REDUCED SYNONYMOUS SUBSTITUTION RATE AT THE START OF ENTEROBACTERIAL GENES

Synonymous codon usage is less biased at the start of Escherichia coli genes than elsewhere. The rate of synonymous substitution between E.c oli and Salmonella typhimurium is substantially reduced near the start of the gene, which suggests the presence of an additional selection p ressure which competes with the selection for codons which are most ra pidly translated. Possible competing sources of selection are the pres ence of secondary ribosome binding sites downstream from the start cod on, the avoidance of mRNA secondary structure near the start of the ge ne and the use of sub-optimal codons to regulate gene expression. We p rovide evidence against the last of these possibilities. We also show that there is a decrease in the frequency of A, and an increase in the frequency of G along the E. coli genes at all three codon positions. We argue that these results are most consistent with selection to avoi d mRNA secondary structure.