Transcriptional organization of the dnaN and recF genes of Escherichia coli K-12.

The dnaN gene of Escherichia coli determines the beta subunit of DNA polymerase III, a multisubunit enzyme responsible for most of the replicative DNA synthesis. The dnaN gene maps between the dnaA and recF genes. We have characterized the regulatory region of the dnaN gene by screening DNA restriction fragments for promoter activity, S1 mapping of mRNAs, deletion analysis, and in vivo dnaN complementation tests. There are at least three dnaN promoters located in the second half of the dnaA coding region. The one closest to the dnaN structural gene is the weakest, but it provides sufficient dnaN expression for complementation when the gene is present on a multicopy plasmid. Deletion of sequences needed for initiation of dnaN translation or introduction of nonsense codons into dnaN causes reduction of recF expression. However, a deletion inactivating dnaN without changing the reading frame of the gene does not affect expression of the recF gene. These results indicate that the dnaN and recF genes are organized in an operon. We have previously shown the presence of termination signals within the dnaN coding region (Armengod, M.E., and Lambíes, E. (1986) Gene (Amst.) 43, 183-196). Therefore, we propose that the polarity produced by nonsense mutations in dnaN is primarily transcriptional. The uncoupling of transcription and translation of the dnaN gene (when translation is interrupted by premature nonsense codons or by other mechanisms) probably results in transcription termination at termination signals in dnaN.