Maki H, Horiuchi T, Sekiguchi M
Proc Natl Acad Sci U S A. 1983 Dec;80(23):7137-41. doi: 10.1073/pnas.80.23.7137.
A 1.6-kilobase-pair DNA fragment derived from the Escherichia coli chromosome was analyzed by Tn3 transposon insertion and deletion mapping to locate a mutator gene, dnaQ (mutD), and the rnh gene that codes for RNase H. When a strong promoter, PL of lambda phage, was placed at the right- and left-side of the cloned DNA fragment, the dnaQ protein and RNase H, respectively were overproduced. These results suggested that the two genes are transcribed in opposite directions and that their promoters are located in a narrow region between the genes. Nucleotide sequence analysis confirmed this and further revealed that transcriptional and translational initiation signals for the two genes overlap. From the sequence data it was deduced that the dnaQ protein and RNase H consist of 243 and 155 triplets and have molecular weights of 27,500 and 17,500, respectively. dnaQ81 amber mutant showed two codon alterations, CAG(glutamine-195) leads to TAG(amber) and ACA(threonine-193) leads to ATA(isoleucine). The dnaQ-lacZ and the rnh-lacZ fused genes were constructed and hybrid proteins with beta-galactosidase activity were produced. From beta-galactosidase levels it was estimated that the promoter for dnaQ is 5 times more active than that for rnh.
对一段来自大肠杆菌染色体的1.6千碱基对DNA片段进行了分析，通过Tn3转座子插入和缺失图谱定位一个突变基因dnaQ（mutD）以及编码核糖核酸酶H的rnh基因。当将λ噬菌体的强启动子PL置于克隆DNA片段的右侧和左侧时，分别过量产生了dnaQ蛋白和核糖核酸酶H。这些结果表明这两个基因以相反方向转录，且它们的启动子位于基因之间的狭窄区域。核苷酸序列分析证实了这一点，并进一步揭示这两个基因的转录和翻译起始信号重叠。从序列数据推断，dnaQ蛋白和核糖核酸酶H分别由243和155个三联体组成，分子量分别为27,500和17,500。dnaQ81琥珀突变体显示两个密码子改变，CAG（谷氨酰胺-195）变为TAG（琥珀），ACA（苏氨酸-193）变为ATA（异亮氨酸）。构建了dnaQ-lacZ和rnh-lacZ融合基因，并产生了具有β-半乳糖苷酶活性的杂合蛋白。根据β-半乳糖苷酶水平估计，dnaQ的启动子活性比rnh的启动子活性高5倍。
