The rate of protein synthesis depends on both the rate of initiation of translation and the rate of elongation of the peptide chain. The rate of initiation depends on the encountering rate between ribosomes and mRNA; this rate in turn depends on the concentration of ribosomes and mRNA. Thus, patterns of codon usage that increase transcriptional efficiency should increase mRNA concentration, which in turn would increase the initiation rate and the rate of protein synthesis. An optimality model of the transcriptional process is presented with the prediction that the most frequently used ribonucleotide at the third codon sites in mRNA molecules should be the same as the most abundant ribonucleotide at the third codon sites in mRNA molecules should be the same as the most abundant ribonucleotide in the cellular matrix where mRNA is transcribed. This prediction is supported by four kinds of evidence. First, A-ending codons are the most frequently used synonymous codons in mitochondria, where ATP is much more abundant than that of the three other ribonucleotides. Second, A-ending codons are more frequently used in mitochondrial genes than in nuclear genes. Third, protein genes from organisms with a high metabolic rate use more A-ending codons and have higher A content in their introns than those from organisms with a low metabolic rate.