Cell cycle-dependent regulation of thymidine kinase activity introduced into mouse LMTK- cells by DNA and chromatin-mediated gene transfer.

We report on the expression of thymidine kinase (tk) activity in synchronized populations of mouse cells that have been transformed to the tk+ phenotype with purified DNA from various sources or with metaphase chromosomes from human cells. The viral (herpes) tk gene is constitutively expressed but, in all other cases examined, the activity is regulated as in normal tk+ mouse cells: There is a dramatic increase at the beginning of the S-phase. This regulation is observed whether the transgenome is stably integrated into the host genome or whether it is still in an unstable nonintegrated state.