A mildly acid-sensitive mutant of bv. WSM710 (WR6-35) produced colonies which were more mucoid in phenotype than the wild-type. Strain WR6-35 contained a single copy of Tn5 and the observed mucoid phenotype, acid sensitivity and Tn5-induced kanamycin resistance were 100% co-transducible using phage RL38. WR6-35 produced threefold more exopolysaccharide (EPS) than the wild-type in minimal medium devoid of a nitrogen source. EPS produced by the mutant and the wild-type was identical as determined by proton NMR spectra. An rhizobial fragment containing Tn5 and flanking rhizobial sequences was cloned from the mutant, restriction mapped and sequenced. There was extensive similarity between the ORF disrupted by Tn5 in bv. WR6-35 and the gene of () Rm1021 (71-3% identity over 892 bp). At the protein level there was 70% identity and 93-3% similarity over 267 amino acids with the ExoR protein of Rm1021. Hydrophilicity profiles of the two proteins from these two rhizobia are superimposable. This gene in bv. was thus designated The data suggest that Tn5 has disrupted a regulatory gene encoding a protein that negatively modulates EPS biosynthesis in bv. WSM710. Despite earlier suggestions that EPS production and acid tolerance might be positively correlated, disruption of in either bv. or and its associated overproduction of EPS does not result in a more acid-tolerant phenotype than the wild-type when cultures are screened on conventional laboratory agar.


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