ExoR is genetically coupled to the ExoS-ChvI two-component system and located in the periplasm of Sinorhizobium meliloti
- PMID: 17462014
- DOI: 10.1111/j.1365-2958.2007.05680.x
ExoR is genetically coupled to the ExoS-ChvI two-component system and located in the periplasm of Sinorhizobium meliloti
Abstract
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.
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