... triphosphate to the 5¢-hydroxyl group of thymidine, forming dTMP. Human TK1 iscytosolic and its activity is absent in resting cells, appears in late G1,increases in S phase coinciding with the increase ... origin [14], the feedback inhibitor dTTP wasbound in the substrate pocket, similar to the bindingof dTTP to the D. melanogaster multi-substratedeoxyribonucleoside kinase [16], despite the ... during the cell cycle, in close correlationwith the rate of DNA synthesis, with low dNTP levels in G1cells increasing during S phase, generally withdTTP being the most abundant and dGTP the...