STRUCTURAL AND FUNCTIONAL CONSERVATION OF THE RHIZOPINE CATABOLISM (MOC) LOCUS IS LIMITED TO SELECTED RHIROBIUM-MELILOTI STRAINS AND UNRELATED TO THEIR GEOGRAPHICAL ORIGIN

Rhizopine (L -3-O-methyl-scyllo -inosamine; 3-O-MSI) synthesis (mos) a nd catabolism (moc) genes were originally isolated from Rhizobium meli loti strain L5-30 Murphy et al., Proc. Natl. Acad. Sci. U.S.A., 84:493 , 1987). These genes have been postulated to give a competitive advant age to this strain in the rhizosphere, since the ability to utilize th e unusual nutritional mediator rhizopine as nitrogen and carbon source appears to be correlated with the ability of Moc(+) bacteria to effic iently infect alfalfa plants, This study examines the distribution of rhizopine catabolism (moc) genes among different soil bacteria. By usi ng oligonucleotide primers homologous to the moc genes and the polymer ase chain reaction (PCR), moc genes were shown to be absent from a ran dom collection of 100 different soil isolates, However, screening 50 d ifferent electrophoretic type strains of a worldwide R. meliloti colle ction (Eardly et al., Appl. Environ, Microbiol, 56:187, 1990) revealed the presence of moc genes in three additional strains, S33, 102F51, a nd 74B3. These three strains were found to be able to synthesize rhizo pine in planta (Mos(+)) and to catabolize it (Moc(+)). To determine th e relatedness of the Mos(+)/Moc(+) strains to each other and to other R. meliloti strains, we used the rep-PCR method to generate genomic fi ngerprints, and to create a phylogenetic tree with the help of an opti cal imaging system and data analysis program (AMBIS), Because of the a pparent infrequent occurrence of moc genes among soil bacteria, we sug gest that the use of moc genes as a selectable marker trait for tracki ng genetically manipulated organisms is feasible.