The periplasmic regulator ExoR inhibits ExoS/ChvI two‐component signalling in Sinorhizobium meliloti

Abstract

SummarySinorhizobium meliloti requires ExoS/ChvI twocomponent signalling to establish a nitrogen-fixing symbiosis with legume hosts. The importance of ExoS/ChvI signalling in microbe-host interactions is underscored by the requirement of ExoS/ChvI orthologues for virulence of the related a-proteobacteria Agrobacterium tumefaciens and Brucella abortus. In S. meliloti, ExoS/ChvI is a key regulator of gene expression for exopolysaccharide synthesis, biofilm formation, motility, nutrient utilization and free-living viability. Previously, we showed that the novel conserved regulator ExoR interacts genetically with both ExoS and ChvI, and localizes to the periplasm of S. meliloti. Here, we show that ExoR physically associates with ExoS and that this association is important for regulating ExoS/ChvI signalling. We have identified point mutations in the Sel1-like repeat region of ExoR that disrupt binding to ExoS and cause a dramatic increase in ExoS/ChvI-dependent gene expression. Furthermore, we have found that physical interaction with ExoS stabilizes the ExoR protein. Together, our results indicate that ExoR binds to ExoS in the periplasm of S. meliloti to inhibit ExoS/ChvI activity, and that ExoR represents a novel periplasmic inhibitor of two-component signalling.

The Sinorhizobium meliloti SyrM Regulon: Effects on Global Gene Expression Are Mediated by syrA and nodD3

“…Since ExoR inhibits activity of the ExoS histidine kinase and, hence, subsequent activation of ChvI, a Ptrp-exoR strain is expected to behave like a chvI-null mutant. For example, a Ptrp-exoR strain is not viable on LB medium unless the exoS96::Tn5 allele that confers increased ExoS activity is present (43,46). Because Ptrp-syrA suppressed known ChvI K214T phenotypes, we asked whether it would allow conjugation of Ptrp-exoR into a wild-type strain.…”

“…As discussed above, Ptrp-syrA and exoR95 and exoS96 strains share two key phenotypes: nonmotility and EPS-I overproduction. The exoR95:: Tn5 (and some exoS alleles) suppress phenotypes of the chvI K214T mutant, presumably by increasing ChvI activity and expression of ChvI target genes (41)(42)(43)46). We tested a Ptrp::syrA chvI K214T strain to see if overexpressing syrA would suppress any of the chvI K214T phenotypes listed above.…”

“…exoR encodes a small periplasmic protein that suppresses ExoS activity, resulting in negative modulation of ChvI ( Fig. 1) (40,43,(46)(47)(48). ExoR autoregulates its own expression through ExoS-ChvI and may be subjected to regulatory proteolysis (47,48).…”

Genetic analysis of signal integration by the Sinorhizobium meliloti sensor kinase FeuQ

“…These auxiliary proteins have been identified in all cellular compartments, and their mechanisms of action vary from one system to another (Buelow & Raivio, 2010). Auxiliary proteins have been shown to link multiple TCSs to one another, to interrupt stimulus detection by the SK, or to modulate SK autophosphorylation, phosphotransfer or phosphatase activities (Chen et al, 2008;Garnerone et al, 1999;Gerken et al, 2009;Jeong et al, 2012;Mitrophanov & Groisman, 2008;Neiditch et al, 2006;Pioszak & Ninfa, 2003a;Szurmant et al, 2008).…”

Function of the N-terminal region of the phosphate-sensing histidine kinase, SphS, in Synechocystis sp. PCC 6803

“…Since many response regulators affect transcription, this provides a way in which the twocomponent system can modulate physiological responses to intra-and extracellular changes. The regulatory pathway is further complicated by the discovery of a third component modulating the activity of specific twocomponent systems (Chen et al, 2008;Salinas et al, 2007). The P i -responsive two-component system is present in a wide variety of bacteria.…”