Characterization of genes for synthesis and catabolism of a new rhizopine induced in nodules by Rhizobium meliloti Rm220-3: extension of the rhizopine concept

Abstract

Rhizopines are selective growth substrates synthesized in nodules only by strains of rhizobia capable of their catabolism. We report the isolation and study of genes for the synthesis and catabolism of a new rhizopine, scyllo-inosamine (sla), from alfalfa nodules induced by Rhizobium melilot Rm220-3. This compound is similar in structure to the previously described rhizopine 3-0-methyl-scyflo-inosamine from R. meliloh L5-30 (P. J. Murphy, N. Heycke, Z. Banfalvi, M. E. Tate, F. J. de Bruoin, A. Kondorosi, J. Tempi, and J. Schell, Proc.Natl. Acad. Sci. USA 84: [493][494][495][496][497] 1987). The synthesis (mos) and catabolism (moc) genes for the Rm220-3 rhizopine are closely linked and located on the nod-nif Sym plasmid. The mos genes are directly controlled by the NifA/NtrA regulatory system. A comparison of the sequence of the 5' regions of the two mos loci shows very extensive conservation of sequence as well as strong homology to the niJH coding region. Restriction mapping and hybridization to DNA from the four open reading frames (ORFs) of the L5-30 mos locus indicate the absence of mosA and presence of the other three ORFs (ORF1 and mosB and -C) in Rm220-3. We suggest that the L5-30 mosA gene product is involved in the conversion ofscyflo-inosamine to 3-O-methyl-scyllo-inosamine. Restriction fragment length polymorphism analysis of the moc regions of both strains shows that they are very similar. Regulation studies indicate that the moc region is not controlled by the common regulatory genes njfA, ntrA, and ntrC. We discuss the striking similarities in gene structure, location, and regulation between these two rhizopine loci in relation to the rhizopine concept.Rhizobia form symbiotic associations with leguminous plants which result in the conversion of atmospheric nitrogen into a form which the plant can utilize. The rhizobia are thought to benefit from the interaction as they are provided with nutrients (derived from plant photosynthates) and a temporary shelter from the soil environment (the nodule). However, for much of the time rhizobia survive as saprophytic organisms in the soil or the rhizosphere in competition with other microorganisms. Many factors, such as the availability of nutrients from plant root exudates, determine which rhizobia survive and eventually predominate in the rhizosphere (5,19).A number of examples of plant-associated products increasing rhizobial growth rate have been described. These include compounds produced by host plants (for example, trigonelline [6,10], which is catabolized by a variety of rhizobia) and certain flavonoids which increase rhizobial growth (26). Other compounds such as calystegins are produced by nonhost plants and are catabolized by only a limited number of rhizobia as well as a variety of other soil microorganisms (58, 59). A more selective compound found in the exudate of pea roots is L-homoserine, an amino acid catabolized by pea-nodulating Rhizobium leguminosarum bv. viciae strains but by few other rhizobia (62).In a number of cases, the locations o...

Evolutionary dynamics of rhizopine within spatially structured rhizobium populations

“…This simpli¢cation is in part justi¢ed because the synthesis (mos) and catabolism (moc) genes are closely linked on the symbiotic plasmid (Murphy et al 1988) and appear to be inherited together (Wexler et al 1996), suggesting that synthesis and catabolism evolve as a single functional unit (Murphy et al 1988;Saint et al 1993). This simpli¢cation also assumes no independent mutation of the two loci.…”

“…Two kinds of rhizopine have been identi¢ed: scylloinosamine (SI or sIa) (Saint et al 1993) and L-3-Omethyl-scyllo-inosamine (3-O-MSI) (Tempe¨et al 1982;Murphy et al 1987). Both compounds consist of a sixcarbon ring decorated with a nitrogen-containing amine group, and can be distinguished by the presence of a methyl group in the latter compound.…”

“…Genes responsible for rhizopine catabolism are expressed in free-living rhizobia (Murphy et al 1987). These two sets of genes are linked on the symbiotic plasmid, so that rhizopine produced by bacteroids in a nodule can be catabolized by reproductively capable cells of the same strain (Tempe¨& Petit 1983;Murphy et al 1988;Saint et al 1993). Non-rhizopine rhizobium and other soil bacteria apparently cannot catabolize rhizopine (Rossbach et al 1995).…”

“…Inositol rhizopines are nitrogen-and carboncontaining compounds produced from plant precursors by bacteroids in nodules and catabolized by reproductive rhizobia (Tempe¨et al 1982;Murphy et al 1987Murphy et al , 1995Saint et al 1993;Rao et al 1995). Rhizopine synthesis is directly regulated by the symbiotic nitrogen-¢xation regulatory gene nifA, so that conditions in the nodule that initiate nitrogen ¢xation also initiate rhizopine synthesis (Murphy et al 1988).…”

Detection of and Response to Signals Involved in Host-Microbe Interactions by Plant-Associated Bacteria

“…Genes involved in rhizopine synthesis (mos genes) and those involved in rhizopine catabolism (moc genes) are closely linked and in S. meliloti are located on the symbiotic megaplasmid pSymA, which also harbors the nitrogen fixation genes (344,435). Since rhizopines are produced in bacteroids, they are most probably synthesized from plant-derived precursors.…”