EXTENSION OF THE RHIZOBIUM-MELILOTI SUCCINOGLYCAN BIOSYNTHESIS GENE-CLUSTER - IDENTIFICATION OF THE EXSA GENE ENCODING AN ABC TRANSPORTER PROTEIN, AND THE EXSB GENE WHICH PROBABLY CODES FOR A REGULATOR OF SUCCINOGLYCAN BIOSYNTHESIS

Two new genes, designated exsA and exsB, were identified adjacent to t he 24 kb exo gene cluster of Rhizobium meliloti, which is involved in succinoglycan (EPS I) biosynthesis. The derived amino acid sequence of ExsA displayed significant homologies to ATP binding cassette (ABC) t ransporter proteins. R. meliloti strains mutated in exsA were characte rized by a decreased ratio of HMW to LMW EPS I, indicating a function for ExsA in EPS I biosynthesis. The R. meliloti NdvA protein, which is involved in the transport of cyclic beta-(1,2)-glucans, was identifie d as the closest homologue of ExsA. R. meliloti exsB mutants produced a three-fold increased amount of EPS I in comparison to the wild-type strain. In contrast, high copy number of exsB resulted in a decrease i n the EPS I level to 20% of wild type, indicating that the exsB gene p roduct can negatively influence EPS I biosynthesis. It was demonstrate d that this influence is not due to transcriptional regulation of the exo genes by the exsB gene product. By plasmid integration it was show n that exsA and exsB represent monocistronic transcription units.