【摘要】 根瘤菌可在几种豆科植物上成瘤，结瘤基因（ｎｏｄ基因）赋予这些细菌以寄主专一性的方式诱导瘤的形成。目前，已被鉴定的三类主要结瘤基因是：＂公共＂结瘤基因，如ｎｏｄＡＢＣ，这类基因是结瘤不可缺少的，并具有高度的结构和功能上的保守性；另一类是寄主专一性基因，这类基因具有种属或菌株的专一性，它们决定了细菌的寄主范围；最后一类是结瘤基因Ｄ（ｎｏｄＤ），这类基因编码一类调节蛋白，并和植物分泌的酚类化合物一道起动其他结瘤操纵子的转录。一些根瘤菌种属只有一个拷贝的ｎｏｄＤ基因，而另一些则可拥有三个拷贝之多。苜蓿根瘤菌（Ｒ．ｍｅｌｉｌｏｔｉ）中存在三个拷贝的ｎｏｄＤ基因，其中的任何一个发生突变都以依赖于寄主的方式影响结瘤效果。在慢生型大豆根瘤菌（Ｂ．ｊａｐｏｎｉｃｕｍ）中，ｎｏｄＤ１突变菌株还可在大豆或其他豆科植物寄主上结瘤，只是稍往后推迟而已，ｎｏｄＤ２基因对ｎｏｄＹＡＢＣＳＵＩＪ操纵子诱导的影响却很小，两个ｎｏｄＤ拷贝突变后尚表现出结瘤活性。在只有一个ｎｏｄＤ基因拷贝的根瘤菌种属中，如豌豆根瘤菌（Ｒ．ｌｅｇｕｍｉｎｏｓａｒｕｍ）和茎瘤固氮菌（Ａ．ｃａｕｌｉｎｏｄａｎｓ）中，ｎｏｄＤ突变后会导致菌株丧失结瘤能力。一般说来，不?更多还原

【Abstract】 Rhizobia can cause the formation of nitrogen-fixing nodules on the roots of several leguminous plants.The nodulation(nod) genes enable bacteria to induce nodule formation in a host-specific manner.Three major types of nod genes have been identified:the"common" nod genes,such as nodABC,which are essential for nodulation,functionally and structurally conserved among all rhizobia thus far studied; host-specificity genes,which are strain-or species-specific and determine the bacterial host range;and finally,the nodD genes,which encode regulatory proteins that in conjunction with plant-derived phenolic compounds activate the transcription of the other nod operons.Some species of rhizobia have a single nodD genes,whereas others may possess as many as three nodD homologues.In R.meliloti,there exist at least three nodD genes,mutations in only one of the three affect nodulation efficiency in a host-dependent manner.In B.japon icum,two copies of nodD have been characterized,nodD1 mutants still nodulate soybean and other legume hosts with only a small delay,nodD2 gene has only a marginal effect on induction of the nodYABCSUIJ operon,and mutant strains deleted for both nodD copies still showed nodulation activity. In rhizobial species that have only one nodD gene,such as R.leguminosarum and A.caulinodans, nodD mutants are nod-.Generally speaking,odD genes from diverse sources differ in their response to various hosts, behaving in a species-specific manner.Interestingly,a nodD-like gene cloned from a Frankia strain At4 in our laboratory can efficiently restored the nodulation of a nodD mutant of R.leguminosarum by.viciae bythe genetic complementation with the nodD-like DNA sequence,and the nodD-like gene showed poor DNA homology with the nodD gene from R.leguminosarum by.vicae.更多还原