Mutational pressure in genomes of human α-herpesviruses
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Genomes of herpes simplex viruses (HSV1 and HSV2) possess highly GC-rich DNA (their G + C content is 0.68 and 0.70, respectively). This suggests a strong mutational GC pressure. The genome of varicella zoster virus (VZV) has a GC content of 0.46, which suggests a weak AT pressure. We have calculated the frequencies of nucleotide substitutions directed by mutational pressure for the genes encoding a major capsid protein (MCP) of human α-herpes viruses capable of infecting humans. For HSV1 the substitution frequencies were calculated from the MCP gene of HSV1 (G + C = 0.68, 3 GC = 0.89) to MCP gene of HSV2 (G + C = 0.70, 3 GC = 0.91) and to the homologous gene (G + C = 0.73, 3 GC = 0.99), which is phylogenetically related to HSV, i.e., primate herpes virus 16. In these two pairs of genes, transversions C → G and G → C were the most frequently observed. Less frequent were transitions in the direction of mutation GC pressure (T → C and A → G). Even less frequently, the transversions A → C and T → G were seen. For VZV, the substitution frequencies were calculated from an MCP gene of VZV (G + C = 0.47, 3GC = 0.41) to an MCP gene (G + C = 0.41, 3GC = 0.28) of the primate herpes virus 9 related to VZV. In this pair of genes, the most frequent transitions were seen in the direction of mutation AT pressure (C → T and G → A). The transversion frequencies of A → T and T → A were fairly lower; however, they exceeded the transversion frequencies of C → A and G → T. For the MCP gene of VZV, the probability of transition frequency induced by mutational pressure in the third codon positions (where the majority of transitions are synonymous) was 2.36-fold higher than in the MCP gene of HSV1 and 3-fold higher than in HSV2. These results tend to explain the rarity of recurrent zoster compared to herpes infection.
KeywordsVaricella Zoster Virus Codon Position Mutational Pressure Equine Herpesvirus Varicella Virus
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