Noel K D, Forsberg L S, Carlson R W
Department of Biology, Marquette University, Milwaukee, Wisconsin 53201-1881, USA.
J Bacteriol. 2000 Oct;182(19):5317-24. doi: 10.1128/JB.182.19.5317-5324.2000.
Judged by migration of its lipopolysaccharide (LPS) in gel electrophoresis, the O antigen of Rhizobium etli mutant strain CE166 was apparently of normal size. However, its LPS sugar composition and staining of the LPS bands after electrophoresis indicated that the proportion of its LPS molecules that possessed O antigen was only 40% of the wild-type value. Its LPS also differed from the wild type by lacking quinovosamine (2-amino-2,6-dideoxyglucose). Both of these defects were due to a single genetic locus carrying a Tn5 insertion. The deficiency in O-antigen amount, but not the absence of quinovosamine, was suppressed by transferring into this strain recombinant plasmids that shared a 7.8-kb stretch of the R. etli CE3 lps genetic region alpha, even though this suppressing DNA did not carry the genetic region mutated in strain CE166. Strain CE166 gave rise to pseudonodules on legume host Phaseolus vulgaris, whereas the mutant suppressed by DNA from lps region alpha elicited nitrogen-fixing nodules. However, the nodules in the latter case developed slowly and were widely dispersed. Two other R. etli mutants that had one-half or less of the normal amount of O antigen also gave rise to pseudonodules on P. vulgaris. The latter strains were mutated in lps region alpha and could be restored to normal LPS content and normal symbiosis by complementation with wild-type DNA from this region. Hence, the symbiotic role of LPS requires near-normal abundance of O antigen and may require a structural feature conferred by quinovosamine.
通过凝胶电泳中脂多糖（LPS）的迁移来判断，菜豆根瘤菌突变株CE166的O抗原大小显然正常。然而，其LPS糖组成以及电泳后LPS条带的染色表明，具有O抗原的LPS分子比例仅为野生型值的40%。其LPS与野生型的不同之处还在于缺乏奎诺糖胺（2-氨基-2,6-二脱氧葡萄糖）。这两个缺陷都是由于一个携带Tn5插入的单一基因座所致。通过将共享菜豆根瘤菌CE3 lps基因区域α的7.8 kb片段的重组质粒转入该菌株，O抗原量的缺陷（而非奎诺糖胺的缺失）得到了抑制，尽管这种抑制性DNA并未携带在菌株CE166中发生突变的基因区域。菌株CE166在豆科宿主菜豆上形成假根瘤，而被来自lps区域α的DNA抑制的突变体则引发固氮根瘤。然而，后一种情况下的根瘤发育缓慢且分布广泛。另外两个O抗原量为正常量一半或更少的菜豆根瘤菌突变体在菜豆上也形成了假根瘤。后一种菌株在lps区域α发生了突变，通过用来自该区域的野生型DNA进行互补，可以恢复到正常的LPS含量和正常共生状态。因此，LPS的共生作用需要O抗原接近正常丰度，并且可能需要奎诺糖胺赋予的结构特征。
