Transcriptional activation of the dnaA gene encoding the initiator for oriC replication by IciA protein, an inhibitor of in vitro oriC replication in Escherichia coli.

Transcription of the Escherichia coli dnaA gene, encoding DnaA protein required for initiation of chromosomal DNA replication at oriC in E. coli, starts from two promoters, 1P and 2P. Gel-shift and DNase I-protection assays revealed that IciA protein, an inhibitor of initiation of in vitro E. coli chromosomal DNA replication at oriC, bound to two sites in the dnaA promoter region. One site is located upstream of promoter 1P, and the second is located downstream of promoter 2P. Whereas IciA protein did not affect transcription from the promoter 2P, transcription from the promoter 1P was specifically enhanced by IciA protein in vivo and in vitro. DnaA protein bound to the DnaA box between the two promoters 1P and 2P, acts as an transcriptional repressor. Under this condition, IciA protein counteracted the repressive effect of DnaA protein on the promoter 1P. These findings suggest that IciA protein may regulate the initiation of chromosomal DNA replication at oriC by controlling expression of the dnaA gene, as well as by inhibiting the initiation of chromosomal DNA replication at oriC.