THE RKPG, RKPH, RKPI AND RKPJ GENES ARE INVOLVED IN CAPSULAR POLYSACCHARIDE PRODUCTION BY RHIZOBIUM-MELILOTI

The first complementation unit of the fix-23 region of Rhizobium melil oti, which comprises six genes (rkpAB-CDEF) exhibiting similarity to f atty acid synthase genes, is required for the production of a novel ty pe of capsular polysaccharide that is involved in root nodule developm ent and structurally analogous to group II K antigens found in Escheri chia coli (G. Petrovics, P. Putnoky, R. Reuhs, J. Kim, T. A, Thorp, K, D. Noel, R. W. Carlson, and A. Kondorosi, Mel. Microbiol. 8:1083-1094 , 1993; B. L. Reuhs, R. W. Carlson, and J. S. Kim, J. Bacteriol. 175:3 570-3580, 1993), Here we present the nucleotide sequence for the other three complementation units of the fix-23 locus, revealing the presen ce of four additional open reading frames assigned to genes rkpGHI and -J. The putative RkpG protein shares similarity with acyltransferases , RkpH is homologous to short-chain alcohol dehydrogenases, and RkpJ s hows significant sequence identity with bacterial polysaccharide trans port proteins, such as KpsS of E. coil, No significant homology was fo und for RkpI. Biochemical and immunological analysis of Tn5 derivative s for each gene demonstrated partial or complete loss of capsular poly saccharides from the cell surface; on this basis, we suggest that all genes in the fix-23 region are required for K-antigen synthesis or tra nsport.