RNase E and RNase J are needed for S-adenosylmethionine homeostasis in Sinorhizobium meliloti

Sinorhizobium meliloti BR-bodies promote fitness during host colonization

Biomolecular condensates, such as the nucleoli or P-bodies, are non-membrane-bound assemblies of proteins and nucleic acids that facilitate specific cellular processes. Like eukaryotic P-bodies, the recently discovered bacterial ribonucleoprotein bodies (BR-bodies) organize the mRNA decay machinery, yet the similarities in molecular and cellular functions across species have been poorly explored. Here, we examine the functions of BR-bodies in the nitrogen-fixing endosymbiont Sinorhizobium meliloti, which colonizes the roots of compatible legume plants. Assembly of BR-bodies into visible foci in S. meliloti cells requires the C-terminal intrinsically disordered region (IDR) of RNase E, and foci fusion is readily observed in vivo, suggesting they are liquid-like condensates that form via mRNA sequestration. Using Rif-seq to measure mRNA lifetimes, we found a global slowdown in mRNA decay in a mutant deficient in BR-bodies, indicating that compartmentalization of the degradation machinery promotes efficient mRNA turnover. While BR-bodies are constitutively present during exponential growth, the abundance of BR-bodies increases upon cell stress, whereby they promote stress resistance. Finally, using Medicago truncatula as host, we show that BR-bodies enhance competitiveness during colonization and appear to be required for effective symbiosis, as mutants without BR-bodies failed to stimulate plant growth. These results suggest that BR-bodies provide a fitness advantage for bacteria during infection, perhaps by enabling better resistance against the host immune response.

The role of RNA regulators, quorum sensing and c-di-GMP in bacterial biofilm formation

Biofilms provide an ecological advantage against many environmental stressors, such as pH and temperature, making it the most common life-cycle stage for many bacteria. These protective characteristics make eradication of bacterial biofilms challenging. This is especially true in the health sector where biofilm formation on hospital or patient equipment, such as respirators, or catheters, can quickly become a source of anti-microbial resistant strains. Biofilms are complex structures encased in a self-produced polymeric matrix containing numerous components such as polysaccharides, proteins, signalling molecules, extracellular DNA and extracellular RNA. Biofilm formation is tightly controlled by several regulators, including quorum sensing (QS), cyclic diguanylate (c-di-GMP) and small non-coding RNAs (sRNAs). These three regulators in particular are fundamental in all stages of biofilm formation; in addition, their pathways overlap, and the significance of their role is strain-dependent. Currently, ribonucleases are also of interest for their potential role as biofilm regulators, and their relationships with QS, c-di-GMP and sRNAs have been investigated. This review article will focus on these four biofilm regulators (ribonucleases, QS, c-di-GMP and sRNAs) and the relationships between them.

Incoherent dual regulation by a SAM-II riboswitch controlling translation at a distance

In Sinorhizobium meliloti, the methionine biosynthesis genes metA and metZ are preceded by S-adenosyl-L-methionine (SAM) riboswitches of the SAM-II class. Upon SAM binding, structural changes in the metZ riboswitch were predicted to cause transcriptional termination, generating the sRNA RZ. By contrast, the metA riboswitch was predicted to regulate translation from an AUG1 codon. However, downstream of the metA riboswitch, we found a putative Rho-independent terminator and an in-frame AUG2 codon, which may contribute to metA regulation. We validated the terminator between AUG1 and AUG2, which generates the sRNA RA1 that is processed to RA2. Under high SAM conditions, the activities of the metA and metZ promoters and the steady-state levels of the read-through metA and metZ mRNAs were decreased, while the levels of the RZ and RA2 sRNAs were increased. Under these conditions, the sRNAs and the mRNAs were stabilized. Reporter fusion experiments revealed that the Shine–Dalgarno (SD) sequence in the metA riboswitch is required for translation, which, however, starts 74 nucleotides downstream at AUG2, suggesting a novel translation initiation mechanism. Further, the reporter fusion data supported the following model of RNA-based regulation: Upon SAM binding by the riboswitch, the SD sequence is sequestered to downregulate metA translation, while the mRNA is stabilized. Thus, the SAM-II riboswitches fulfil incoherent, dual regulation, which probably serves to ensure basal metA and metZ mRNA levels under high SAM conditions. This probably helps to adapt to changing conditions and maintain SAM homoeostasis.

