YabA of Bacillus subtilis controls DnaA-mediated replication initiation but not the transcriptional response to replication stress

P>yabA encodes a negative regulator of replication initiation in Bacillus subtilis and homologues are found in many other Gram-positive species. YabA interacts with the beta-processivity clamp (DnaN) of DNA polymerase and with the replication initiator and transcription factor DnaA. Because of these interactions, YabA has been proposed to modulate the activity of DnaA. We investigated the role of YabA in regulating replication initiation and the activity of DnaA as a transcription factor. We found that YabA function is mainly limited to replication initiation at oriC. Loss of YabA did not significantly alter expression of genes controlled by DnaA during exponential growth or after replication stress, indicating that YabA is not required for modulating DnaA transcriptional activity. We also found that DnaN activates replication initiation apparently through effects on YabA. Furthermore, association of GFP-YabA with the replisome correlated with the presence of DnaN at replication forks, but was independent of DnaA. Our results are consistent with models in which YabA inhibits replication initiation at oriC, and perhaps DnaA function at oriC, but not with models in which YabA generally modulates the activity of DnaA in response to replication stress.