Amino acid composition is correlated with protein abundance in Escherichia coli: can this be due to optimization of translational efficiency?

Amino acid occurrence frequencies were found for four groups of Escherichia coli proteins with different abundance levels in the cell. These frequencies decrease with increasing protein abundance for amino acids whose codons are translated by tRNAs present at low concentrations (e.g., Cys, Trp, Ser, etc.); the opposite tendency was observed for amino acids translated by abundant tRNAs (Lys, Val, etc.). The efficiency (rate and accuracy) of codon translation is expected to be proportional to the concentration of the cognate tRNA. Therefore, the observed constraints on amino acid composition may be explained as resulting from evolutionary pressure optimizing the translational efficiency of a gene (the same pressure is responsible for the nonrandom choice of synonymous codons).