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The rkp-3 Gene Region of Sinorhizobium meliloti Rm41 Contains Strain-Specific Genes that Determine K Antigen Structure

¹Institute of Genetics, Biological Research Center, Hungarian Academy of Sciences, P.O. Box 521, H-6701 Szeged, Hungary; ²Complex Carbohydrate Research Center, University of Georgia, Athens 30602, U.S.A.; ³Institut des Sciences Végétales, CNRS, Avenue de la Terrasse, F-91198 Gif-sur-Yvette Cedex, France; ⁴Department of Genetics and Molecular Biology, University of Pécs, P.O. Box 266, H-7601 Pécs, Hungary

The rkp-3 region is indispensable for capsular polysaccharide (K antigen) synthesis in Sinorhizobium meliloti Rm41. Strain Rm41 produces a K antigen of strain-specific structure, designated as the K_R5 antigen. The data in this report show that the rkp-3 gene region comprises 10 open reading frames involved in bacterial polysaccharide synthesis and export. The predicted amino acid sequences for the rkpL-Q gene products are homologous to enzymes involved in the production of specific sugar moieties, while the putative products of the rkpRST genes show a high degree of similarity to proteins required for transporting polysaccharides to the cell surface. Southern analysis experiments using gene-specific probes suggest that genes involved in the synthesis of the precursor sugars are unique in strain Rm41, whereas sequences coding for export proteins are widely distributed among Sinorhizobium species. Mutations in the rkpL-Q genes result in a modified K antigen pattern and impaired symbiotic capabilities. On this basis, we suggest that these genes are required for the production of the K_R5 antigen that is necessary for S. meliloti Rm41 exoB (AK631)-alfalfa (Medicago sativa) symbiosis.

Additional keywords: pseudaminic acid.