THE RHIZOBIUM-MELILOTI RHIZOPINE MOS LOCUS IS A MOSAIC STRUCTURE FACILITATING ITS SYMBIOTIC REGULATION

The Rhizobium meliloti L5-30 mos locus, encoding biosynthesis of the r hizopine 3-O-methyl-scyllo-inosamine, is shown to be a mosaic structur e. The mos locus consists of four open reading frames (ORFs) (ORF1 and mosABC) arranged in an operon structure. Within this locus, several d omains of homology with other prokaryotic symbiotic genes (nifH,fixA,f ixU, and nifT) are present, suggesting that this locus may represent a hot spot for rearrangement of symbiotic genes. Unusually, these domai ns are present in the coding as well as noncoding regions of the mos l ocus. Proteins corresponding to those encoded by mosABC, but not ORF1, have been detected in nodule extracts by using antibodies. As ORF1 sh ows extensive homology with the 5' region of the nifH gene (P. J. Murp hy, N. Heycke, S. P. Trenz, P. Ratet, F. J. de Bruijn, and J. Schell, Proc. Natl. Acad. Sci. USA 85:9133-9137, 1988) and a frameshift mutati on indicates that expression of this ORF is not required for mos activ ity, we propose that the mos locus has acquired a duplicated copy of n ifH, including the promoter region, in order to become symbiotically r egulated. Surprisingly, since the functions are likely different, MosA has an amino acid sequence similar to that of the DapA protein of Esc herichia coli. The central domain of MosB has extensive homology with a range of diverse proteins involved with carbohydrate metabolism in e ither antibiotic or outer-cell-wall biosynthesis. This region is also common to the regulatory proteins DegT and Dnrj, suggesting a regulato ry role for MosB. The structure of MosC is consistent with its being a membrane transport protein.