Restoration of hypoxanthine phosphoribosyl transferase activity in mouse 1R cells after fusion with chick-embryo fibroblasts.

Fusion of the 1R mouse cell, which lacks activity of hypoxanthine phosphoribosyl transferase (EC 2.4.2.8), with chick-embryo fibroblasts yielded progeny cells that survived in hypoxanthine-aminopterin-thymidine selective medium. This property and the failure of the progeny to survive in 8-azaguanine indicated that hypoxanthine phosphoribosyl transferase activity was present. Electrophoretic analysis revealed that the enzyme was of mouse, not chick, origin. These observations are consistent with the operation of a regulator gene responsible for the absence of hypoxanthine phosphoribosyl-transferase activity in the 1R cell and its presence in the progeny.