Influence of mutational bias in codon usage and amino-acid usage. (a) Percentage use of AT-rich codons in the four marine picocyanobacteria. Amino acids are ranked according to AT content of their respective codons. Methionine and tryptophan, which are both encoded by only one codon, have been discarded from the analysis. (b) Percentage use of amino acids in marine picocyanobacteria.