REGULATION OF EXOPOLYSACCHARIDE PRODUCTION IN RHIZOBIUM-LEGUMINOSARUMBIOVAR VICIAE WSM710 INVOLVES EXOR

A mildly acid-sensitive mutant of Rhizobium leguminosarum bv. viciae W SM710 (WR6-35) produced colonies which were more mucoid in phenotype t han the wild-type. Strain WR6-35 contained a single copy of Tn5 and th e observed mucoid phenotype, acid sensitivity and Tn5-induced kanamyci n resistance were 100% co-transducible using phage RL38. WR6-35 produc ed threefold more exopolysaccharide (EPS) than the wild-type in minima l medium devoid of a nitrogen source. EPS produced by the mutant and t he wild-type was identical as determined by proton NMR spectra. An Eco RI rhizobial fragment containing Tn5 and flanking rhizobial sequences was cloned from the mutant, restriction mapped and sequenced. There wa s extensive similarity between the ORF disrupted by Tn5 in R. legumino sarum by. viciae WR6-35 and the exoR gene of Rhizobium (Sinorhizobium) meliloti Rm1021 (71.3% identity over 892 bp). At the protein level th ere was 70% identity and 93.3% similarity over 267 amino acids with th e ExoR protein of R. meliloti Rm1021. Hydrophilicity profiles of the t wo proteins from these two rhizobia are superimposable. This gene in R . leguminosarum by. viciae was thus designated exoR. The data suggest that Tn5 has disrupted a regulatory gene encoding a protein that negat ively modulates EPS biosynthesis in R. leguminosarum by. viciae WSM710 . Despite earlier suggestions that EPS production and acid tolerance m ight be positively correlated, disruption of exoR in either R. legumin osarum by. viciae or R. meliloti and its associated overproduction of EPS does not result in a more acid-tolerant phenotype than the wild-ty pe when cultures are screened on conventional laboratory agar.