Plasmid ColE3 specifies a lysis protein.

Tn5 insertion mutations in plasmid ColE3 were isolated and characterized. Several of the mutants synthesized normal amounts of active colicin E3 but, unlike wild-type colicinogenic cells, did not release measurable amounts of colicin into the culture medium. Cells bearing the mutant plasmids were immune to exogenous colicin E3 at about the same level as wild-type colicinogenic cells. All of these lysis mutants mapped near, but outside of, the structural genes for colicin E3 and immunity protein. Cells carrying the insertion mutations which did not release colicin E3 into the medium were not killed by UV exposure at levels that killed cells bearing wild-type plasmids. The protein specified by the lysis gene was identified in minicells and in mitomycin C-induced cells. A small protein, with a molecular weight between 6,000 and 7,000, was found in cells which released colicin into the medium, but not in mutant cells that did not release colicin. Two mutants with insertions within the structural gene for colicin E3 were also characterized. They produced no colicin activity, but both synthesized a peptide consistent with their map position near the middle of the colicin gene. These two insertion mutants were also phenotypically lysis mutants--they were not killed by UV doses lethal to wild-type colicinogenic cells and they did not synthesize the small putative lysis protein. Therefore, the lysis gene is probably in the same operon as the structural gene for colicin E3.