Sequence analysis of the Escherichia coli dnaE gene

We have determined the sequence of a 4,350-nucleotide region of the Escherichia coli chromosome that contains dnaE, the structural gene for the alpha subunit of DNA polymerase III holoenzyme. The dnaE gene appeared to be part of an operon containing at least three other genes: 5'-lpxB-ORF23-dnaE-ORF37-3' (ORF, open reading frame). The lpxB gene encodes lipid A disaccharide synthase, an enzyme essential for cell growth and division (M. Nishijima, C.E. Bulawa, and C.R.H. Raetz, J. Bacteriol. 145:113-121, 1981). The termination codons of lpxB and ORF23 overlapped the initiation codons of ORF23 and dnaE, respectively, suggesting translational coupling. No rho-independent transcription termination sequences were observed. A potential internal transcriptional promoter was found preceding dnaE. Deletion of the -35 region of this promoter abolished dnaE expression in plasmids lacking additional upstream sequences. From the deduced amino acid sequence, alpha had a molecular weight of 129,920 and an isoelectric point of 4.93 for the denatured protein. ORF23 encoded a more basic protein (pI 7.11) with a molecular weight of 23,228. In the accompanying paper (D.N. Crowell, W.S. Reznikoff, and C.R.H. Raetz, J. Bacteriol. 169:5727-5734, 1987), the sequence of the upstream region that contains lpxA and lpxB is reported.