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A Model for the Catabolism of Rhizopine in Rhizobium leguminosarum Involves a Ferredoxin Oxygenase Complex and the Inositol Degradative Pathway

November 1998 , Volume 11 , Number  11
Pages  1,057 - 1,068

Masoud Bahar , 1 John de Majnik , 1 Margaret Wexler , 1 Judith Fry , 2 Philip S. Poole , 2 and Peter J. Murphy 1

1Department of Crop Protection, University of Adelaide, Waite Campus, Glen Osmond, South Australia 5064, Australia; 2School of Animal and Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading, U.K.

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Accepted 15 July 1998.

Rhizopines are nodule-specific compounds that confer an intraspecies competitive nodulation advantage to strains that can catabolize them. The rhizopine (3-O-methyl-scyllo-inosamine, 3-O-MSI) catabolic moc gene cluster mocCABRDE(F) in Rhizobium leguminosarum bv. viciae strain 1a is located on the Sym plasmid. MocCABR are homologous to the mocCABR gene products from Sinorhizobium meliloti. MocD and MocE contain motifs corresponding to a TOL-like oxygenase and a [2Fe-2S] Rieske-like ferredoxin, respectively. The mocF gene encodes a ferredoxin reductase that would complete the oxygenase system, but is not essential for rhizopine catabolism. We propose a rhizopine catabolic model whereby MocB transports rhizopine into the cell and MocDE and MocF (or a similar protein elsewhere in the genome), under the regulation of MocR, act in concert to form a ferredoxin oxygenase system that demethylates 3-O-MSI to form scyllo-inosamine (SI). MocA, an NAD(H)-dependent dehydrogenase, and MocC continue the catabolic process. Compounds formed then enter the inositol catabolic pathway.

Additional keywords: nitrogen fixation, toluene.

© 1998 The American Phytopathological Society