Exopolysaccharide II production is regulated by salt in the halotolerant strain Rhizobium meliloti EFB1

The halotolerant strain Rhizobium meliloti EFB1 modifies the production of extracellular polysaccharides in response to salt. EFB1 colonies grown in the presence of 0.3 M NaCl show a decrease in mucoidy, and in salt- supplemented liquid medium this organism produces 40% less exopolysaccharides. We isolated transposon-induced mutant that, when grown in the absence of salt, had a colony morphology (nonmucoid) similar to the colony morphology of the wild type grown in the presence of salt. Calcofluor fluorescence, proton nuclear magnetic resonance spectroscopy, and genetic analysis of the mutant indicated that galactoglucan, which is not produced under normal conditions by other R. meliloti strains, is produced by strain EFB1 and that production of this compound decreases when the organism is grown in the presence of salt. The mutant was found to be affected in a genetic region highly homologous to genes for galactoglucan production in R. meliloti Rm2011 (expE genes). However, sequence divergence occurs in a putative expE promoter region. A transcriptional fusion of the promoter with lacZ demonstrated that, unlike R. meliloti Rm2011, galactoglucan is produced constitutively by EFB1 and that its expression is reduced 10-fold during exponential growth in the presence of salt.