RNase J1 and J2 Are Host-Encoded Factors for Plasmid Replication.

Plasmids need to ensure their transmission to both daughter-cells when their host divides, but should at the same time avoid overtaxing their hosts by directing excessive host-resources toward production of plasmid factors. Naturally occurring plasmids have therefore evolved regulatory mechanisms to restrict their copy-number in response to the volume of the cytoplasm. In many plasmid families, copy-number control is mediated by a small plasmid-specified RNA, which is continuously produced and rapidly degraded, to ensure that its concentration is proportional to the current plasmid copy-number. We show here that pSA564 from the RepA_N-family is regulated by a small antisense RNA (RNA1), which, when over-expressed in trans, blocks plasmid replication and cures the bacterial host. The 5' untranslated region (5'UTR) of the plasmid replication initiation gene (repA) potentially forms two mutually exclusive secondary structures, ON and OFF, where the latter both sequesters the repA ribosome binding site and acts as a rho-independent transcriptional terminator. Duplex formation between RNA1 and the 5'UTR shifts the equilibrium to favor the putative OFF-structure, enabling a single small RNA to down-regulate repA expression at both transcriptional and translational levels. We further examine which sequence elements on the antisense RNA and on its 5'UTR target are needed for this regulation. Finally, we identify the host-encoded exoribonucleases RNase J1 and J2 as the enzymes responsible for rapidly degrading the replication-inhibiting section of RNA1. This region accumulates and blocks RepA expression in the absence of either RNase J1 or J2, which are therefore essential host factors for pSA564 replication in Staphylococcus aureus.

Medicago-Sinorhizobium-Ralstonia: A Model System to Investigate Pathogen-Triggered Inhibition of Nodulation.

How plants deal with beneficial and pathogenic microorganisms and how they can tolerate beneficial ones and face pathogens at the same time are questions that remain puzzling to plant biologists. Legume plants are good models to explore those issues, as their interactions with nitrogen-fixing bacteria called rhizobia results in a drastic and easy-to-follow phenotype of nodulation. Intriguingly, despite massive and chronic infection, legume defense reactions are essentially suppressed during the whole symbiotic process, raising a question about a potential negative effect of plant immune responses on the establishment of nodulation. In the present study, we used the model legume, Medicago truncatula, coinoculated with mutualistic and phytopathogenic bacteria, Sinorhizobium medicae and Ralstonia solanacearum, respectively. We show that the presence of R. solanacearum drastically inhibits the nodulation process. The type III secretion system of R. solanacearum, which is important for the inhibition of pathogen-associated molecular pattern-triggered immunity (PTI), strongly contributes to inhibit nodulation. Thus, our results question the negative effect of PTI on nodulation. By including a pathogenic bacterium in the interaction system, our study provides a new angle to address the influence of the biotic environment on the nodulation process.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Medicago-Sinorhizobium-Ralstonia Co-infection Reveals Legume Nodules as Pathogen Confined Infection Sites Developing Weak Defenses.

Legumes have the capacity to develop root nodules hosting nitrogen-fixing bacteria, called rhizobia. For the plant, the benefit of the symbiosis is important in nitrogen-deprived conditions, but it requires hosting and feeding massive numbers of rhizobia. Recent studies suggest that innate immunity is reduced or suppressed within nodules [1-10]; this likely maintains viable rhizobial populations. To evaluate the potential consequences and risks associated with an altered immuni`ty in the symbiotic organ, we developed a tripartite system with the model legume Medicago truncatula [11, 12], its nodulating symbiont of the genus Sinorhizobium (syn. Ensifer) [13, 14], and the pathogenic soil-borne bacterium Ralstonia solanacearum [15-18]. We show that nodules are frequent infection sites where pathogen multiplication is comparable to that in the root tips and independent of nodule ability to fix nitrogen. Transcriptomic analyses indicate that, despite the presence of the hosted rhizobia, nodules are able to develop weak defense reactions against pathogenic R. solanacearum. Nodule defense response displays specificity compared to that activated in roots. In agreement with nodule innate immunity, optimal R. solanacearum growth requires pathogen virulence factors. Finally, our data indicate that the high susceptibility of nodules is counterbalanced by the existence of a diffusion barrier preventing pathogen spreading from nodules to the rest of the plant.

The Nitrate Assimilatory Pathway in Sinorhizobium meliloti: Contribution to NO Production.

The interaction between rhizobia and their legume host plants culminates in the formation of specialized root organs called nodules in which differentiated endosymbiotic bacteria (bacteroids) fix atmospheric nitrogen to the benefit of the plant. Interestingly, nitric oxide (NO) has been detected at various steps of the rhizobium-legume symbiosis where it has been shown to play multifaceted roles. It is recognized that both bacterial and plant partners of the Sinorhizobium meliloti-Medicago truncatula symbiosis are involved in NO synthesis in nodules. S. meliloti can also produce NO from nitrate when living as free cells in the soil. S. meliloti does not possess any NO synthase gene in its genome. Instead, the denitrification pathway is often described as the main driver of NO production with nitrate as substrate. This pathway includes the periplasmic nitrate reductase (Nap) which reduces nitrate into nitrite, and the nitrite reductase (Nir) which reduces nitrite into NO. However, additional genes encoding putative nitrate and nitrite reductases (called narB and nirB, respectively) have been identified in the S. meliloti genome. Here we examined the conditions where these genes are expressed, investigated their involvement in nitrate assimilation and NO synthesis in culture and their potential role in planta. We found that narB and nirB are expressed under aerobic conditions in absence of ammonium in the medium and most likely belong to the nitrate assimilatory pathway. Even though these genes are clearly expressed in the fixation zone of legume root nodule, they do not play a crucial role in symbiosis. Our results support the hypothesis that in S. meliloti, denitrification remains the main enzymatic way to produce NO while the assimilatory pathway involving NarB and NirB participates indirectly to NO synthesis by cooperating with the denitrification pathway.

Post-transcriptional control of virulence gene expression in Staphylococcus aureus.

Opportunistic pathogens have to be ready to change life-style whenever the occasion arises, and therefore need to keep tight control over the expression of their virulence factors. Doubly so for commensal bacteria, such as Staphylococcus aureus, which should avoid harming their hosts when they are in a state of peaceful co-existence. S. aureus carries very few sigma factors to help define the transcriptional programs, but instead uses a plethora of small RNA molecules and RNA-RNA interactions to regulate gene expression post-transcriptionally. The endoribonucleases RNase III and RNase Y contribute to this regulatory diversity, and provide a link to RNA-decay and intra-cellular spatiotemporal control of expression. In this review we describe some of these post-transcriptional mechanisms as well as some of the novel transcriptomic approaches that have been used to find and to study them.