Masaki H, Ohta T
J Mol Biol. 1985 Mar 20;182(2):217-27. doi: 10.1016/0022-2836(85)90340-7.
A DNA segment of plasmid ColE3-CA38 was cloned into pBR328 and its nucleotide sequence was determined. This segment contains the putative promoter-operator region, the structural genes of protein A (gene A) and protein B (gene B) of colicin E3, and a part of gene H. Just behind the promoter region, there is an inverted repeat structure of two 'SOS boxes', the specific binding site of the lexA protein. This suggests that the expression of colicin E3 is regulated directly by the lexA protein. Genes A and B face the same direction, with an intergenic space of nine nucleotides between them. ColE3-CA38 and ColE1-K30 are homologous in their promoter-operator regions, but hardly any homology was found in their structural genes. On the other hand, ColE3-CA38 is fairly homologous to CloDF13 throughout the regions sequenced, with some exceptions including putative receptor-binding regions. By deletion mapping of the immunity gene and recloning of gene B, it was shown genetically that protein B itself is the actual immunity substance of colicin E3. It was also found that the expression of E3 immunity partially depends on the recA function. Thus, we propose two modes of expression of E3 immunity: in the uninduced state, only a slight amount of protein B is produced constitutively to protect the cell from being attacked by the exogenous colicin; and in the SOS-induced state, a large amount of protein B is produced to protect the protein synthesis system of the host cell from ribosome inactivation by endogenously produced colicin E3.
将质粒ColE3-CA38的一段DNA片段克隆到pBR328中，并测定了其核苷酸序列。该片段包含推定的启动子-操纵子区域、大肠杆菌素E3的蛋白A（基因A）和蛋白B（基因B）的结构基因，以及基因H的一部分。在启动子区域的紧后方，有两个“SOS框”的反向重复结构，这是lexA蛋白的特异性结合位点。这表明大肠杆菌素E3的表达直接受lexA蛋白调控。基因A和B方向相同，它们之间有9个核苷酸的基因间隔区。ColE3-CA38和ColE1-K30在其启动子-操纵子区域是同源的，但在它们的结构基因中几乎没有发现同源性。另一方面，ColE3-CA38在整个测序区域与CloDF13相当同源，但在一些区域包括推定的受体结合区域存在例外。通过对免疫基因的缺失定位和基因B的重新克隆，遗传学研究表明蛋白B本身就是大肠杆菌素E3的实际免疫物质。还发现E3免疫的表达部分依赖于recA功能。因此，我们提出E3免疫的两种表达模式：在未诱导状态下，仅组成性地产生少量蛋白B以保护细胞免受外源大肠杆菌素的攻击；在SOS诱导状态下，产生大量蛋白B以保护宿主细胞的蛋白质合成系统免受内源性产生的大肠杆菌素E3导致的核糖体失活。
