THE SINORHIZOBIUM-MELILOTI MUCR PROTEIN, WHICH IS ESSENTIAL FOR THE PRODUCTION OF HIGH-MOLECULAR-WEIGHT SUCCINOGLYCAN EXOPOLYSACCHARIDE, BINDS TO SHORT DNA REGIONS UPSTREAM OF EXOH AND EXOY

Sinorhizobium meliloti (Rhizobium meliloti) is able to produce two dif ferent exopolysaccharides, succinoglycan and galactoglucan. Mutations in the mucR gene of S. meliloti result in the stimulation of galactogl ucan synthesis, while the type of succinoglycan produced is modified. In culture supernatants of a mucR mutant, low-molecular-weight succino glycan is present, whereas no high-molecular-weight succinoglycan coul d be detected. The biosynthesis of succinoglycan is directed by the pr oducts of the exo gene cluster. Two DNA fragments from this cluster, o ne located in front of the exoH gene and one in the intergenic region between the divergently transcribed genes exoX and exoY, were shown to represent effective binding sites for MucR. Whereas the latter bindin g site contains an inverted repeat motif, the former does not. However , the binding of MucR did not strongly modify the transcription of the exo genes involved. In the mucR mutant the expression levels of exoH- lacZ and exoX-lacZ transcriptional fusions were found to be increased 1.5- and 1.7-fold, respectively. On the other hand, the expression lev el of an exoY-lacZ transcriptional fusion was found to be 1.5-fold low er in the mucR mutant than in the wild-type background. Comparison of the DNA sequences of MucR-binding sites provides insight into the stru ctural requirements for binding of MucR.