Evidence for Use of Rare Codons in the dnaG Gene and Other Regulatory Genes of Escherichia coli

Amino acid sequence and composition data of Escherichia coli dnaG primase protein and its tryptic peptides have confirmed that the dnaG gene contains an unusually high number of codons that are not frequently used in most E. coli genes. In 25 E. coli proteins analyzed the codons AUA, UCG, CCU, CCC, ACG, CAA, AAT, and AGG are infrequently used, occurring as 4% of the total codons in the reading frame and 11% and 10% in the nonreading frames. In dnaG they occur as 11% in the reading frame and 12% in the nonreading frames. The rpsU and rpoD genes, which flank the dnaG gene [Smiley, B. L., Lupski, J. R., Svec, P. S., McMacken, R. & Godson, G. N. (1982) Proc. Natl. Acad. Sci. USA 79, 4550-4554], however, have normal codon usage. Translational modulation using isoaccepting tRNA availability may therefore be part of the mechanism of keeping the dnaG gene expression low, while expression of the adjacent rpsU and rpoD genes on the same mRNA transcript is high.