Persistence of Thymidine Kinase Activity in Mitochondria of a Thymidine Kinase-Deficient Derivative of Mouse L Cells

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Abstract
The cell line LM(TK - ) Cl 1D, a derivative of mouse L fibroblasts deficient in thymidine kinase (EC 2.7.1.21) that shows very little thymidine kinase activity in extracts of whole cells as compared to the parental line, and that does not incorporate thymidine or 5-bromodeoxyuridine into nuclear DNA, has maintained the capacity to incorporate these precursors into mitochondrial DNA at a substantial rate. The amount of [ methyl - 3 H]thymidine incorporated into mitochondrial DNA of LM(TK - ) Cl 1D cells after long-term labeling has been conservatively estimated in different experiments to be between 30 and 60% of that incorporated into mitochondrial DNA or nuclear DNA of strain A9, an L-cell derivative without any thymidine-kinase dificiency; by contrast, the incorporation of thymidine into nuclear DNA of Cl 1D cells is less than 1% of that in A9 cells. These results strongly suggest that the loss of thymidine kinase activity in the extramitochondrial compartment of LM(TK - ) Cl 1D cells has not been accompanied by the loss of the mitochondria-associated enzyme activity, pointing to a different genetic or epigenetic control of the extramitochondrial and mitochondrial enzymes.