Studies on DNA replication and cell division in bacteria.

Eleven independent methods for estimating the replication time (C) of the bacterial chromosome are described and discussed. Four of these methods are used to demonstrate that the replication velocity (which is inversely proportional to C) in two non-related thymine-requiring strains of Escherichia coli is a function of the thymine concentration present in their growth medium. By varying the concentration of thymine C can be varied over a two- to three-fold rang, without causing any significant change in the doubling time of the cells. This demonstrates the non-coupling between replication velocity of the chrome-somes and growth rate of the culture. The best absolute values of C obtained in this study are derived from measurement of the rate-stimulation-factor. This is the factor by which the rate of DNA synthesis in the culture is stimulated after a period of thymine starvation equivalent to one mass doubling. However, the most reliable estimates of the relative replication velocities are obtained from the relative DNA/mass ratios in steady-state exponentially-growing cultures. The rate of change of C with the thymine concentration ia similar in glycerol-grown cells to that in glucose-grown cultures. Average cell size in glucose-grown cultures of both strains (but not in glycerol-grown cultures) increases steadily with time. A partial analysis of this phenomenon lad to the hypothesis that an irreversible event, which interferes with call division occurs with a probability that is inversely proportional to the thymine concentration. Evidence is presented suggesting that the time between completion of a round of chromosome replication and the subsequent cell division is coupled in some unknown way to C.