The level of cytosine is usually much higher than the level of guanine in two-fold degenerated sites from third codon positions of genes from Simplex- and Varicelloviruses with G+C higher than 50%.

We studied usage of cytosine and guanine in 914 genes from completely sequenced genomes of five Simplex- and seven Varicelloviruses. In genes with total GC-content higher than 50% usage of cytosine is usually higher than usage of guanine (an average difference for genes with G+C higher than 70% reaches 4.0%). This difference is caused mostly by the elevated usage of cytosine in two-fold degenerated sites situated in third codon positions relatively to the usage of guanine in two-fold degenerated sites situated in third codon positions (an average difference for genes with G+C higher than 70% is equal to 28.2%). The usage of amino acids that are encoded by codons containing cytosine in two-fold degenerated sites situated in third codon positions (AA2TC) is much higher than the usage of amino acids encoded by codons containing guanine in two-fold degenerated sites situated in third codon positions (AA2AG). The usage of AA2AG declines much more steeply with the growth of GC-content than the usage of AA2TC. This effect is the consequence of the nature of genetic code and of the negative selection. In GC-rich genes the usage of cytosine in four-fold degenerated sites is only a little (but significantly) higher than the usage of guanine (in genes with G+C higher than 70% an average difference is equal to 4.3%). This difference may be caused by transcription-associated mutational pressure.