The BtuB Group Col Plasmids and Homology Between the Colicins They Encode

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Colicins A, E1, E2, E3, E4, E5, E6, and E7 exhibited reduced activity against BtuB mutants of Escherichia coli K-12 and also against wild-type cells in the presence of vitamin B12. Plasmids encoding representatives of these colicins were specifically immune to high levels of the homologous colicin. Col+ cells grown in media containing mitomycin C accumulated large amounts of colicin polypeptide. ColE2+, ColE3+, ColE4+, ColE5+, and ColE6+ cultures also synthesized large amounts of second, lower-molecular-weight protein under these conditions. Colicins E2 through E7, but not A or E1, reacted with antiserum raised against purified colicin E3. Colicins E2 and E7 induced synthesis of β-galactosidase encoded by lacZ under the control of the colicin Ib gene promotor on a derivative of Col plasmid ColIb.P9. This promotor is usually active only when the cells are treated with agents which damage DNA or block replication. Plasmids encoding various mutant forms of colicin E3 (M. Mock and M. Schwartz, J. Bacteriol. 142:384-390, 1980) recombined with ColE2, ColE4, ColE5, or ColE6 plasmids at a frequency of 10−4 per cell to produce a colicin active against ColE2+, E4+, E5+, or E6+ cells. ColE5 and ColE6 plasmids recombined with ColE3 plasmids bearing mutations affecting colicin E3 receptor recognition, envelope penetration, and catalytic activities. ColE2 and ColE4 plasmids recombined only with ColE3 plasmids bearing mutations affecting receptor recognition and envelope penetration. Recombinants between mutant ColE3 plasmids and ColA, ColE1, or ColE7 plasmids were not detected. We propose the designation BtuB group for the colicins described here, and we divide the group into two classes comprising colicins A and E1, which act on the cytoplasmic membrane, and the related colicins E2 through E7, which have known or putative nuclease activities.

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