The A+T-rich genome of Herpesvirus saimiri contains a highly conserved gene for thymidylate synthase.

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Abstract
Herpesvirus saimiri (HVS) is the prototype member of a distinctive subset of lymphotropic herpesviruses (the .gamma.2 subgroup) with A + T-rich coding sequences. In this paper, we show that cells productively infected with HVS contain high concentrations of a virus-specified thymidylate synthase, (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45); we identify the active polypeptide and present the sequence of the virus gene. The predicted amino acid sequence of the 294-residue subunit of the virus enzyme is 70% homologous with the sequence of the human enzyme and about 50% homologous with prokaryotic thymidylate synthases, illustrating the remarkable structural constraints imposed by the thymidylate synthase function. However, the presence of the enzyme is not a conserved property of herpesviruses. We find no evidence for a virus-encoded thymidylate synthase activity (or a homology to a thymidylate synthase sequence) in G + C-rich representatives of .alpha.1 (e.g., herpes simplex viruses, 66-68% G + C), .beta.(i.e., human cytomegalovirus, 58-59% G + C), and .gamma.1 (i.e., Epstein-Barr virus, 60% G + C) herpesvirus subgroups. The production of excess thymidylate by a virus thymidylate synthase in cells infected with an A + T-rich herpesvirus would provide one plausible source of biased mutations by the virus-encoded replicative enzymes, which we have previously suggested as the likely general cause of differences in the mean nucleotide compositions of herpesvirus genomes.