Codon and Amino Acid Usage in Retroviral Genomes Is Consistent with Virus-Specific Nucleotide Pressure

Retroviral RNA genomes are known to have a biased nucleotide composition. For instance, the plus-strand RNA of human immunodeficiency virus (HIV) is A-rich, and the genome of human T cell leukemia virus(HTLV) is C-rich, and other retroviruses have a U-rich or G-rich genome. The biased composition of these genomes is most likely caused by directional mutational pressure of the respective reverse transcriptase enzymes. Using a set of retroviral genomes with a distinct nucleotide composition, we performed skew analyses of the nucleotide bias along the complete viral genome. Distinct nucleotide signatures were apparent, and these typical patterns were generally conserved across the viral genome. Furthermore, it is demonstrated that this typical nucleotide bias, combined with a profound discrimination against the CpG dinucleotide sequence, strongly influences the codon usage of the retroviruses in a direct manner, and their amino acid usage in an indirect manner. The fact that both codon usage and amino acid usage are so closely entwined with the genome composition has important practical implications. For instance, the typical trends in nucleotide usage could influence the molecular phylogenetic reconstruction of the family Retroviridae.