ExoR is genetically coupled to the ExoS–ChvI two‐component system and located in the periplasm of Sinorhizobium meliloti

Summary Sinorhizobium meliloti enters into a symbiotic relationship with legume host plants, providing fixed nitrogen in exchange for carbon and amino acids. In S. meliloti , exoR and the exoS–chvI two‐component system regulate the biosynthesis of succinoglycan, an exopolysaccharide important for host invasion. It was previously reported that a loss‐of‐function mutation in exoR and a gain‐of‐function mutation in exoS cause overproduction of succinoglycan and loss of motility, indicating that ExoR negatively regulates and ExoS–ChvI positively regulates downstream genes. However, a relationship between exoR and exoS–chvI has never been clearly established. By identification and detailed characterization of suppressor strains, we provide genetic evidence that exoR and exoS–chvI control many similar phenotypes. These include succinoglycan production, symbiosis, motility, and previously uncharacterized prototrophy and biofilm formation, all of which are co‐ordinately restored by suppressors. We further demonstrate that ExoR is located in the periplasm, suggesting that it functions to regulate downstream genes in a novel manner. In pathogenic bacteria closely related to S. meliloti , exoS–chvI homologues are required for virulence and the regulation of cell envelope composition. Our data suggest that periplasmically localized ExoR and ExoS–ChvI function together in a unique and critical regulatory system associated with both free‐living and symbiotic states of S. meliloti .