González J E, Reuhs B L, Walker G C
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
Proc Natl Acad Sci U S A. 1996 Aug 6;93(16):8636-41. doi: 10.1073/pnas.93.16.8636.
Effective invasion of alfalfa by Rhizobium meliloti Rm1021 normally requires the presence of succinoglycan, an exopolysaccharide (EPS) produced by the bacterium. However, Rm1021 has the ability to produce a second EPS (EPS II) that can suppress the symbiotic defects of succinoglycan-deficient strains. EPS II is a polymer of modified glucose-(beta-1,3)-galactose subunits and is produced by Rm1021 derivatives carrying either an expR101 or mucR mutation. If the ability to synthesize succinoglycan is blocked genetically, expR101 derivatives of Rm1021 are nodulation-proficient, whereas mucR derivatives of Rm1021 are not. The difference in nodulation proficiency between these two classes of EPS II-producing strains is due to the specific production of a low molecular weight form of EPS II by expR101 strains. A low molecular weight EPS II fraction consisting of 15-20 EPS II disaccharide subunits efficiently allows nodule invasion by noninfective strains when present in amounts as low as 7 pmol per plant, suggesting that low molecular weight EPS II may act as a symbiotic signal during infection.
苜蓿中华根瘤菌Rm1021对苜蓿的有效侵染通常需要琥珀聚糖的存在，琥珀聚糖是该细菌产生的一种胞外多糖（EPS）。然而，Rm1021有能力产生第二种EPS（EPS II），它可以抑制琥珀聚糖缺陷型菌株的共生缺陷。EPS II是由修饰的葡萄糖-（β-1,3）-半乳糖亚基组成的聚合物，由携带expR101或mucR突变的Rm1021衍生物产生。如果通过基因手段阻断合成琥珀聚糖的能力，Rm1021的expR101衍生物能够结瘤，而Rm1021的mucR衍生物则不能。这两类产生EPS II的菌株在结瘤能力上的差异是由于expR101菌株特异性产生了低分子量形式的EPS II。当每株植物中低分子量EPS II组分以低至7皮摩尔的量存在时，由15 - 20个EPS II二糖亚基组成的该组分能够有效地使非感染性菌株侵入根瘤，这表明低分子量EPS II可能在感染过程中作为一种共生信号起作用。
