REGULATION OF PURINE BIOSYNTHESIS IN G1 PHASE ARRESTED MAMMALIAN CELLS: 215

Abstract

The effects of G1 phase growth arrest on purine biosynthesis were studied in cultured S49 T lymphoma cells. Incubations of wildtype cells for 18 hr with dibutyryl cyclic AMP, which induces G1 arrest, reduced the rates of purine biosynthesis by 95%. Time course and concentration dependence studies indicated that the decrease in rates of purine biosynthesis correlated with the extent of G1 phase arrest. Similar studies with somatic cell mutants deficient in some component of cyclic AMP action or metabolism indicated that the depression in purine synthetic rates required G1 arrest and did not result from cell death. Rates of RNA and DNA synthesis were also markedly diminished in the growth arrested cells. Measurements of purine synthesis in the presence of azaserine indicated that there was an early block in purine biosynthesis prior to the formation of phosphoribosylformylglycinamide. Additionally, the activities of adenylosuccinate synthetase and IMP dehydrogenase were diminished in G1 arrested cells. The levels of all controlling enzymes, substrates, and cofactors, however, were similar in cycling and G1 arrested cells. Despite diminished rates of purine biosynthesis, the levels of intracellular nucleotides in G1 cells were not reduced. These results suggest that perturbations in the consumption of nucleotides via inhibition of nucleic acid synthesis have profound effects on the purine pathway and indicate the importance of feedback inhibition by nucleotides in the regulation of purine synthesis in situ.