DIVISION SEPTATION IN THE ABSENCE OF CHROMOSOME TERMINATION IN BACILLUS-SUBTILIS

Germinating spores of the temperature-sensitive DNA initiation mutants of Bacillus subtilis, TsB134 and dna-1(Ts), were allowed to undergo a single round of replication by shifting to the restrictive temperature shortly after its initiation. To monitor the progress of the round 5-bromouracil was added at various times and DNA extracted after a further time, sufficient to allow completion of the chromosome. Average replication was measured from the relative amounts of LL and LH material in Cs2SO4 gradients. The replication state of origin (purA), intermediate (leuA) and terminus (metB) markers at the times of 5-bromouracil addition were obtained from genetic analysis of the density species fractionated in gradients of CsCl. The DNA replication inhibitor, 6-(p-hydroxyphenylazo)-uracil (HPUra), was added at various stages of the single round and the outgrown cells examined at later times for the frequency and type of septation. Under the conditions of the experiment, central division septation was blocked if HPUra (20 μm) was added before 70% (approximately) of the chromosome was replicated. Using higher concentrations of HPUra, 40 and 100 μm, it was shown that central division septation would occur at about its normal time if replication was blocked after this 70% stage but before termination. In these circumstances there was a distinct tendency for the DNA to remain close to the septum on both sides of it. The B. subtilis spore contains a single chromosome, which means that the central septum that forms in the absence of termination must pass through a partially completed chromosome. Electron microscopic evidence for such a situation has already been described (Van Iterson & Aten, 1976). It is concluded that, at least under the restrictive conditions of the present experiments, termination of chromosome replication is not obligatory for the formation of the division septum with which it is normally coupled. © 1979.