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 different exopolysaccharides, succinoglycan and galactoglucan. Mutations in the mucR gene of S. meliloti result in the stimulation of galactoglucan synthesis, while the type of succinoglycan produced is modified. In culture supernatants of a mucR mutant, low-molecular-weight succinoglycan is present, whereas no high-molecular-weight succinoglycan could be detected. The biosynthesis of succinoglycan is directed by the products of the exo gene cluster. Two DNA fragments from this cluster, one 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 binding 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 level of an exoY-lacZ transcriptional fusion was found to be 1.5-fold lower in the mucR mutant than in the wild-type background. Comparison of the DNA sequences of MucR-binding sites provides insight into the structural requirements for binding of MucR.