Obligatory Coupling of Colicin Release and Lysis in Mitomycin-treated Col+ Escherichia coli

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Abstract
E. coli K12 strains carrying the small, high copy number ColE2-P9 plasmid produce large amounts of colicin and then lyse and release colicin when grown in broth culture containing mitomycin C. Strains carrying the larger, low copy number ColIa-CA53 plasmid produced much less colicin and did not lyse or discharge > 15% of their colicin when grown under the same conditions. Naturally-occurring Col+ strains and E. coli K12 derivatives carrying different Col plasmids could be classified either as ColE2+-like or ColIa+-like according to whether or not they produced large amounts of colicin and lysed and discharged colicin when grown in the presence of mitomycin, and also by the size and presumed copy number of the Col plasmid they carried. Strains carrying multiple copies of the cloned colicin Ia structural gene produced large amounts of colicin but did not lyse or release colicin when grown in the presence of mitomycin. This result rules out the possibility that high level accumulation of colicin is sufficient to cause lysis. Conditions were sought under which colicin Ia could be released from the producing cells. Mitomycin-treated cultures of strains carrying both ColE2 and ColIa plasmids released both colicins when they lysed, although colicin Ia release occurred later than colicin E2 release. Colicin Ia-laden cells also released their colicin when diluted into fresh culture medium.