Riboregulation in Nitrogen-Fixing Endosymbiotic Bacteria

Small non-coding RNAs (sRNAs) are ubiquitous components of bacterial adaptive regulatory networks underlying stress responses and chronic intracellular infection of eukaryotic hosts. Thus, sRNA-mediated regulation of gene expression is expected to play a major role in the establishment of mutualistic root nodule endosymbiosis between nitrogen-fixing rhizobia and legume plants. However, knowledge about this level of genetic regulation in this group of plant-interacting bacteria is still rather scarce. Here, we review insights into the rhizobial non-coding transcriptome and sRNA-mediated post-transcriptional regulation of symbiotic relevant traits such as nutrient uptake, cell cycle, quorum sensing, or nodule development. We provide details about the transcriptional control and protein-assisted activity mechanisms of the functionally characterized sRNAs involved in these processes. Finally, we discuss the forthcoming research on riboregulation in legume symbionts.

Links between S-adenosylmethionine and Agr-based quorum sensing for biofilm development in Listeria monocytogenes EGD-e

Listeria monocytogenes is the causative agent of human listeriosis which has high hospitalization and mortality rates for individuals with weakened immune systems. The survival and dissemination of L. monocytogenes in adverse environments can be reinforced by the formation of biofilms. Therefore, this study aimed to understand the mechanisms underlying listerial biofilm development. Given that both nutrient availability and quorum sensing (QS) have been known as the factors influencing biofilm development, we hypothesized that the signal from a sentinel metabolite S‐adenosylmethionine (SAM) and Agr‐based QS could be synchronous in L. monocytogenes to modulate nutrient availability, the synthesis of extracellular polymeric substances (EPSs), and biofilm formation. We performed biofilm assays and quantitative real‐time PCR to investigate how biofilm volumes and the expression of genes for the synthesis of EPS were affected by SAM supplementation, agr deletion, or both. We found that exogenously applied SAM induced biofilm formation and that the expression of genes encoding the EPS synthesis machineries was regulated by SAM and/or Agr QS. Moreover, the gene transcription of components acting in the methyl cycle for SAM synthesis and Agr QS was affected by the signals from the other system. In summary, we reveal an interconnection at the transcriptional level between metabolism and QS in L. monocytogenes and highlight the critical role of metabolite‐oriented QS in biofilm development.

Regulatory Tools for Controlling Gene Expression in Cyanobacteria

Cyanobacteria are attractive hosts for converting carbon dioxide and sunlight into desirable chemical products. To engineer these organisms and manipulate their metabolic pathways, the biotechnology community has developed genetic tools to control gene expression. Many native cyanobacterial promoters and related sequence elements have been used to regulate genes of interest, and heterologous tools that use non-native small molecules to induce gene expression have been demonstrated. Overall, IPTG-based induction systems seem to be leaky and initially demonstrate small dynamic ranges in cyanobacteria. Consequently, a variety of other induction systems have been optimized to enable tighter control of gene expression. Tools require significant optimization because they function quite differently in cyanobacteria when compared to analogous use in model heterotrophs. We hypothesize that these differences are due to fundamental differences in physiology between organisms. This review is not intended to summarize all known products made in cyanobacteria nor the performance (titer, rate, yield) of individual strains, but instead will focus on the genetic tools and the inherent aspects of cellular physiology that influence gene expression in cyanobacteria.

Sinorhizobium meliloti RNase III: Catalytic Features and Impact on Symbiosis

Members of the ribonuclease (RNase) III family of enzymes are metal-dependent double-strand specific endoribonucleases. They are ubiquitously found and eukaryotic RNase III-like enzymes include Dicer and Drosha, involved in RNA processing and RNA interference. In this work, we have addressed the primary characterization of RNase III from the symbiotic nitrogen-fixing α-proteobacterium Sinorhizobium meliloti. The S. meliloti rnc gene does encode an RNase III-like protein (SmRNase III), with recognizable catalytic and double-stranded RNA (dsRNA)-binding domains that clusters in a branch with its α–proteobacterial counterparts. Purified SmRNase III dimerizes, is active at neutral to alkaline pH and behaves as a strict metal cofactor-dependent double-strand endoribonuclease, with catalytic features distinguishable from those of the prototypical member of the family, the Escherichia coli ortholog (EcRNase III). SmRNase III prefers Mn²⁺ rather than Mg²⁺ as metal cofactor, cleaves the generic structured R1.1 substrate at a site atypical for RNase III cleavage, and requires higher cofactor concentrations and longer dsRNA substrates than EcRNase III for optimal activity. Furthermore, the ultraconserved E125 amino acid was shown to play a major role in the metal-dependent catalysis of SmRNase III. SmRNase III degrades endogenous RNA substrates of diverse biogenesis with different efficiency, and is involved in the maturation of the 23S rRNA. SmRNase III loss-of-function neither compromises viability nor alters morphology of S. meliloti cells, but influences growth, nodulation kinetics, the onset of nitrogen fixation and the overall symbiotic efficiency of this bacterium on the roots of its legume host, alfalfa, which ultimately affects plant growth. Our results support an impact of SmRNase III on nodulation and symbiotic nitrogen fixation in plants.

