Battisti L, Lara J C, Leigh J A
Department of Microbiology, University of Washington, Seattle 98195.
Proc Natl Acad Sci U S A. 1992 Jun 15;89(12):5625-9. doi: 10.1073/pnas.89.12.5625.
Rhizobium meliloti strain SU47 produces both high molecular weight (HMW) and low molecular weight (LMW) forms of an acidic exopolysaccharide, succinoglycan. Genetic studies have shown that succinoglycan is required for alfalfa root nodule invasion. We found that LMW succinoglycan, when applied exogenously to alfalfa roots, restored nodule invasion to exoA, exoB, exoF, and exoH mutants. Nodule initiation signals were not involved, since LMW succinoglycan from R. meliloti nodD1D2D3 and nodA mutants and from luteolin-induced wild-type cultures elicited effects similar to LMW succinoglycan from the uninduced wild-type strain. In contrast, LMW fractions from an exoA mutant, nonsuccinylated LMW succinoglycan, and HMW succinoglycan did not promote invasion, nor did LMW exopolysaccharides from R. leguminosarum bv. trifolii and Rhizobium sp. strain NGR234. LMW succinoglycan could be separated by anion-exchange chromatography into several distinct subfractions differing in repeating subunit multiplicities (monomer, trimer, and tetramer) and charge. When tested singly, only the most charged, tetrameric form was active. These results show that a specific oligosaccharide form of succinoglycan promotes nodule invasion in alfalfa. The implications for the mode of action of succinoglycan are discussed.
苜蓿中华根瘤菌菌株SU47能产生高分子量（HMW）和低分子量（LMW）两种形式的酸性胞外多糖——琥珀聚糖。遗传学研究表明，苜蓿根瘤侵染需要琥珀聚糖。我们发现，将LMW琥珀聚糖外源施加到苜蓿根上时，能恢复exoA、exoB、exoF和exoH突变体的根瘤侵染能力。根瘤起始信号不参与其中，因为来自苜蓿中华根瘤菌nodD1D2D3和nodA突变体以及木犀草素诱导的野生型培养物中的LMW琥珀聚糖所产生的效应，与未诱导的野生型菌株产生的LMW琥珀聚糖类似。相比之下，exoA突变体的LMW级分、非琥珀酰化的LMW琥珀聚糖和HMW琥珀聚糖均不能促进侵染，豆科根瘤菌三叶草生物型和根瘤菌属菌株NGR234的LMW胞外多糖也不能促进侵染。LMW琥珀聚糖可通过阴离子交换色谱法分离成几个不同的亚级分，这些亚级分在重复亚基多重性（单体、三聚体和四聚体）和电荷方面存在差异。单独测试时，只有电荷最多的四聚体形式具有活性。这些结果表明，琥珀聚糖的一种特定寡糖形式能促进苜蓿根瘤的侵染。本文还讨论了琥珀聚糖作用模式的相关意义。
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