POSITIVE AND NEGATIVE REGULATORY ELEMENTS IN THE DNAA-DNAN-RECF OPERON OF ESCHERICHIA-COLI

The recF gene of E coli lies within a cluster of genes which play essential roles in DNA replication; the gene order is dnaA dnaN recF gyrB. Each of these genes has its own promoters which, with the exception of dnaA promoters, reside entirely within the translated region of the respective preceding gene. In this report, we analyze the effect of the dnaA and dnaN promoters on recF expression by translational fusions between recF and the lacZ reporter gene. Our results indicate that recF is a distal gene of the dnaA operon, and support the previous proposal that dnaN and recF constitute a transcriptional unit under control of the dnaN promoters. They also suggest that dnaA, dnaN and recF are predominantly expressed from the same mRNA although transcriptional and/or post-transcriptional mechanisms should be specifically involved in lowering expression of the recF gene. Recently, we have localized 3 tandem transcription termination sites in the second half of the dnaN gene, downstream from the recF promoters. Neither of them shows the typical features of simple terminators and apparently they do not work in a minimal system of in vitro transcription. In this report, we present evidence that only one of them is dependent on the Rho protein. Although the operon structure allows coordinate expression of dnaA, dnaN and recF, the presence of internal promoters (the dnaN and recF promoters), which appear to be inducible by DNA damage, and intracistronic terminators, whose activity is inversely proportional to the efficiency of translation, permits expression of individual genes to be independently regulated in response to altered growth conditions.