Induction of thymidylate synthetase activity in Tetrahymena by cyclic guanosine monophosphate

The induction of TMP synthetase activity in Tetrahymena pyriformis depended upon growth conditions. Enzymatic activity was low in cells grown in complex medium, and was high in cells grown in, or shifted to, defined medium. TMP synthetase activity rose 5 hours after the shift from complex to defined medium using uracil as the pyrimidine source. The time of induction was decreased to 3 1 2 hours using dUMP as the pyrimidine source. cGMP or its dibutyryl derivative, but not cAMP, caused the induction of TMP synthetase activity in cells grown in complex medium. Caffeine, but not theophylline, mimicked the cGMP response. cAMP decreased both the cGMP and caffeine mediated increases in TMP synthetase activity. This is the first demonstration of an effect of cGMP on induction of an enzyme of pyrimidine metabolism in any cellular system.