ACQUISITION OF CHICK CYTOSOL THYMIDINE KINASE ACTIVITY BY THYMIDINE KINASE-DEFICIENT MOUSE FIBROBLAST CELLS AFTER FUSION WITH CHICK ERYTHROCYTES

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Abstract
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, [3H]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 [3H]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.