Multiple genetic controls on Rhizobium meliloti syrA, a regulator of exopolysaccharide abundance

Exopolysaccharides (EPS) are produced by a wide assortment of bacteria including plant pathogens and rhizobial symbionts. Rhizobium meliloti mutants defective in EPS production fail to invade alfalfa nodules. Production of EPS in R. meliloti is likely controlled at several levels. We have characterized a new gene of this regulatory circuit. syrA was identified by its ability to confer mucoid colony morphology and by its ability to suppress the colonial phenotype of an exoD mutant. Here we show that syrA encodes a 9-kD hydrophobic protein that has sequence similarity to two other EPS regulatory proteins: ExoX of Rhizobium NGR234 and R. meliloti, and Psi of R. leguminosarum bv. phaseoli. The syrA transcription start site lies 522 nucleotides upstream of a non-canonical TTG start codon. The syrA promoter region is similar to the promoter region of the nodulation regulatory protein, nodD3. We found that in free-living bacteria, syrA expression is activated by the regulatory locus, syrM, but not by nodD3. In planta, syrM is not required for expression of syrA. Instead, expression of the nitrogen fixation (nifHDKE) genes upstream of syrA plays a role. Specific and distinct sets of genetic controls may operate at different times during nodule invasion.