Induction of replication by thymine starvation at the chromosome origin in Escherichia coli

Abstract

The sequential replication of the DNA of Escherichia coli 15T− (557) observed during exponential growth was compared with that observed after thymine starvation. Thymine starvation was shown to alter the sequence, initiating replication prematurely at the chromosome origin, i.e. that point on the chromosome at which replication commences after a period of amino acid starvation. After thymine starvation most cells will replicate the chromosome simultaneously from two points—the origin and the growth point present prior to starvation. This abnormal situation per se is not lethal and the replication sequence returns to normal after about one generation. The mechanism of recovery has not been determined. The results show that amino acid starvation prevents premature initiation of DNA synthesis at the origin rather than a terminal step necessary to complete the chromosome.