Co‐ordinating DNA replication with cell division in bacteria: a link between the early stages of a round of replication and mid‐cell Z ring assembly

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Abstract
Spores of a thymine-requiring strain of Bacillus subtilis 168, which is also temperature sensitive for the initiation of chromosome replication, were germinated and allowed to grow out at the permissive temperature in a minimal medium containing no added thymine. Under these conditions, there was no or very limited progression into the elongation phase of the first round of replication. In a significant proportion of the outgrown cells, a Z ring formed precisely at mid-cell and over the centrally positioned nucleoid, leading eventually to the formation of a mature division septum. When initiation of the first round of replication was blocked through a temperature shift and with thymine present, the Z ring was positioned acentrally. The central Z ring that formed in the absence of thymine was blocked by the presence of a DNA polymerase III inhibitor. It is concluded that the very early stages of a round of replication (initiation plus possibly limited progression into the elongation phase) play a key role in the precise positioning of the Z ring at mid-cell and between replicating daughter chromosomes.