ABSTRACT
The control of growth, cell division, and of the cell cycle is a subject which has interested biologists for a long time in connection with development, oncogenesis, and mechanisms programming decisions in living cells. The great majority of studies have dealt with growth control rather than with cell cycle control. The reason for this is mainly a practical one. Thymidine kinase (TK) is an enzyme of the salvage pathway of DNA precursor production. It phosphorylates thymidine at the expense of ATP. Since thymidine nucleotides are normally not produced by this reaction, but rather by methylation of uridine nucleotides by the enzyme thymidylate synthase, the physiological role of TK in normal cells is not understood. Upstream sequences containing the promoter for the TK genes were determined. A comparison of these sequences from genes of different origin shows little homology.
