Detection of DNA encoding colicins E1 and Ia in 6 plasmid DNA samples (out of 12 randomly picked and analyzed colicin E1 producers). Panel A: an agarose gel with total plasmid DNA stained with ethidium bromide; panel B: Southern blot analysis of the same plasmid DNA samples with colicin E1 probe; panel C: Southern blot analysis with colicin Ia probe. The 1 kb DNA ladder (New England Biolabs, Ipswitch, MA) was used as the DNA marker (panel A, lane 13). Lanes 1 and 2, plasmid DNA isolated of strain B399 (digested with Eco RI and undigested, respectively). Lanes 3 and 4, strain B954 (producer of colicins E1, Ia, and microcin V); lanes 5 and 6, strain B961; lanes 7 and 8, strain B2041; lanes 9 and 10, strain B953; lanes 11 and 12, strain B830. Strains B399, B954, B2041 and B830 were all producers of colicins E1, Ia, and microcin V. Strain B961 produced colicins E1, Ia, E7, K and microcin V. Strain B953 produced colicins E1, Ia, and microcins V and H47. Please note that patterns of undigested plasmid DNA were different in panel B and C, respectively, indicating that colicin Ia and E1 genes are located on separate plasmids.