Zhan H J, Levery S B, Lee C C, Leigh J A
Department of Microbiology, University of Washington, Seattle 98195.
Proc Natl Acad Sci U S A. 1989 May;86(9):3055-9. doi: 10.1073/pnas.86.9.3055.
Rhizobium meliloti strain SU47 produces the calcofluor-binding exopolysaccharide, succinoglycan, that is required for alfalfa root nodule invasion. Strains derived from R. meliloti SU47 secreted an acidic exopolysaccharide, EPSb, that replaced succinoglycan in nodule invasion. EPSb, which has not formerly been identified among the Rhizobiaceae, consisted of the repeating unit 4,6-O-(1-carboxyethylidene)-alpha-D-Galp1----3(X-O-Ac)-beta-D-G lcp1----3. EPSb synthesis occurred either in strains containing a mutation in a locus designated mucR or in strains with a recombinant cosmid pMuc. mucR mapped slightly counterclockwise from pyr49 on the chromosome, while pMuc contained genes mapping to the megaplasmid pRmeSU47b. In exoA, -F, and -H mutants, which are deficient in normal succinoglycan secretion and nodule invasion, a transposon Tn5 insertion in mucR or the presence of pMuc resulted in EPSb secretion and a restoration of nodule invasion on Medicago sativa and Melilotus alba. Mutants in exoB and exoC were incapable of succinoglycan and EPSb secretion as well as nodule invasion. A mutant that secreted succinoglycan but was incapable of EPSb secretion invaded nodules normally.
苜蓿中华根瘤菌菌株SU47产生与苜蓿根瘤侵染所需的能结合荧光增白剂的胞外多糖琥珀聚糖。源自苜蓿中华根瘤菌SU47的菌株分泌一种酸性胞外多糖EPSb，它在根瘤侵染中取代了琥珀聚糖。EPSb以前在根瘤菌科中未被鉴定出来，其由重复单元4,6-O-(1-羧基亚乙基)-α-D-吡喃半乳糖1----3(X-O-乙酰基)-β-D-吡喃葡萄糖1----3组成。EPSb的合成发生在含有一个名为mucR的位点发生突变的菌株中，或者发生在含有重组粘粒pMuc的菌株中。mucR在染色体上从pyr49稍微逆时针方向定位，而pMuc包含定位到巨型质粒pRmeSU47b上的基因。在正常琥珀聚糖分泌和根瘤侵染缺陷的exoA、-F和-H突变体中，mucR中的转座子Tn5插入或pMuc的存在导致EPSb分泌，并恢复了对紫花苜蓿和白花草木樨的根瘤侵染。exoB和exoC突变体无法分泌琥珀聚糖和EPSb以及进行根瘤侵染。一个分泌琥珀聚糖但无法分泌EPSb的突变体正常侵染根瘤。
