Colicin Factor DNA: a Single Non-Homologous Region in Col E2–E3 Heteroduplex Molecules

Abstract

COLICIN factors E2 and E3 (Col E2 and Col E3) are stably maintained extrachromosomal genetic elements carried by Escherichia coli that have been identified as small circular deoxyribonucleic acid (DNA) molecules with the same molecular weights (5 × 10⁶)¹. Cells carrying one of these Col factors can produce a specific antibiotic protein², colicin E2 or colicin E3, and also exhibit an “immunity” to the lethal action of the specific colicin it produces³. While the mechanisms of action of colicin E2 and E3 are quite different^4–6, structural and immunological comparisons suggest the proteins are related² as does the shared receptor sites for colicin adsorption^7–9. The partial immunity that cells colicinogenic for Col E2 exhibit for colicin E3¹⁰ suggests another possible relation may exist between the two colicin factors. In order to further understand the relationships of these two small colicin factors, an examination of the presence and location of homologous base sequences in their DNA molecules was undertaken. The experiments reported here include the electron microscopic examination of heteroduplex molecules formed by renaturing mixtures of denatured Col E2 and Col E3 DNA in formamide solutions^11–14, as well as measurements of competitive annealing between both DNA molecules using DNA-DNA membrane filter hybridization techniques¹⁵.