Molecular comparisons of plasmids isolated from colicinogenic strains of Escherichia coli

The plasmid content of 14 colicinogenic strains of Escherichia coli has been examined. Four strains contained miniplasmids (1.2-2.0 kb). Small plasmids (4-7 kb) were detected in all the strains specifying group A colicins (colicins A, E1, E2, E3 and K; group I plasmids). Larger plasmids (55-130 kb) were detected in seven out of nine strains specifying group B colicins (colicins B, H, Ia, Ib, M, V and S1; group II plasmids). DNA-DNA hybridization with group II plasmids showed a wide variation in the degree of DNA sequence homology among its members. In contrast little (if any) DNA sequence homology was detected with the group I plasmids when the same group II plasmid DNAs were used as hybridization probes. The results of DNA-DNA hybridization studies with the two small group II plasmids (pcolG-CA46 and pcolD-CA23) suggested that these plasmids are equivalent to deleted forms of larger group II plasmids. Our hybridization data thus support the division of colicin plasmids into the two groups (I and II) that have been previously defined from genetic and physiological studies.