Overreplication of the Origin Region in the dnaB37 Mutant of Bacillus subtilis: Postinitiation Control of Chromosomal Replication

When the Bacillus subtilis dnaB37 mutant, defective in initiation, is returned to permissive temperature after accumulation of initiation proteins at 45 degrees C, we have shown, by extensive DNA· DNA hybridization analysis, that the origin region is replicated in excess (≈ 2-fold). However, this replication is limited to a region of about 120-175 kilobases on either side of the origin. This has been confirmed by autoradiographic analysis of the overreplicated region. During the second round of synchronized replication at 30 degrees C, replication in fact appears to resume from the stalled forks on either side of the origin. We propose that in B. subtilis, in addition to a first level of control at the origin, a second level of control exists downstream of the origin in order to limit overreplication of the chromosome. These two controls might normally be tightly coupled. We suggest that the second level of control is exerted through the reversible inhibition of replisome movement at specific regions on either side of the origin.