Codon-specific missense errors in vivo.

We have developed a simple method for measuring the missense substitution of amino acids at specified positions in proteins synthesized in vivo. We find that the frequency of cysteine substitution for the single arginine in Escherichia coli ribosomal protein L7/L12 is close to 10(-3) for wild-type bacteria, decreases to 4 x 10(-4) in streptomycin-resistant bacteria containing mutant S12 (rpsL), and is virtually unchanged in Ram bacteria containing mutant S4 (rpsD). We have also found that the frequency of the cysteine substitution for the single tryptophan in E. coli ribosomal protein S6 is 3-4 x 10(-3) for wild-type bacteria, decreases to 6 x 10(-4) in streptomycin-resistant bacteria and is elevated to nearly 10(-2) in Ram bacteria.