Only some codons and amino acids respond to GC content. (a) Plot of codon frequency within coding sequences versus total GC content, for the arginine codons CGA (white squares) and CGC (black circles) in bacteria and archaea. Linear regression lines are shown in black for CGC and gray for CGA. (b) A similar plot for the amino acids threonine (white squares) and arginine (black circles) in bacteria and archaea. The plots show that whereas CGC and arginine clearly correlate with GC content, CGA and threonine do not. The three relevant parameters for the response, slope, intercept and correlation coefficient, are all highly correlated with each other (see Table 1).