Studies on Escherichia coli sex factors: evidence that covalent circles exist within cells and the general problem of isolation of covalent circles.

We examined in detail conditions necessary for making reproducible and for maximizing the amount of deoxyribonucleic acid obtained from a sex factor-containing cell as covalent circles. The results argue that under optimal conditions covalent circles are neither created nor lost during the isolation procedure. The causes of the culture-to-culture variation in recovery of covalent circular deoxyribonucleic acid were investigated but an understanding of this is not yet at hand. Some commonly used conditions which drastically reduce the recovery of covalent circles are described.