA negatively regulates the activity of RNase E via protein-protein interaction in various bacteria. Recently, RraAS1 and RraAS2, which are functional homologs of RraA from Escherichia coli, were identified in the Gram-positive species Streptomyces coelicolor. RraAS1 and RraAS2 inhibit RNase ES ribonuclease activity in S. coelicolor. RraAS1 and RraAS2 have a C-terminal extension region unlike typical bacterial RraA proteins. In this study, we present the crystal structure of RraAS2, exhibiting a hexamer arranged in a dimer of trimers, consistent with size exclusion chromatographic results. Importantly, the C-terminal extension region formed a long α-helix at the junction of the neighboring subunit, which is similar to the trimeric RraA orthologs from Saccharomyces cerevisiae. Truncation of the C-terminal extension region resulted in loss of RNase ES inhibition, demonstrating its crucial role. Our findings present the first bacterial RraA that has a hexameric assembly with a C-terminal extension α-helical region, which plays an essential role in the regulation of RNase ES activity in S. coelicolor.

RraAS1 inhibits the ribonucleolytic activity of RNase ES by interacting with its catalytic domain in Streptomyces coelicolor

RraA is a protein inhibitor of RNase E, which degrades and processes numerous RNAs in Escherichia coli. Streptomyces coelicolor also contains homologs of RNase E and RraA, RNase ES and RraAS1/RraAS2, respectively. Here, we report that, unlike other RraA homologs, RraAS1 directly interacts with the catalytic domain of RNase ES to exert its inhibitory effect. We further show that rraAS1 gene deletion in S. coelicolor results in a higher growth rate and increased production of actinorhodin and undecylprodigiosin, compared with the wild-type strain, suggesting that RraAS1-mediated regulation of RNase ES activity contributes to modulating the cellular physiology of S. coelicolor.

RraAS2 requires both scaffold domains of RNase ES for high-affinity binding and inhibitory action on the ribonucleolytic activity

RraA is a protein inhibitor of RNase E (Rne), which catalyzes the endoribonucleolytic cleavage of a large proportion of RNAs in Escherichia coli. The antibiotic-producing bacterium Streptomyces coelicolor also contains homologs of RNase E and RraA, designated as RNase ES (Rns), RraAS1, and RraAS2, respectively. Here, we report that RraAS2 requires both scaffold domains of RNase ES for high-affinity binding and inhibitory action on the ribonucleolytic activity. Analyses of the steady-state level of RNase E substrates indicated that coexpression of RraAS2 in E. coli cells overproducing Rns effectively inhibits the ribonucleolytic activity of full-length RNase ES, but its inhibitory effects were moderate or undetectable on other truncated forms of Rns, in which the N- or/and C-terminal scaffold domain was deleted. In addition, RraAS2 more efficiently inhibited the in vitro ribonucleolytic activity of RNase ES than that of a truncated form containing the catalytic domain only. Coimmunoprecipitation and in vivo cross-linking experiments further showed necessity of both scaffold domains of RNase ES for high-affinity binding of RraAS2 to the enzyme, resulting in decreased RNA-binding capacity of RNase ES. Our results indicate that RraAS2 is a protein inhibitor of RNase ES and provide clues to how this inhibitor affects the ribonucleolytic activity of RNase ES.