Codon discrimination due to presence of abundant non-cognate competitive tRNA

It has been thought that preferential use of synonymous codons provides high efficiency and fidelity of protein synthesis through specific codon-anticodon interactions. In yeast genes, some codon boxes seem to prefer a codon which is unsuited for its cognate anticodon. Now, we propose that codon usage biases may arise due to presence of abundant non-cognate competitive tRNA capable of misreading a codon by C-U or G-U pairing in the middle position.