6-(-Hydroxyphenylazo)uracil (HPUra), which is thought to be a specific inhibitor of DNA replication in many Gram-positive bacteria, has been used to investigate the relationship between DNA replication and the induction of sporulation in Gene frequency analysis of samples removed from sporulating cultures indicates that sporulating cells terminate their final round of DNA replication at the same time as they escape from the inhibitory effect of HPUra. If the rate of DNA replication during starvation is slowed down by the use of sub-inhibitory concentrations of HPUra, the onset of this escape is delayed. When cells containing only completed chromosomes are transferred from a rich growth medium to a poor sporulation medium they initiate new rounds of DNA replication in the sporulation medium and then subsequently sporulate. The results are consistent with the hypothesis of Mandelstam & Higgs (1974) that induction of sporulation occurs 15 to 20 min after the initiation of a round of DNA replication.


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