The adjacent dnaZ and dnaX genes of Escherichia coli are contained within one continuous open reading frame.

The dnaZ and dnaX loci of Escherichia coli have been genetically defined as separate genes, both of which are essential for DNA replication (1). The 2.1 kb region of DNA that complements mutations in both genes has a maximum coding capacity of approximately 80,000 daltons. Two protein products are produced from this region with molecular weights of 77,000 and 52,000 (2,3). We have sequenced a 2.7 kb fragment containing the dnaZ and dnaX genes and determined that it contains only one open reading frame of sufficient length to encode either of these proteins. This open reading frame may encode a protein of 71,147 daltons or of 68,451 daltons depending on which potential translational initiation codon is utilized. There are two transcriptional promoters preceding the gene as well as a ribosome binding site preceding the two potential initiation codons. Both the promoters and ribosome binding sites are predicted to be weak, perhaps contributing to the low expression of these genes.