Operon structure of dnaT and dnaC genes essential for normal and stable DNA replication of Escherichia coli chromosome.

dnaT and dnaC of Escherichia coli, whose products are components of the primosome, encode genes essential for both normal chromosomal replication and stable DNA replication induced by SOS signals. dnaT and dnaC together with an unknown gene (P-18), which encodes an 18-kDa polypeptide present downstream of dnaC, are cotranscribed from a promoter located 104 base pairs upstream of dnaT. A portion of the transcripts are processed 16 or 27 base pairs upstream of the dnaC coding region and 20% of the transcripts extending through dnaC are terminated at a terminator 103 base pairs downstream of dnaC. The rest of the transcripts pass through this terminator and are terminated downstream of the P-18 gene. The predicted amino acid sequence of the P-18 protein indicate the presence of a 26-amino acid signal peptide at its N terminus, suggesting that this protein may be secreted into the membrane or a periplasmic space. Another gene, which encodes a 14-kDa hydrophobic protein (P-14), was identified upstream of dnaT. The P-14 gene is transcribed from an upstream promoter, and its transcript extends through dnaT and dnaC. These results indicate that dnaT and dnaC constitute an operon with two unknown genes whose products may be associated with a membrane.