Townsend Guy E, Forsberg Lennart S, Keating David H
Department of Microbiology and Immunology, Loyola University Chicago, Building 105, 2160 S. First Ave., Maywood, IL 60153, USA.
J Bacteriol. 2006 Dec;188(24):8560-72. doi: 10.1128/JB.01035-06. Epub 2006 Oct 6.
Leguminous plants and bacteria from the family Rhizobiaceae form a symbiotic relationship, which culminates in novel plant structures called root nodules. The indeterminate symbiosis that forms between Sinorhizobium meliloti and alfalfa requires biosynthesis of Nod factor, a beta-1,4-linked lipochitooligosaccharide that contains an essential 6-O-sulfate modification. S. meliloti also produces sulfated cell surface polysaccharides, such as lipopolysaccharide (LPS). The physiological function of sulfated cell surface polysaccharides is unclear, although mutants of S. meliloti with reduced LPS sulfation exhibit symbiotic abnormalities. Using a bioinformatic approach, we identified a homolog of the S. meliloti carbohydrate sulfotransferase, LpsS, in Mesorhizobium loti. M. loti participates in a determinate symbiosis with the legume Lotus japonicus. We showed that M. loti produces sulfated forms of LPS and capsular polysaccharide (KPS). To investigate the physiological function of sulfated polysaccharides in M. loti, we identified and disabled an M. loti homolog of the sulfate-activating genes, nodPQ, which resulted in undetectable amounts of sulfated cell surface polysaccharides and a cysteine auxotrophy. We concomitantly disabled an M. loti cysH homolog, which disrupted cysteine biosynthesis without reducing cell surface polysaccharide sulfation. Our experiments demonstrated that the nodPQ mutant, but not the cysH mutant, showed an altered KPS structure and a diminished ability to elicit nodules on its host legume, Lotus japonicus. Interestingly, the nodPQ mutant also exhibited a more rapid growth rate and appeared to outcompete wild-type M. loti for nodule colonization. These results suggest that sulfated cell surface polysaccharides are required for optimum nodule formation but limit growth rate and nodule colonization in M. loti.
豆科植物与根瘤菌科细菌形成共生关系，最终形成称为根瘤的新型植物结构。苜蓿中华根瘤菌与苜蓿之间形成的不定型共生关系需要合成结瘤因子，即一种β-1,4-连接的脂壳寡糖，其含有必不可少的6-O-硫酸化修饰。苜蓿中华根瘤菌还产生硫酸化的细胞表面多糖，如脂多糖（LPS）。尽管苜蓿中华根瘤菌中脂多糖硫酸化程度降低的突变体表现出共生异常，但硫酸化细胞表面多糖的生理功能尚不清楚。我们采用生物信息学方法，在百脉根中鉴定出苜蓿中华根瘤菌碳水化合物硫酸转移酶LpsS的一个同源物。百脉根与豆科植物日本百脉根参与一种定型共生关系。我们发现百脉根产生硫酸化形式的脂多糖和荚膜多糖（KPS）。为了研究硫酸化多糖在百脉根中的生理功能，我们鉴定并使硫酸激活基因nodPQ的一个百脉根同源物失活，这导致无法检测到硫酸化细胞表面多糖，并出现半胱氨酸营养缺陷。我们同时使百脉根cysH同源物失活，这破坏了半胱氨酸的生物合成，但没有降低细胞表面多糖的硫酸化程度。我们的实验表明，nodPQ突变体而非cysH突变体，其荚膜多糖结构发生改变，在其宿主豆科植物日本百脉根上引发根瘤的能力减弱。有趣的是，nodPQ突变体还表现出更快的生长速度，并且在根瘤定殖方面似乎比野生型百脉根更具竞争力。这些结果表明，硫酸化细胞表面多糖是最佳根瘤形成所必需的，但会限制百脉根的生长速度和根瘤定殖。
