B i osyn t hes i s of the exopo I ysacc ha r i de galactoglucan in Sinorhizobiurn rneliloti is subject to a complex control by the phosphate = dependent regulator PhoB and the proteins

Lehrstuhl fur Genetik, Fakultat fur Biologie, Universitdt Bielefeld, Postfach 100131, D-33501 Bielefeld, Germany The soil bacterium Sinorhizobium meliloti (Rhizobium meliloti) has the ability to produce the alternative exopolysaccharide galactoglucan (EPS II) in addition to succinoglycan (EPS I). In the wild-type strain EPS II production is induced by phosphate-limiting conditions or by extra copies of the exp gene cluster. Based on similarities to transcriptional regulators of the MarR family, an additional putative regulatory gene, expG, was identified in the exp gene cluster. Using exp-lacZ transcriptional fusions, a stimulating effect of extra copies of this expG gene on the transcription of al l exp complementation groups was determined. Phosphate limitation also resulted in increased expression of the exp-lac2 fusions. This increase was reduced in strains characterized by a deletion of expG. The previously reported high level of exp gene transcription in a mud? mutant was further elevated under phosphate-limiting conditions. The expA, ex@, expG and expE promoters contain sequences with similarities to the PHO box known as the PhoB-binding site in phosphate-regulated promoters in Escherichia coli. The 5. meliloti phoB gene was required for the activation of exp gene expression under phosphate limitation, but not for induction of exp expression by MucR or ExpG.