EXOPOLYSACCHARIDE-II PRODUCTION IS REGULATED BY SALT IN THE HALOTOLERANT STRAIN RHIZOBIUM-MELILOTI EFB1

The halotolerant strain Rhizobium meliloti EFB1 modifies the productio n 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 exopo lysaccharides. 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 spectrosco py, and genetic analysis of the mutant indicated that galactoglucan, w hich is not produced under normal conditions by other R. meliloti stra ins, is produced by strain EFB1 and that production of this compound d ecreases when the organism is grown in the presence of salt. The mutan t was found to be affected in a genetic region highly homologous to ge nes for galactoglucan production in R. meliloti Rm2011 (expE genes), H owever, 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 b y EFB1 and that its expression is reduced 10-fold during exponential g rowth in the presence of salt.