Little J W, Mount D W, Yanisch-Perron C R
Proc Natl Acad Sci U S A. 1981 Jul;78(7):4199-203. doi: 10.1073/pnas.78.7.4199.
Escherichia coli shows a pleiotropic response (the SOS response) to treatments that damage DNA or inhibit DNA replication. Previous evidence has suggested that the product of the lexA gene is involved in regulating the SOS response, perhaps as a repressor, and that it is sensitive to the recA protease. We show here that lexA protein is a repressor of at least two genes, recA and lexA. Purified protein bound specifically to the regulatory regions of the two genes, as judged by DNase I protection experiments, and it specifically inhibited in vitro transcription of both genes. The binding sites in recA and lexA were found to be about 20 base pairs (bp) and 40 bp long, respectively. The 40-bp sequence in lexA was composed of two adjacent 20-bp sequences, which had considerable homology to one another and to the corresponding recA sequence. These 20-bp sequences, which we term "SOS boxes," show considerable inverted repeat structure as well. These features suggest that each box represents a single repressor binding site. Finally, we found that purified lexA protein was a substrate for the recA protease in a reaction requiring ATP or an analogue, adenosine 5'-[gamma-thio]triphosphate, and denatured DNA.
大肠杆菌对损伤DNA或抑制DNA复制的处理表现出多效性反应（SOS反应）。先前的证据表明，lexA基因的产物参与调控SOS反应，可能作为一种阻遏物，并且它对recA蛋白酶敏感。我们在此表明，LexA蛋白是至少两个基因recA和lexA的阻遏物。通过DNase I保护实验判断，纯化的蛋白特异性结合到这两个基因的调控区域，并且它特异性抑制这两个基因的体外转录。发现recA和lexA中的结合位点分别约为20个碱基对（bp）和40个bp长。lexA中的40-bp序列由两个相邻的20-bp序列组成，它们彼此之间以及与相应的recA序列具有相当大的同源性。这些我们称为“SOS框”的20-bp序列也显示出相当大的反向重复结构。这些特征表明每个框代表一个单一的阻遏物结合位点。最后，我们发现纯化的LexA蛋白在需要ATP或类似物腺苷5'-[γ-硫代]三磷酸以及变性DNA的反应中是recA蛋白酶的底物。
