Molecular Analysis of the Rhizobium meliloti mucR Gene Regulating the Biosynthesis of the Exopolysaccharides Succinoglycan and Galactoglucan. Mathias Keller. Lehrstuhl fur Genetik, Fakultat fur Biologie, Universitat Bielefeld, P.O. Box 100131, 33501 Bielefeld, Federal Republic of Germany. Andreas Roxlau, Wen M. Weng, Michael Schmidt, Jurgen Quandt, Karsten Niehaus, Doris Jording, Walter Arnold, and Alfred Punier, Lehrstuhl fur Genetik, Fakultat fur Biologie, Universitat Bielefeld, P.O. Box 100131, 33501 Bielefeld, Federal Republic of Germany. MPMI 8:267-277. Accepted 21 November 1994. Copyright 1995 The American Phytopathological Society.

The Rhizobium meliloti Tn5 mutant Rm3131, producing galactoglucan (EPS II) instead of succinoglycan (EPS I), was complemented by a 3.6-kb EcoRl-fragment of the Rhizobium meliloti genome. Sequencing of this fragment revealed six open reading frames (ORFs). The ORF found to be affected in the mutant Rm3131 codes for a putative protein of 15.7 kDa and forms a monocistronic transcrip-tional unit. Further genetic analysis revealed that the gene mutated in Rm3131 is identical to the previously described R. meliloti mucR gene (H. Zhan, S. B. Levery, C. C. Lee, and J. A. Leigh, 1989, Proc. Natl. Acad. Sci. USA 86:3055-3059). By hybridization it was shown that a mucR homologous gene is present in several rhizobacteria. The deduced amino acid sequence of MucR showed nearly 80% identity to the Agrobacterium tumefaciens Ros protein, a negative regulator of vir genes and necessary for succinoglycan production. MucR contains like Ros a putative zinc finger sequence of the C2H2 type. Transcrip-tional fusions of genes for EPS I and EPS II synthesis, the so-called exo and exp genes, with the marker gene lacZ were used to delineate the role of mucR for exo and exp gene expression. It was found that exp genes are negatively regulated by MucR on the transcriptional level, whereas a posttranscriptional regulation by MucR is assumed for exo genes. Furthermore, mucR is negatively regulating its own transcription.