Synonymous codon usage, accuracy of translation, and gene length in Caenorhabditis elegans

In many unicellular organisms, invertebrates, and plants, synonymous codon usage biases result from a coadaptation between codon usage and tRNAs abund ance to optimize the efficiency of protein synthesis. However, it remains u nclear whether natural selection acts at the level of the speed or the accu racy of mRNAs translation. Here we show that codon usage can improve the fi delity of protein synthesis in multicellular species. As predicted by the m odel of selection for translational accuracy, we find that the frequency of codons optimal for translation is significantly higher at codons encoding for conserved amino acids than at codons encoding for nonconserved amino ac ids in 548 genes compared between Caenorhabditis elegans and Homo sapiens. Although this model predicts that codon bias correlates positively with gen e length, a negative correlation between codon bias and gene length has bee n observed in eukaryotes. This suggests that selection for fidelity of prot ein synthesis is not the main factor responsible for codon biases. The rela tionship between codon bias and gene length remains unexplained. Exploring the differences in gene expression process in eukaryotes and prokaryotes sh ould provide new insights to understand this key question of codon usage.