FIG. 4.

Distribution and composition of mutations. Mutations are listed by region (a), amino acid substitution (b), all base changes (c), contribution of G-to-A transitions to frequent substitutions (d), and codon usage for the frequent Arg and Lys substitutions (e). In panel a, V region mutations are indicated by solid fill and C region mutations are indicated by lighter fill. In panel d, the number of amino acid changes is indicated in solid fill and the number generated by G-to-A mutations is indicated in lighter fill. Three E-to-R substitutions were generated by two base changes, one of which was G to A. Panel e shows the observed (solid fill) and expected (lighter fill) codon use for the frequent Arg and Lys substitutions. The expected codon use was calculated based on the minimum number of base changes required to encode Arg or Lys based on the original codon in the parental isolate.