Buendia A M, Enenkel B, Köplin R, Niehaus K, Arnold W, Pühler A
Lehrstuhl für Genetik, Universität Bielefeld, Germany.
Mol Microbiol. 1991 Jun;5(6):1519-30. doi: 10.1111/j.1365-2958.1991.tb00799.x.
A 2.6 kb ClaI-BamHI DNA fragment of megaplasmid 2 of Rhizobium meliloti 2011 was found to carry genes involved in exopolysaccharide synthesis and infection of alfalfa nodules. The analysis of the nucleotide sequence of this DNA fragment revealed the existence of two open reading frames (ORFs) running in opposite directions. Plasmid integration mutagenesis showed that these ORFs are organized as two monocistronic transcription units. One of the ORFs represents a new exo gene designated exoZ, which is involved in, but not essential for, the production of acidic exopolysaccharide. However, exoZ is not necessary for nodule formation with alfalfa. The ExoZ protein was found to show homology (23.3%) to the NodX protein of the R. leguminosarum biovar viciae strain TOM, known to be essential for nodulating the primitive Afghanistan pea. The second identified ORF corresponds to the exoB locus. The deduced amino acid sequence of the ExoB protein is homologous (39.6%) to that of the Escherichia coli GalE protein. In R. meliloti, exoB codes for a UDP-glucose 4-epimerase. A deficiency in the activity of this enzyme fully accounts for all the multiple carbohydrate defects that have been observed in exoB mutants.
苜蓿中华根瘤菌2011的大质粒2上一个2.6 kb的ClaI - BamHI DNA片段被发现携带参与胞外多糖合成和苜蓿根瘤侵染的基因。对该DNA片段核苷酸序列的分析揭示存在两个方向相反的开放阅读框（ORF）。质粒整合诱变表明这些ORF被组织成两个单顺反子转录单元。其中一个ORF代表一个新的外切基因，命名为exoZ，它参与酸性胞外多糖的产生，但并非必需。然而，exoZ对于与苜蓿形成根瘤并非必需。发现ExoZ蛋白与豌豆根瘤菌生物变种野豌豆菌株TOM的NodX蛋白具有同源性（23.3%），已知该蛋白对于使原始阿富汗豌豆结瘤至关重要。第二个鉴定出的ORF对应于exoB位点。推导的ExoB蛋白氨基酸序列与大肠杆菌GalE蛋白的序列具有同源性（39.6%）。在苜蓿中华根瘤菌中，exoB编码一种UDP - 葡萄糖4 - 差向异构酶。该酶活性的缺陷完全解释了在exoB突变体中观察到的所有多种碳水化合物缺陷。
