Evolution of herpesvirus thymidine kinases from cellular deoxycytidine kinase

The thymidine kinases encoded by herpesviruses of higher vertebrates form a distinct group and are unrelated to the thymidine kinases (TKs) of other organisms. Their evolutionary source has not been identified, but our analysis has revealed a clear relationship with a sequence of human deoxycytidine kinase (dCK) published recently. We report the sequence of the putative TK of channel catfish virus, a herpesvirus of a lower vertebrate, and show that it is also related to dCK. We propose, therefore, that the TKs of herpesviruses of higher and lower vertebrates have evolved, either independently or successively, from a cellular dCK.