Plasmids of Shigella dysenteriae Y6R: a defective Col factor

The six plasmids of Shigella dysenteriae Y6R were separated by sucrose gradients into five fractions containing deoxyribonucleic acid (DNA), having contour lengths (expressed in units equal to the fraction of the length of the replicative form of phiX174), respectively, of 0.29, 0.35, 0.74, 1.08, and a mixture of 5.7 and 7.2. DNA-DNA hybridization on nitrocellulose filters between each of the plasmids and between plasmid-free S. dysenteriae Y6R host DNA and plasmids was investigated. There was a high degree of homology between the 0.29- and 0.35-unit plasmids. No significant homology was found between any of the other pairs of plasmids. Homologous DNA to the extent of 2.4 copies of the 1.08-unit plasmid was found in the host genome. Homology between the other plasmids and the host genome is very slight, but appears to be significant. About 0.7 of the 1.08-unit plasmid is homologous to the ColE1 façtor of Escherichia coli JC411 (ColE1). This plasmid may be defective ColE1 factor with the immunity function intact, but with a defect in the gene leading to the production of active colicin. Electron microscope examination of heteroduplexes formed between the two smallest plasmids and between the 1.08-unit plasmid and the ColE1 factor yielded independent determinations of the extent of homology in agreement with the values determined by hybridization. In the latter case, two nonhomologous regions of substitution of DNA were detected.