Mid-cell Z ring assembly in the absence of entry into the elongation phaseof the round of replication in bacteria: co-ordinating chromosome replication with cell division

We have shown previously that, when spores of a thymine-requiring strain of Bacillus subtilis were grown out in the absence of thymine, mid-cell Z rin gs formed over the nucleoid and much earlier than might be expected with re spect to progression into the round of replication. It is now shown that su ch conditions allow no replication of oriC. Rather than replication, partia l degradation of the oriC region occurs, suggesting that the status of this region is connected with the 'premature' mid-cell Z ring assembly. A corre lation was observed between entry into the replication elongation phase and a block to mid-cell Z rings. The conformation of the nucleoid under variou s conditions of DNA replication inhibition or limitation suggests that reli ef of nucleoid occlusion is not primarily responsible for mid-cell Z ring f ormation in the absence of thymine. We propose the existence of a specific structure at mid-cell that defines the Z ring nucleation site (NS). It is s uggested that this NS is normally masked by the replisome upon initiation o f replication or soon after entry into the elongation phase, and subsequent ly unmasked relatively late in the round. During spore outgrowth in the abs ence of thymine, this checkpoint control over mid-cell Z ring assembly brea ks down prematurely.