A FUNCTIONAL MYOINOSITOL CATABOLISM PATHWAY IS ESSENTIAL FOR RHIZOPINE UTILIZATION BY SINORHIZOBIUM-MELILOTI

Rhizopine (L-3-O-methyl-scyllo-inosamine) is a symbiosis-specific comp ound found in alfalfa nodules induced by specific Sinorhizobium melilo ti strains. It has been postulated that rhizobial strains able to synt hesize and catabolize rhizopine gain a competitive advantage in the rh izosphere. The pathway of rhizopine degradation is analysed here. Sinc e rhizopine is an inositol derivative, it was tested whether inositol catabolism is involved in rhizopine utilization. A genetic locus requi red for the catabolism of inositol as sole carbon source was cloned fr om S. meliloti. This locus was delimited by transposon Tn5 mutagenesis and its DNA sequence was determined. Based on DNA similarity studies and enzyme assays, this genetic region was shown to encode an S. melil oti myo-inositol dehydrogenase. Strains that harboured a mutation in t he myo-inositol dehydrogenase gene (idhA) did not display myoinositol dehydrogenase activity, were unable to utilize myo-inositol as sole ca rbon/energy source, and were unable to catabolize rhizopine. Thus, myo inositol dehydrogenase activity is essential for rhizopine utilization in S. meliloti.