Regulation of chromosomal replication initiation by oriC-proximal DnaA-box clusters in Bacillus subtilis

Bacterial chromosome replication is initiated by binding of DnaA to a DnaA-box cluster (DBC) within the replication origin (oriC). In Bacillus subtilis, six additional DBCs are found outside of oriC and some are known to be involved in transcriptional regulation of neighboring genes. A deletion mutant lacking the six DBCs (Δ6) initiated replication early. Further, inactivation of spo0J in Δ6 cells yielded a pleiotropic phenotype, accompanied by severe growth inhibition. However, a spontaneous suppressor in soj or a deletion of soj, which stimulates DnaA activity in the absence of Spo0J, counteracted these effects. Such abnormal phenotypic features were not observed in a mutant background in which replication initiation was driven by a plasmid-derived replication origin. Moreover, introduction of a single DBC at various ectopic positions within the Δ6 chromosome partly suppressed the early-initiation phenotype, but this was dependent on insertion location. We propose that DBCs negatively regulate replication initiation by interacting with DnaA molecules and play a major role, together with Spo0J/Soj, in regulating the activity of DnaA.