Campbell G R, Reuhs B L, Walker G C
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
J Bacteriol. 1998 Oct;180(20):5432-6. doi: 10.1128/JB.180.20.5432-5436.1998.
For Sinorhizobium meliloti (also known as Rhizobium meliloti) AK631 to establish effective symbiosis with alfalfa, it must be able to synthesize a symbiotically active form of its K antigen, a capsular polysaccharide containing a Kdo (3-deoxy-D-manno-octulosonic acid) derivative. Previously isolated mutants defective in the synthesis of K antigen are resistant to bacteriophage phi16-3. By screening ca. 100,000 Tn5-mutagenized R. meliloti bacteria for resistance to bacteriophage phi16-3, we isolated 119 mutants, 31 of which could not be complemented by genes previously identified as being required for K-antigen synthesis. Of these 31 new mutants, 13 were symbiotically defective and lacked the K antigen. Through genetic and phenotypic analyses, we have grouped these mutants into four distinct classes. Although all of these mutants lack the K antigen, many also have altered lipopolysaccharides (LPS), suggesting that the biochemical pathways for the synthesis of K antigen and LPS have common enzymatic steps. In addition, we have found that these and other classes of K-antigen-defective mutants of S. meliloti AK631 exhibit unique patterns of sensitivities to phage strains to which the parental strain was resistant. Our studies have identified new classes of genes required for both the synthesis of K antigen and the symbiotic proficiency of S. meliloti AK631. Some of these classes of genes also play a role in LPS synthesis.
为了使苜蓿中华根瘤菌（也称为苜蓿根瘤菌）AK631与苜蓿建立有效的共生关系，它必须能够合成其K抗原的共生活性形式，即一种含有Kdo（3-脱氧-D-甘露糖辛酸）衍生物的荚膜多糖。先前分离出的在K抗原合成方面有缺陷的突变体对噬菌体phi16-3具有抗性。通过筛选约100,000个经Tn5诱变的苜蓿根瘤菌细菌对噬菌体phi16-3的抗性，我们分离出了119个突变体，其中31个不能被先前鉴定为K抗原合成所需的基因互补。在这31个新突变体中，有13个在共生方面存在缺陷且缺乏K抗原。通过遗传和表型分析，我们将这些突变体分为四个不同的类别。尽管所有这些突变体都缺乏K抗原，但许多突变体的脂多糖（LPS）也发生了改变，这表明K抗原和LPS合成的生化途径有共同的酶促步骤。此外，我们发现苜蓿中华根瘤菌AK631的这些以及其他类别的K抗原缺陷突变体对亲本菌株具有抗性的噬菌体菌株表现出独特的敏感性模式。我们的研究确定了苜蓿中华根瘤菌AK631的K抗原合成和共生能力所需的新基因类别。其中一些基因类别在LPS合成中也发挥作用。
