Intracellular mosaicism (nuclear-/mitochondrial+) for thymidine kinase in mouse l cells

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Abstract
An electron-microscope and ultracentrifuge examination of mitochondria and mitochondrial DNA from Cl-I-d cells has demonstrated that mitochondria possess the enzyme(s) necessary for incorporation of thymidine and its analogues. These cells lack the nuclear thymidine kinase and are routinely grown in high levels of 5-bromodeoxyuridine (Budr) to select against wildtype revenants. When these cells are grown in the presence of 5-bromodeoxyuridine the mitochondrial DNA incorporates the thymidine analogue into both strands of the DNA duplex while the nuclear DNA shows no incorporation. Approximately 13 % of the total thymidines of the M-DNA are replaced by 5-bromodeoxyuridine and the DNA exhibits the form expected for normal mouse mitochondrial DNA. If the cells are grown in normal medium the mitochondrial DNA regains its normal base composition. Comparison of mitochondrial morphology under these 2 conditions reveals that the presence of 5-bromodeoxyuridine results in a distortion of the cristae similar to the effect induced by chloramphenicol. It is hypothesized that the incorporated mutagen (5-bromodeoxyuridine) may be altering the protein products of mitochondrial DNA. The results suggest (1) the existence of a separate and distinct thymidine kinase in the mitochondria of mouse L cells and (2) mutagenic but reversible effects of Budr incorporated into mitochondrial DNA.