Mulligan J T, Long S R
Department of Biological Sciences, Stanford University, California 94305-5020.
Genetics. 1989 May;122(1):7-18. doi: 10.1093/genetics/122.1.7.
Nodulation (nod) gene expression in Rhizobium meliloti requires plant inducers and the activating protein product of the nodD gene. We have examined three genes in R. meliloti which have nodD activity and sequence homology. These three nodD genes are designated nodD1, nodD2 and nodD3, and have distinctive properties. The nodD1 gene product activates expression of the nodABC operon, as measured by a nodC-lacZ fusion or by transcript analysis, in the presence of crude seed or plant wash or the inducer, luteolin. The nodD3 gene product can cause a high basal (uninduced) level of nodC-lacZ expression and nodABC transcripts which is relatively unaffected by inducers. The effect of nodD3 is dependent on the presence of another gene, syrM (symbiotic regulator). By primer extension analysis we determined that the transcription start site is the same for nodD1 plus luteolin or nodD3-syrM mediated expression of nodA and nodH mRNAs. syrM also enhances the expression of another symbiotically important trait, production of extracellular polysaccharide. This regulatory effect of syrM requires locus syrA, which is linked to nodD3 and syrM. The syrM-syrA mediated increase in polysaccharide production requires at least some of the previously identified exo genes and may be a parallel regulatory event to the syrM-nodD3 control of nod promoters.
苜蓿根瘤菌中结瘤（nod）基因的表达需要植物诱导物和nodD基因的激活蛋白产物。我们研究了苜蓿根瘤菌中三个具有nodD活性和序列同源性的基因。这三个nodD基因分别命名为nodD1、nodD2和nodD3，具有不同的特性。在存在粗种子提取物或植物洗涤液或诱导物木犀草素的情况下，通过nodC-lacZ融合或转录分析测量，nodD1基因产物可激活nodABC操纵子的表达。nodD3基因产物可导致nodC-lacZ表达和nodABC转录本的高水平基础（未诱导）表达，且相对不受诱导物影响。nodD3的作用取决于另一个基因syrM（共生调节因子）的存在。通过引物延伸分析，我们确定nodD1加木犀草素或nodD3-syrM介导的nodA和nodH mRNA表达的转录起始位点相同。syrM还增强了另一个共生重要性状——细胞外多糖产生的表达。syrM的这种调节作用需要与nodD3和syrM连锁的syrA位点。syrM-syrA介导的多糖产量增加至少需要一些先前鉴定的外切基因，并且可能是与syrM-nodD3对nod启动子的控制平行的调节事件。
