CYTOSINE DEAMINASE GENE AS A POSITIVE SELECTION MARKER

Cytosine deaminase (EC 3.5.4.1), a non-mammalian enzyme, catalyzes the deamination of cytosine and 5-fluorocytosine to form uracil and 5-flu orouracil, respectively. Eukaryotic cells have been genetically modifi ed with a bacterial cytosine deaminase gene to express a functional en zyme, When the genetically modified cells are combined with 5-fluorocy tosine, it creates a potent negative selection system, which may have important applications in cancer gene therapy, In this paper, we intro duce a novel positive selection method based upon the expression of th e cytosine deaminase gene, This method utilizes inhibitors in the pyri midine de novo synthesis pathway to create a condition in which cells are dependent on the conversion of pyrimidine supplements to uracil by cytosine deaminase, Thus, only cells expressing the cytosine deaminas e gene can be rescued in a positive selection medium.