Pulse-labeled Components in the Replication of Mitochondrial Deoxyribonucleic Acid

Mouse L cells lacking the cytoplasmic thymidine kinase (EC 2.7.1.21) retain a mitochondrial specific thymidine kinase. This results in exclusive labeling of mitochondrial DNA when the cells are grown in the presence of radioactive thymidine. Pulse-labeling experiments demonstrate that the replicating forms of mitochondrial DNA contain a substantial fraction of pulse-label. A surprisingly large fraction of the pulse-label is contained in nonreplicating closed circular DNA and is not covalently bound to the closed circular molecules. This suggests a repetitive synthesis of the initiation sequence in mitochondrial DNA replication.