Spermidine determines the sensitivity to the calmodulin antagonist, chlorpromazine, for the circadian conidiation rhythm but not for the mycelial growth in Neurospora crassa.

The gene that suppresses the phenotype of the cpz-2 mutation, which results in changing the sensitivity to chlorpromazine in relation to mycelial growth and circadian rhythms, was cloned in Neurospora crassa. This gene is not the cpz-2 gene itself but rather is identical to the spe-3 gene that encodes spermidine synthase in Neurospora. The intracellular content of spermidine was lowered in the cpz-2 strain compared to that of the wild-type strain. By integration of the spe-3 gene or by the addition of spermidine into culture medium, the temperature sensitivity of mycelial growth was lost and the conidiation rhythm became sensitive to chlorpromazine in the cpz-2 strain, as was observed in the wild-type strain, but the hypersensitivity of mycelial growth on chlorpromazine in the cpz-2 strain was not affected. Therefore, it appears that spermidine determines only the sensitivity of the conidiation rhythm to chlorpromazine.