Structural and functional conservation of the rhizopine catabolism (moc) locus is limited to selected Rhizobium meliloti strains and unrelated to their geographical origin.

Rhizopine (L-3-O-methyl-scyllo-inosamine; 3-O-MSI) synthesis (mos) and catabolism (moc) genes were originally isolated from Rhizobium meliloti 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 advantage to this strain in the rhizosphere, since the ability to utilize the unusual nutritional mediator rhizopine as nitrogen and carbon source appears to be correlated with the ability of Moc+ bacteria to efficiently infect alfalfa plants. This study examines the distribution of rhizopine catabolism (moc) genes among different soil bacteria. By using oligonucleotide primers homologous to the moc genes and the polymerase chain reaction (PCR), moc genes were shown to be absent from a random collection of 100 different soil isolates. However, screening 50 different electrophoretic type strains of a worldwide R. meliloti collection (Eardly et al., Appl. Environ. Microbiol. 56:187, 1990) revealed the presence of moc genes in three additional strains, S33, 102F51, and 74B3. These three strains were found to be able to synthesize rhizopine in planta (Mos+) and to catabolize it (Moc+). To determine the relatedness of the Mos+/Moc+ strains to each other and to other R. meliloti strains, we used the rep-PCR method to generate genomic fingerprints, and to create a phylogenetic tree with the help of an optical imaging system and data analysis program (AMBIS). Because of the apparent infrequent occurrence of moc genes among soil bacteria, we suggest that the use of moc genes as a selectable marker trait for tracking genetically manipulated organisms is feasible.