Reuhs B L, Williams M N, Kim J S, Carlson R W, Côté F
Complex Carbohydrate Research Center, University of Georgia, Athens 30602, USA.
J Bacteriol. 1995 Aug;177(15):4289-96. doi: 10.1128/jb.177.15.4289-4296.1995.
The rhizobial production of extracellular polysaccharide (EPS) is generally required for the symbiotic infection of host plants that form nodules with an apical meristem (indeterminate nodules). One exception is Rhizobium meliloti AK631, an exoB mutant of Rm41, which is deficient in EPS production yet infects and fixes nitrogen (i.e., is Fix+) on alfalfa, an indeterminate nodule-forming plant. A mutation of lpsZ in AK631 results in a Fix- strain with altered phage sensitivity, suggesting that a cell surface factor may substitute for EPS in the alfalfa-AK631 symbiosis. Biochemical analyses of the cell-associated polysaccharides of AK631 and Rm5830 (AK631 lpsZ) demonstrated that the lpsZ mutation affected the expression of a surface polysaccharide that is analogous to the group II K polysaccharides of Escherichia coli; the polysaccharide contains 3-deoxy-D-manno-2-octulosonic acid or a derivative thereof in each repeating unit. Rm5830 produced a polysaccharide with altered chromatographic and electrophoretic properties, indicating a difference in the molecular weight range. Similar results were obtained in a study of Rm1021, a wild-type isolate that lacks the lpsZ gene: the introduction of lpsZ into Rm1021 exoB (Rm6903) both suppresses the Fix- phenotype and results in a modified expression of the K polysaccharide. Chromatography and electrophoresis analysis showed that the polysaccharide extracted from Rm6903 lpsZ+ differed from that of Rm6903 in molecular weight range. Importantly, the effect of LpsZ is not structurally specific, as the introduction lpsZ+ into Rhizobium fredii USDA257 also resulted in a molecular weight range change in the structurally distinct K polysaccharide produced by that strain. This evidence suggests that LpsZ has a general effect on the size-specific expression of rhizobial K polysaccharides.
根瘤菌产生胞外多糖（EPS）通常是与具有顶端分生组织的宿主植物（形成不定型根瘤）进行共生感染所必需的。一个例外是苜蓿根瘤菌AK631，它是Rm41的exoB突变体，缺乏EPS产生能力，但仍能感染苜蓿（一种形成不定型根瘤的植物）并固氮（即Fix+）。AK631中lpsZ的突变导致了一个Fix-菌株，其噬菌体敏感性发生改变，这表明在苜蓿 - AK631共生关系中，一种细胞表面因子可能替代了EPS。对AK631和Rm5830（AK631 lpsZ）的细胞相关多糖进行生化分析表明，lpsZ突变影响了一种表面多糖的表达，这种表面多糖类似于大肠杆菌的II型K多糖；该多糖在每个重复单元中都含有3 - 脱氧 - D - 甘露 - 2 - 辛酮糖酸或其衍生物。Rm5830产生的多糖具有改变的色谱和电泳性质，表明分子量范围存在差异。在对缺乏lpsZ基因的野生型分离株Rm1021的研究中也得到了类似结果：将lpsZ引入Rm1021 exoB（Rm6903）既能抑制Fix-表型，又能导致K多糖表达的改变。色谱和电泳分析表明，从Rm6903 lpsZ+中提取的多糖在分子量范围上与Rm6903不同。重要的是，LpsZ的作用在结构上并非特异性的，因为将lpsZ+引入费氏中华根瘤菌USDA257也导致了该菌株产生的结构不同的K多糖分子量范围发生变化。这一证据表明，LpsZ对根瘤菌K多糖的大小特异性表达具有普遍影响。
