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. P roduction of EPS in R. meliloti is likely controlled at several levels . We have characterized a new gene of this regulatory circuit. syrA wa s 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 sequenc e similarity to two other EPS regulatory proteins: ExoX of Rhizobium N GR234 and R. meliloti, and Psi of R. leguminosarum by. phaseoli. The s yrA transcription start site lies 522 nucleotides upstream of a non-ca nonical TTG start codon. The syrA promoter region is similar to the pr omoter region of the nodulation regulatory protein, nodD3. We found th at in free-living bacteria, syrA ex activated by the regulatory locus, syrM, but not by nodD3. In planta, syrM is not required for expressio n of syrA. Instead, expression of the nitrogen fixation (nifHDKE) gene s upstream of syrA plays a role. Specific and distinct sets of genetic controls may operate at different times during nodule invasion.