Rhizobium meliloti exoG and exoJ mutations affect the exoX-exoY system for modulation of exopolysaccharide production.

R. meliloti Rm1021 normally produces an acidic Calcofluor-binding exopolysaccharide, called succinoglycan or EPS I, which is required for successful nodulation of alfalfa by this strain. At least 13 loci affecting production of EPS I were previously mapped to a cluster on the second of two symbiotic megaplasmids in Rm1021, pRmeSU47b. A putative regulatory region was originally defined by the exoG and exoJ mutations. exoG and exoJ mutants produced less exopolysaccharide than wild-type strains and induced nitrogen-fixing nodules on alfalfa with reduced efficiency compared with the wild type. These mutants appeared to produce only a low-molecular-weight form of EPS I. Mutations called exoX cause an increase in exopolysaccharide production and map in the same region as the exoG and exoJ mutations. The DNA sequence of this region reveals that it contains two open reading frames, called exoX and exoY, which have homologs in other Rhizobium species. Interestingly, the exoG insertion mutations fall in an intergenic region and may affect the expression of exoX or exoY. The exoJ mutation falls in the 3' portion of the exoX open reading frame and is probably an allele of exoX that results in altered function. exoG and exoJ mutations limit EPS I production in the presence of exoR95 or exoS96 mutations, which cause overproduction of EPS I. Gene regulation studies suggest that ExoX and ExoY constitute a system that modulates exopolysaccharide synthesis at a posttranslational level. The deduced sequence of ExoY is homologous to a protein required for an early step in xanthan gum biosynthesis, further suggesting that the modulatory system may affect the exopolysaccharide biosynthetic apparatus.