The Sinorhizobium meliloti ExoR protein is required for the downregulation of lpsS transcription and succinoglycan biosynthesis in response to divalent cations.

The Sinorhizobium meliloti lpsS gene encodes a sulfotransferase that modifies lipopolysaccharide. Mutants lacking lpsS display no defect in lipopolysaccharide sulfation when assayed under laboratory conditions, but exhibit an abnormal symbiosis with alfalfa. These results suggest that lpsS is transcriptionally repressed under laboratory conditions, but upregulated during symbiosis. Here, it is shown that lpsS, as well as exo genes required for the biosynthesis of succinoglycan, are transcriptionally repressed in laboratory media containing divalent cations. Furthermore, the divalent cation-dependent transcriptional downregulation of lpsS is dependent on the exoR gene, which encodes a global regulator of transcription.