Gibert I, Calero S, Barbé J
Department of Genetics and Microbiology, Faculty of Sciences, Autonomous University, Barcelona, Spain.
Mol Gen Genet. 1990 Feb;220(3):400-8. doi: 10.1007/BF00391745.
By using a promoter probe plasmid we investigated expression of the linked nrdA and nrdB genes coding for the two different subunits of the ribonucleoside diphosphate reductase enzyme of Escherichia coli. For this reason, nrdA-lacZ, nrdAB-lacZ and nrdB-lacZ fusions were constructed. Results obtained indicate that the nrdB gene has a promoter from which it may be transcribed independently of the nrdA gene. Furthermore, the nrdB gene may also be transcribed from the nrdA promoter. The expression of the nrdB gene is about 14-fold higher from the nrdA promoter than from its own promoter. The induction of both nrdA and nrdB genes by DNA-damaging agents in the wild-type strain as well as in several SOS mutants was also studied; nrdA gene expression was increased by these treatments in RecA+, RecA-, and LexAInd- strains, although in both RecA- and LexAInd- mutants the nrdA gene expression was considerably lower than that in RecA+ cells. nrdB gene expression was stimulated by DNA damage only when its transcription was from the nrdA promoter, but there was no effect when nrdB was transcribed from its own promoter. In addition, the basal level of nrdA-lacZ and nrdAB-lacZ fusions was reduced in strains containing either RecA- and LexAInd- mutations or a multicopy plasmid carrying the lexA+ gene, whereas the presence of a LexA51Def mutation increased the constitutive expression of both fusions. On the contrary, the basal level of the nrdB-lacZ fusion remained constant in all these strains. Together these results indicate that induction of the SOS response enhances expression of the nrd genes from the nrdA promoter.
通过使用启动子探针质粒，我们研究了编码大肠杆菌核糖核苷二磷酸还原酶两种不同亚基的相连nrdA和nrdB基因的表达。因此，构建了nrdA - lacZ、nrdAB - lacZ和nrdB - lacZ融合体。所得结果表明，nrdB基因有一个启动子，它可以独立于nrdA基因进行转录。此外，nrdB基因也可以从nrdA启动子转录。nrdB基因从nrdA启动子的表达比从其自身启动子的表达高约14倍。还研究了野生型菌株以及几个SOS突变体中DNA损伤剂对nrdA和nrdB基因的诱导作用；在RecA +、RecA - 和LexAInd - 菌株中，这些处理增加了nrdA基因的表达，尽管在RecA - 和LexAInd - 突变体中，nrdA基因的表达都明显低于RecA + 细胞中的表达。只有当nrdB基因从nrdA启动子转录时，DNA损伤才会刺激其表达，而当nrdB从其自身启动子转录时则没有影响。此外，在含有RecA - 和LexAInd - 突变或携带lexA + 基因的多拷贝质粒的菌株中，nrdA - lacZ和nrdAB - lacZ融合体的基础水平降低，而LexA51Def突变的存在增加了两种融合体的组成型表达。相反，nrdB - lacZ融合体的基础水平在所有这些菌株中保持不变。这些结果共同表明，SOS反应的诱导增强了nrdA启动子对nrd基因的表达。
