Discoordinate gene expression in the dnaA-dnaN operon of Escherichia coli

The dnaN gene of Escherichia coli encodes the beta-subunit of the DNA polymerase III holoenzyme. Previous work has established that dnaN lies immediately downstream of dnaA and that both genes may be cotranscribed from the dnaA promoters; no promoter for dnaN has been described. We investigated the in vivo regulation of transcription of the dnaN gene by transcriptional fusions to the galK gene, translational fusion to the lacZ gene and S1 mapping analysis. We found that there are at least three dnaN promoters residing entirely in the reading frame of the preceding dnaA gene, and that transcription from these promoters can occur independently of dnaA transcription which, however, extends at least up to dnaN. Furthermore, we found evidence for the inducibility of the dnaN promoters in a dam background under conditions of simultaneously reduced dnaA transcription. These results are consistent with the hypothesis that although dnaA and dnaN are organized in an operon considerable discoordinate transcription can occur, thus uncoupling dnaN and dnaA regulation, when needed.