Abstract
Vertebrate immunoglobulin (Ig) mRNAs reveal intraspecies variation in codon usage distinct from that seen with yeast or bacterial genes. Comparison of all available Ig gene sequences shows that %(G+C) in codon position III is consistently lower in variable (V) segments than in constant (C) segments. I find an even lower %(G+C) in the hypervariable domains of V segments. This analysis suggests that base substitution in Ig genes correlates positively with local A+T content.