The processed pseudogene of mouse thymidine kinase is active after transfection

Aside of the gene coding for cytoplasmic thymidine kinase, the genome of mouse cells carries two pseudogenes. Both are inactive in situ. One of the pseudogenes is a processed pseudogene in which a two base pair deletion caused a shift of the reading frame and a shortening of the gene product from the 233 amino acids of thymidine kinase to 177 amino acids in the pseudogene product. We report here that introduction of this pseudogene into LTK- cells gave rise to cells with a thymidine kinase positive phenotype. The transformed cells carried multiple copies of the pseudogene the upstream region of which exhibited low but measurable promoter activity. Replacement of the upstream region of the pseudogene by a promoter of Simian virus 40 or of the mammary tumor virus resulted in high transfection efficiencies and in cell lines exhibiting high thymidine kinase activities.