RNase E cleavage shapes the transcriptome ofRhodobacter sphaeroidesand strongly impacts phototrophic growth

This study identifies the cleavage sites of the endoribonuclease RNase E in the Rhodobacter sphaeroides transcriptome and demonstrates its effect on oxidative stress resistance and phototrophic growth.

Bacteria adapt to changing environmental conditions by rapid changes in their transcriptome. This is achieved not only by adjusting rates of transcription but also by processing and degradation of RNAs. We applied TIER-Seq (transiently inactivating an endoribonuclease followed by RNA-Seq) for the transcriptome-wide identification of RNase E cleavage sites and of 5′ RNA ends, which are enriched when RNase E activity is reduced in Rhodobacter sphaeroides. These results reveal the importance of RNase E for the maturation and turnover of mRNAs, rRNAs, and sRNAs in this guanine-cytosine-rich α-proteobacterium, some of the latter have well-described functions in the oxidative stress response. In agreement with this, a role of RNase E in the oxidative stress response is demonstrated. A remarkably strong phenotype of a mutant with reduced RNase E activity was observed regarding the formation of photosynthetic complexes and phototrophic growth, whereas there was no effect on chemotrophic growth.

RNA silencing in plant symbiotic bacteria: Insights from a protein-centric view

Extensive work in model enterobacteria has evidenced that the RNA chaperone Hfq and several endoribonucleases, such as RNase E or RNase III, serve pivotal roles in small RNA-mediated post-transcriptional silencing of gene expression. Characterization of these protein hubs commonly provide global functional and mechanistic insights into complex sRNA regulatory networks. The legume endosymbiont Sinorhizobium meliloti is a non-classical model bacterium with a very complex lifestyle in which riboregulation is expected to play important adaptive functions. Here, we discuss current knowledge about RNA silencing in S. meliloti from the perspective of the activity of Hfq and a recently discovered endoribonuclease (YbeY) exhibiting unprecedented catalytic versatility for the cleavage of single- and double-stranded RNA molecules.

Enzymatic activity necessary to restore the lethality due to Escherichia coli RNase E deficiency is distributed among bacteria lacking RNase E homologues

Escherichia coli RNase E (Eco-RNase E), encoded by rne (Eco-rne), is considered the global RNA decay initiator. Although Eco-RNase E is an essential gene product in E. coli, some bacterial species, such as Bacillus subtilis, do not possess Eco-RNase E sequence homologues. B. subtilis instead possesses RNase J1/J2 (Bsu-RNase J1/J2) and RNase Y (Bsu-RNase Y) to execute RNA decay. Here we found that E. coli lacking the Eco-rne gene (Δrne E. coli) was viable conditional on M9 minimal media by introducing Bsu-RNase J1/J2 or Bsu-RNase Y. We also cloned an extremely short Eco-RNase E homologue (Wpi-RNase E) and a canonical sized Bsu-RNase J1/J2 homologue (Wpi-RNase J) from Wolbachia pipientis, an α-proteobacterial endosymbiont of arthropods. We found that Wpi-RNase J restored the colony-forming ability (CFA) of Δrne E. coli, whereas Wpi-RNase E did not. Unexpectedly, Wpi-RNase E restored defective CFA due to lack of Eco-RNase G, a paralogue of Eco-RNase E. Our results indicate that bacterial species that lack Eco-RNase E homologues or bacterial species that possess Eco-RNase E homologues which lack Eco-RNase E-like activities have a modest Eco-RNase E-like function using RNase J and/or RNase Y. These results suggest that Eco-RNase E-like activities might distribute among a wide array of bacteria and that functions of RNases may have changed dynamically during evolutionary divergence of bacterial lineages.