MOLECULAR AND GENETIC-CHARACTERIZATION OF THE RHIZOPINE CATABOLISM (MOCABRC) GENES OF RHIZOBIUM-MELILOTI L5-30

Rhizopine (L-3-O-methyl-scyllo-inosamine, 3-O-MSI) is a symbiosis-spec ific compound, which is synthesized in nitrogen-fixing nodules of Medi cago sativa induced by Rhizobium meliloti strain L5-30. 3-O-MSI is tho ught to function as an unusual growth substrate for R. meliloti L5-30, which carries a locus (mos) responsible for its synthesis closely lin ked to a locus (moc) responsible for its degradation. Here, the essent ial moc genes were delimited by Tn5 mutagenesis and shown to be organi zed into two regions, separated by 3 kb of DNA. The DNA sequence of a 9-kb fragment spanning the two moc regions was determined, and four ge nes were identified that play an essential role in rhizopine catabolis m (mocABC and mocR). The analysis of the DNA sequence and the amino ac id sequence of the deduced protein products revealed that MocA resembl es NADH-dependent dehydrogenases. MocB exhibits characteristic feature s of periplasmic-binding proteins that are components of high-affinity transport systems. MocC does not share significant homology with any protein in the database. MocR shows homology with the GntR class of ba cterial regulator proteins. These results suggest that the mocABC gene s are involved in the uptake and subsequent degradation of rhizopine, whereas mocR is likely to play a regulatory role.