Glazebrook J, Walker G C
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
Cell. 1989 Feb 24;56(4):661-72. doi: 10.1016/0092-8674(89)90588-6.
We have found that R. meliloti strain Rm1021, which is known to synthesize a Calcofluor-binding exopolysaccharide (EPS I), also has a cryptic capacity to synthesize a second exopolysaccharide (EPS II). Structural analysis of EPS II has shown that it differs in many respects from EPS I. Genetic analysis indicates that EPS II synthesis requires the products of at least seven loci on the second symbiotic megaplasmid of R. meliloti, and is induced by a mutation, expR101, which causes increased transcription of these genes. Synthesis of EPS II suppresses the symbiotic defects of EPS I-deficient strains on Medicago sativa (alfalfa), but not on four other plants that are normally hosts for Rm1021. These observations suggest that structural features of bacterial exopolysaccharides are involved in the determination of host range. The implications of these results for models of exopolysaccharide function, such as serving as signals to the plant or shielding the bacteria from plant defense responses, are discussed.
我们发现，已知能合成与荧光增白剂结合的胞外多糖（EPS I）的苜蓿中华根瘤菌菌株Rm1021，也具有合成第二种胞外多糖（EPS II）的潜在能力。对EPS II的结构分析表明，它在许多方面与EPS I不同。遗传分析表明，EPS II的合成需要苜蓿中华根瘤菌第二个共生大质粒上至少七个基因座的产物，并由一个突变体expR101诱导，该突变导致这些基因的转录增加。EPS II的合成抑制了EPS I缺陷菌株在紫花苜蓿上的共生缺陷，但对通常作为Rm1021宿主的其他四种植物没有作用。这些观察结果表明，细菌胞外多糖的结构特征参与了宿主范围的确定。本文还讨论了这些结果对胞外多糖功能模型的影响，例如作为向植物发出的信号或保护细菌免受植物防御反应的影响。
