Fernández De Henestrosa A R, Ogi T, Aoyagi S, Chafin D, Hayes J J, Ohmori H, Woodgate R
Section on DNA Replication, Repair and Mutagenesis, Building 6, Room 1A13, National Institute of Child Health and Human Development, National Institutes of Health, 9000 Rockville Pike, Bethesda, MD 20892-2725, USA.
Mol Microbiol. 2000 Mar;35(6):1560-72. doi: 10.1046/j.1365-2958.2000.01826.x.
Exposure of Escherichia coli to a variety of DNA-damaging agents results in the induction of the global 'SOS response'. Expression of many of the genes in the SOS regulon are controlled by the LexA protein. LexA acts as a transcriptional repressor of these unlinked genes by binding to specific sequences (LexA boxes) located within the promoter region of each LexA-regulated gene. Alignment of 20 LexA binding sites found in the E. coli chromosome reveals a consensus of 5'-TACTG(TA)5CAGTA-3'. DNA sequences that exhibit a close match to the consensus are said to have a low heterology index and bind LexA tightly, whereas those that are more diverged have a high heterology index and are not expected to bind LexA. By using this heterology index, together with other search criteria, such as the location of the putative LexA box relative to a gene or to promoter elements, we have performed computational searches of the entire E. coli genome to identify novel LexA-regulated genes. These searches identified a total of 69 potential LexA-regulated genes/operons with a heterology index of <15 and included all previously characterized LexA-regulated genes. Probes were made to the remaining genes, and these were screened by Northern analysis for damage-inducible gene expression in a wild-type lexA+ cell, constitutive expression in a lexA(Def) cell and basal expression in a non-inducible lexA(Ind-) cell. These experiments have allowed us to identify seven new LexA-regulated genes, thus bringing the present number of genes in the E. coli LexA regulon to 31. The potential function of each newly identified LexA-regulated gene is discussed.
将大肠杆菌暴露于多种DNA损伤剂会导致全局性“SOS应答”的诱导。SOS调节子中的许多基因的表达受LexA蛋白控制。LexA通过与位于每个LexA调节基因启动子区域内的特定序列（LexA框）结合，作为这些不连锁基因的转录阻遏物。对大肠杆菌染色体中发现的20个LexA结合位点进行比对，揭示出5'-TACTG(TA)5CAGTA-3'的共有序列。与该共有序列紧密匹配的DNA序列被认为具有低异源性指数，并能紧密结合LexA，而那些差异较大的序列则具有高异源性指数，预计不会结合LexA。通过使用这种异源性指数，结合其他搜索标准，如假定的LexA框相对于基因或启动子元件的位置，我们对整个大肠杆菌基因组进行了计算搜索，以鉴定新的LexA调节基因。这些搜索共鉴定出69个潜在的LexA调节基因/操纵子，其异源性指数<15，包括所有先前已表征的LexA调节基因。针对其余基因制备了探针，并通过Northern分析对野生型lexA+细胞中损伤诱导型基因表达、lexA(Def)细胞中的组成型表达以及非诱导型lexA(Ind-)细胞中的基础表达进行筛选。这些实验使我们能够鉴定出七个新的LexA调节基因，从而使大肠杆菌LexA调节子中目前的基因数量达到31个。讨论了每个新鉴定的LexA调节基因的潜在功能。
