Karyotypic and enzymatic characteristics of a somatic hybrid cell line originating from dwarf hamsters.

YBRIDS between mammalian cells with known biochemical differences permit the study of the interactions of the two parental genomes in a single cell (EPHRUSSI 1965; HARRIS 1966). Resistance to 5-bromo-2’-deoxyuridine (BUdR) in mammalian cell lines was accompanied by an inability of the resistant cell to incorporate BUdR into DNA, presumably as a result o€ the simultaneous loss of thymidine kinase activity (DUBBS and KIT 1964; KIT et al. 1963, 1966; LITTLEPIELD 1964,1965). Following the intraspecific fusion of cells from a BUdRresistant cell line and an 8-azaguanine resistant line, hybrid cells were selected for their ability to utilize thymidine and hypoxanthine in the presence of aminopterine (LITTLEFIELD 1964). A wide range in the activities of thymidine kinase, low in the BUdR-resistant parent, and of guanylic-inosinic pyrophcsphorylase, low in the 8-azaguanine resistant parent, was observed in various hybrid clonal sublines. This suggested the presence of controls for these enzymes since some clones exhibited higher enzyme levels than those observed for the drug sensitive parent line. Thymidine kinase activity is subject to feedback control (BREITMAN 1963; BRESNICK and KARJOLA 1964; BRESNICK et al. 1964; IVES et al. 1963; OKAZAKI and KORNBERG 1964). The cell cycle variation of thymidine kinase activity in cell cultures (EKER 1965, 1966; KIT et al. 1965; LITTLEFIELD 1966; STUBBLEFIELD and MUELLER 1965; WEISSMAN, SMELLIE and PAUL 1960) and the elevated activity in regenerating rat liver following partial hepatectomy (BEHKI and MORGAN 1964; BIANCHI et al. 1962; BOJARSKI and HIATT 1960; BOLLUM and POTTER 1959; OKAZAKI and KORNBERG 1964; WEISSMAN, SMELLIE and PAUL 1960) suggest a control of the turnover rate of this enzyme. The repression of the expression of thymidine kinase activity of the SV40 genome in m K S mouse cells, resistant to BUdR, also supports the concept that the activity of this enzyme can be controlled (DUBBS et al. 1967). In the present studies viable somatic cell hybrids have been obtained between