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Chick-mouse heterokaryons were obtained by UV-Sendai virus-induced fusion of chick erythrocytes with thymidine (dT) kinase-deficient mouse fibroblast [LM(TK(-))] cells. Autoradiographic studies demonstrated that 1 day after fusion, [(3)H]dT was incorporated into both red blood cell and LM(TK(-)) nuclei of 23% of the heterokaryons. Self-fused LM(TK(-)) cells failed to incorporate [(3)H]dT into nuclear DNA. 15 clonal lines of chick-mouse somatic cell hybrids [LM(TK(-))/CRB] were isolated from the heterokaryons by cultivating them in selective hypoxanthine-aminopterin-thymidine-glycine medium. LM(TK(-)) and chick erythrocytes exhibited little, if any, cytosol dT kinase activity. In contrast, all 15 LM(TK(-))/CRB lines contained levels of cytosol dT kinase activity comparable to that found in chick embryo cells. Disk polyacrylamide gel electrophoresis and isoelectric focusing analyses demonstrated that the LM(TK(-))/CRB cells contained chick cytosol, but not mouse cytosol dT kinase. The LM(TK(-))/CRB cells also contained mouse mitochondrial, but not chick mitochondrial dT kinase. Hence, the clonal lines were somatic cell hybrids and not LM(TK(-)) cell revertants. The experiments demonstrate that chick erythrocyte cytosol dT kinase can be activated in heterokaryons and in hybrid cells, most likely as a result of functions supplied by mouse fibroblast cells.

Distinctive thymidine (dT) kinase molecular forms are present in mouse, human, and monkey mitochondria. Disk polyacrylamide gel electrophoresis (disk PAGE) analyses have shown that the mitochondrial-specific dT kinases differ from cytosol dT kinases in relative electrophoretic mobilities (Rm). Furthermore, the mouse mitochondrial dT kinase differs in Rm value from primate mitochondrial dT kinases. The mouse and primate cytosol dT kinases can also be distinguished. Disk PAGE analyses have been carried out on the cytosol and mitochondrial dT kinases of human-mouse (WIL-8) and monkey-mouse (mK.CV(III)) somatic cell hybrids in order to learn whether the mitochondria of the hybrid cells contained murine mitochondrial-specific, primate mitochondrial-specific, or both dT kinases. WIL-8 cells were derived from cytosol dT kinase-negative, mitochondrial dT kinase-positive mouse fibro blasts and from cytosol dT kinase-positive, mitochondrial dT kinase-positive human embryonic lung cells; they contained mostly mouse chromosomes and a few human chromosomes, including the determinant for human cytosol dT kinase. The mK.CV(III) cells were derived from cytosol dT kinase-negative, mitochondrial dT kinase-positive mouse kidney cells and from cytosol dT kinase-positive, mitochondrial dT kinase-positive monkey kidney cells; they contained mostly mouse chromosomes and a few monkey chromosomes, including the determinant for monkey cytosol dT kinase. Disk PAGE analyses demonstrated that the mitochondria of human-mouse and monkey-mouse somatic cell hybrids contained the mouse-specific mitochondrial dT kinase but not the human- or monkey-specific mitochondrial dT kinase. These findings suggest that primate cytosol and mitochondrial thymidine kinase genes are coded on different chromosomes.