Studies on the relationship between chromosome replication and cell division in Escherichia coli.

The relationship between chromosome replication and cell division has been investigated in a thymineless mutant of E.coli B/r. The investigation is based upon the ability to vary the rate of DNA chain elongation by manipulation of the thymine concentration in the growth medium. Examination of the changes in average cell mass and DNA content of exponential cultures arising from a reduction in replication velocity suggest that the resulting increase in the replication time C is accompanied by a decrease in the period, D, between termination of the round of replication and the subsequent cell division. Observations on the pattern of DNA synthesis during the division cycle are consistent with this relationship. Nevertheless, the kinetics of transition of exponential cultures moving between steady-states of growth with differing replication velocities provides evidence to support the view that the time of cell division is determined by termination of rounds of replication under steady-state conditions. Measurements taken from electron micrographs of cells growing with differing replication velocities show that the increase in average cell mass is accommodated by an increase in cell width rather than length. The morphological changes are consistent with the predictions of a model for cell growth whereby the number of sites for cell elongation, which operate at a constant rate, are equivalent to the number of chromosome termini per cell. Preliminary studies on cell wall murein synthesis indicate an increasing rate with cell age.