Initiation of deoxyribonucleic acid replication in Escherichia coli B: uncoupling from mass/deoxyribonucleic acid ratio.

In Escherichia coli growing at different rates, the ratio of cell mass to the number of chromosome origins tended to be constant at the time of the initiation of deoxyribonucleic acid (DNA) replication. This observation led to the assumption that the initiation event is controlled in some way by cell mass, e.g., by a growth-dependent synthesis of an initiator or dilution of a repressor. We have now found that the initiation of DNA synthesis can be uncoupled from cell mass. We used a synchronous culture of newly divided cells of E. coli B which was obtained by the membrane elution technique (C.E. Helmstetter, J. Mol. Biol. 24: 417-427, 1967) and was starved for an amino acid. Upon restoration of the amino acid, the cells not only divided at a size that was smaller than normal, but also initiated DNA replication long before they could increase their masses to reach the expected ratio of mass/DNA presumably required for initiation.