Galbraith Mark P, Feng Szi Fei, Borneman James, Triplett Eric W, de Bruijn Frans J, Rossbachl Silvia
Department of Biological Sciences, Western, Michigan UniversityKalamazoo, MI 49008USA.
Department of Agronomy and Center for the Study of Nitrogen Fixation, University of Wisconsin- MadisonMadison, WI 53706USA.
Microbiology (Reading). 1998 Oct;144 ( Pt 10):2915-2924. doi: 10.1099/00221287-144-10-2915.
Rhizopine (L-3-O-methyl-scyllo-inosamine) is a symbiosis-specific compound found in alfalfa nodules induced by specific Sinorhizobium meliloti strains. It has been postulated that rhizobial strains able to synthesize and catabolize rhizopine gain a competitive advantage in the rhizosphere. The pathway of rhizopine degradation is analysed here. Since rhizopine is an inositol derivative, it was tested whether inositol catabolism is involved in rhizopine utilization. A genetic locus required for the catabolism of inositol as sole carbon source was cloned from 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. meliloti myo-inositol dehydrogenase. Strains that harboured a mutation in the myo-inositol dehydrogenase gene (idhA) did not display myo-inositol dehydrogenase activity, were unable to utilize myo-inositol as sole carbon/energy source, and were unable to catabolize rhizopine. Thus, myo-inositol dehydrogenase activity is essential for rhizopine utilization in S. meliloti.
根瘤碱（L-3-O-甲基-scyllo-肌醇胺）是在由特定苜蓿中华根瘤菌菌株诱导形成的苜蓿根瘤中发现的一种共生特异性化合物。据推测，能够合成和分解代谢根瘤碱的根瘤菌菌株在根际获得竞争优势。本文分析了根瘤碱的降解途径。由于根瘤碱是一种肌醇衍生物，因此测试了肌醇分解代谢是否参与根瘤碱的利用。从苜蓿中华根瘤菌中克隆了以肌醇作为唯一碳源进行分解代谢所需的一个基因位点。该基因位点通过转座子Tn5诱变进行定位，并测定了其DNA序列。基于DNA相似性研究和酶活性测定，该基因区域被证明编码一种苜蓿中华根瘤菌的肌醇脱氢酶。在肌醇脱氢酶基因（idhA）中发生突变的菌株不表现出肌醇脱氢酶活性，不能利用肌醇作为唯一的碳/能源，也不能分解代谢根瘤碱。因此，肌醇脱氢酶活性对于苜蓿中华根瘤菌利用根瘤碱至关重要。
