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

Spores of a thymine-requiring strain of Bacillus subtilis 168, which is als o temperature sensitive for the initiation of chromosome replication, were germinated and allowed to grow out at the permissive temperature in a minim al medium containing no added thymine. Under these conditions, there was no or very limited progression into the elongation phase of the first round o f replication. In a significant proportion of the outgrown cells, a Z ring formed precisely at mid-cell and over the centrally positioned nucleoid, le ading eventually to the formation of a mature division septum. When initiat ion of the first round of replication was blocked through a temperature shi ft and with thymine present, the Z ring was positioned acentrally. The cent ral Z ring that formed in the absence of thymine was blocked by the presenc e of a DNA polymerase III inhibitor. It is concluded that the very early st ages of a round of replication (initiation plus possibly limited progressio n into the elongation phase) play a key role in the precise positioning of the Z ring at mid-cell and between replicating daughter chromosomes.