Replication of the Escherichia coli chromosome following thymine starvation.

The replication of the chromosome of E. coli 15T(-) (thymine-requiring) has been followed for approximately four generations after a premature initiation of DNA synthesis by thymine starvation. Various labelings of the DNA with (14)C, (3)H, and density media allowed discrimination of the strands preexistent to thymine starvation from those which were made during the subsequent generations. Results were: (1) There is very little, if any, DNA degradation following 45 min of thymine starvation. (2) The growing points which existed prior to thymine starvation proceeded to the end of the chromosomes after resumption of DNA synthesis and then disappeared. (3) The premature DNA synthesis initiated by thymine starvation involved, symmetrically, all free origins; the entire chromosomal population was subjected to this process. (4) All the DNA which was synthesized during the progression of the premature growing points was normally replicated, i.e., with no delay or loss, during the next three generations.