Lloret J, Wulff B B, Rubio J M, Downie J A, Bonilla I, Rivilla R
Departamento de Biología, Universidad Autónoma de Madrid, Spain.
Appl Environ Microbiol. 1998 Mar;64(3):1024-8. doi: 10.1128/AEM.64.3.1024-1028.1998.
The halotolerant strain Rhizobium meliloti EFB1 modifies the production of extracellular polysaccharides in response to salt. EFB1 colonies grown in the presence of 0.3 M NaCl show a decrease in mucoidy, and in salt-supplemented liquid medium this organism produces 40% less exopolysaccharides. We isolated transposon-induced mutant that, when grown in the absence of salt, had a colony morphology (nonmucoid) similar to the colony morphology of the wild type grown in the presence of salt. Calcofluor fluorescence, proton nuclear magnetic resonance spectroscopy, and genetic analysis of the mutant indicated that galactoglucan, which is not produced under normal conditions by other R. meliloti strains, is produced by strain EFB1 and that production of this compound decreases when the organism is grown in the presence of salt. The mutant was found to be affected in a genetic region highly homologous to genes for galactoglucan production in R. meliloti Rm2011 (expE genes). However, sequence divergence occurs in a putative expE promoter region. A transcriptional fusion of the promoter with lacZ demonstrated that, unlike R. meliloti Rm2011, galactoglucan is produced constitutively by EFB1 and that its expression is reduced 10-fold during exponential growth in the presence of salt.
耐盐菌株苜蓿根瘤菌EFB1会根据盐浓度改变胞外多糖的产量。在0.3M NaCl存在的情况下生长的EFB1菌落，其黏液性降低，并且在添加了盐的液体培养基中，该菌株产生的胞外多糖减少了40%。我们分离出了转座子诱导突变体，该突变体在无盐条件下生长时，其菌落形态（非黏液状）与野生型在有盐条件下生长时的菌落形态相似。对该突变体进行的荧光增白剂荧光、质子核磁共振光谱分析和遗传分析表明，其他苜蓿根瘤菌菌株在正常条件下不产生的半乳葡聚糖，由EFB1菌株产生，并且当该菌株在有盐条件下生长时，这种化合物的产量会降低。发现该突变体在一个与苜蓿根瘤菌Rm2011中半乳葡聚糖产生基因（expE基因）高度同源的遗传区域受到影响。然而，在假定的expE启动子区域存在序列差异。启动子与lacZ的转录融合表明，与苜蓿根瘤菌Rm2011不同，EFB1组成型产生半乳葡聚糖，并且在有盐条件下指数生长期间其表达降低了10倍。
