Dependence of single-stranded DNA length on stage of growth cycle in Escherichia coli B-r and effect of irradiation.

Alkaline sucrose density gradient profiles of DNA from log phase Escherichia coli B/r (CSH) show a main peak with sedimentation coefficient at approximately 130S and a shoulder or second peak at approximately 90S. Incorporation of radioactive precursors into the 90S peak precedes their appearance in the main peak. The size of the second peak appears to be directly related to the rate of replication and it is not present in profiles of nondividing stationary phase cultures. The decrease in weight average molecular weight (Mw) of DNA produced by X-rays is also directly related to the rate of replication. It is greatest in log phase E. coli B/r and least in stationary phase cells, because of the efficiency of rejoining of radiation-induced single strand breaks in DNA of the latter cells.