Identification of chick thymidine kinase determinant in somatic cell hybrids of chick erythrocytes and thymidine kinase-deficient mouse cells

Disc polyacrylamide gel electrophoresis (disc PAGE) analyses of chick-mouse somatic cell hybrids LM(TK^?)/CRB]isolated from fusion mixtures of chick erythrocytes and thymidine (TdR) kinase-deficient mouse LM(TK^?)]cells have demonstrated that the somatic cell hybrids contain only chick cytosol TdR kinase F and mouse mitochondrial TdR kinase A activities. Karyotypes were analysed by the method which sequentially reveals Q- and C-bands. Four hybrid clones contained the full complement of mouse chromosomes and 1 to 3 chick micro-chromosomes. Counterselection of the LM(TK^?)/CRB hybrids in 5-bromodeoxyuridine (BUdR) medium resulted in the loss of chick cytosol TdR kinase F activity and at least one of the chick chromosomes, but mouse mitochondrial TdR kinase A activity was unaffected. Unlike the LM(TK^?)/CRB somatic cell hybrids, the BUdR-resistant clones could not grow in HATG (hypoxanthine-aminopte-rin-thymidine-glycine) medium. The results demonstrate that: (1) the chick cytosol TdR kinase F gene is on a member of the micro-chromosomes; and (2) selection in HATG- and BUdR-containing medium involves only cytosol TdR kinase F.