Colicin E3 and its immunity genes.

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.