Effects of Membrane ATPase Inhibitors on Light-Induced Phase Shifting of the Circadian Clock in Neurospora crassa

Effects of several membrane ATPase inhibitors on light-induced phase shifting of the circadian conidiation rhythm in Neurospora crassa were examined using mycelial discs in liquid culture. Suppression of phase shifting by the inhibitors was strongly dependent on the pH of the liquid medium in which the discs were cultured during the time from light-dark transition (beginning of free-run) to light irradiation. When discs were cultured in pH 6.7 medium, azide, the inhibitors of plasma membrane ATPase (diethylstilbestrol and N, N'-dicyclohexylcarbodiimide), and ethanol completely suppressed the effect of light on the clock. In contrast, mycelial discs cultured in pH 5.7 medium were fully phase-shifted by light in the presence of the same and even higher concentrations of the chemicals. However, sensitivity to light of the discs cultured in relatively acidic medium was eight times higher than that of the discs cultured at neutral pH. Oligomycin and venturicidin, inhibitors of mitochondrial ATPase, did not suppress phase shifting by light at either pH.