Post-initiation control of chromosomal replication in Bacillus subtilis: a mechanism for limiting over-replication or for duplicating key growth and sporulation genes?

We used the Bacillus subtilis dnaB37 mutant, which is defective in initiation, to synchronize DNA replication in order to identify the first fragments to be replicated following initiation and to study the control of this process under various conditions. We show by DNA/DNA hybridization analysis that, after returning the mutant from 45 degrees C to the permissive temperature (30 degrees C), the origin region relative to other sequences is over-replicated (approximately 2-fold) during the first round. This was confirmed by autoradiographic analysis. The over-replicated region is however limited to about 190 kb on the left and right arms. Replication apparently resumes from these positions during the following round of replication. We propose that, in B. subtilis, in addition to the first level of control at the origin, there is a second level or post-initiation control downstream of the origin which limits DNA replication resulting from premature initiation. We believe that these two levels of control are tightly coupled under conditions of balanced growth. Using the same system, we have now shown that DNA replication is subject to "stringent control", an important regulatory network in bacteria. These studies demonstrate that the inhibition of replication induced during the "stringent response" does not occur at the primary origin. In fact, by DNA/DNA hybridization, replication forks were found to be blocked at similar positions to the post-initiation control sites described above. Moreover, replication appears to resume from regions close to the stalled replisomes upon removal of the stringent response.(ABSTRACT TRUNCATED AT 250 WORDS)