Essential role of the gyrB gene product in the transcriptional event coupled to dnaA-dependent initiation of Escherichia coli chromosome replication.

When a culture of the gyrB41-ts mutant is shifted to the nonpermissive temperature, DNA synthesis is arrested at the initiation phase of chromosome replication. After thermal inactivation of the gyrB gene product reinitiation occurs in the presence of chloramphenicol but not in the presence of rifampicin. This suggests that the B subunit of DNA gyrase may regulate synthesis of an "initiator RNA". An rpoB202 mutation has been isolated which suppresses both the DnaA-initiation phenotype and the inhibitory action of antibiotics which are known to result in relaxation of chromosomal DNA in vivo. We propose that DNA tertiary structure rather than DNA gyrase itself plays an essential regulatory function in the dnaA-dependent transcription which precedes the initiation of chromosome replication.