IDENTIFICATION OF GENES IN RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII WHOSE PRODUCTS ARE HOMOLOGS TO A FAMILY OF ATP-BINDING PROTEINS

The specific interaction between rhizobia and their hosts requires man y genes that influence both early and late steps in symbiosis. Three n ew genes, designated prsD, prsE (protein secretion) and orf3, were ide ntified adjacent to the exo133 mutation in a cosmid carrying the genom ic DNA of Rhizobium leguminosarum by, trifolii TA1. The prsDE genes sh are significant homology to the genes encoding ABC transporter protein s PrtDE from Erwinia chrysanthemi and AprDE from Pseudomonas aeruginos a which export the proteases in these bacteria. PrsD shows at least fi ve potential transmembrane hydrophobic regions and a large hydrophilic domain containing an ATP/GTP binding cassette. PrsE has only one pote ntial transmembrane hydrophobic domain in the N-terminal part and is p roposed to function as an accessory factor in the transport system. OR F3, like PrtF and AprF, has a typical N-terminal signal sequence but h as no homology to these proteins. The insertion of a kanamycin resista nce cassette into the prsD gene of the R. leguminosarum by. trifolii T A1 wild-type strain created a mutant which produced a normal amount of exopolysaccharide but was not effective in the nodulation of clover p lants.