SINGLE-STRAND BREAKS IN BACTERIAL DNA ASSOCIATED WITH THYMINE STARVATION

A thymine-requiring strain of Escherichia coli containing a sex factor has been starved of thymine and examined by zone centrifugation in alkaline sucrose gradients for survival of circular sex factor DNA molecules. Circles are lost at a constant rate during the period of thymine starvation. This loss does not occur if either glucose, phosphate, or hypoxanthine (in a hypoxanthine-requirer) is absent and the loss is only slightly slower if required amino acids are eliminated. When the rates of loss are compared for two different sex factors, it is found that the rate is proportional to DNA molecular weight which indicates that the strand breaks which are responsible for circle loss do not occur at a unique site in the DNA molecule. It is hypothesized that the strand breaks are the result of some (unknown) normal process and are usually repaired by DNA ligase and that thymine starvation results in the production of some substance which inhibits DNA ligase. © 1969.