Intracellular and Extracellular Cyclic Nucleotides in Wild-Type and White Collar Mutant Strains of Neurospora crassa: Temperature Dependent Efflux of Cyclic AMP from Mycelia

Cyclic AMP and cyclic GMP were released into the growth medium of mycelia of Neurospora crassa wild-type strains St.L.74A and Em5297a and by white collar-1 and white collar-2 mutant strains. After growth for 6 days at 18°C, there were 2.19 (St.L.74A), 5.83 (Em5297a), 1.38 (white collar-1), and 1.10 (white collar-2) nanomoles of cyclic AMP per gram dry weight of mycelia in the growth medium. These values corresponded to concentrations of cyclic AMP of between approximately 10 and 50 nanomolar. The corresponding values for extracellular cyclic GMP were typically less than 6% of the values for cyclic AMP. Following transfer to fresh medium, cyclic AMP efflux was demonstrated for each of the strains, and the amount of cyclic AMP exported into the fresh medium was greater at 25°C than 6°C. Intracellular cyclic AMP and cyclic GMP were also measured in each of the strains. The values for cyclic AMP were in the same range as those in the literature (approximately 0.5 to 1.5 nanomoles per gram dry weight of mycelia). However, the corresponding intracellular cyclic GMP values were less than 1% of the cyclic AMP values, i.e. more than 50 times lower than the value previously reported for the St.L.74A wild-type. Transfer of mycelia after 6 days at 18°C to fresh media and incubation for 2 hours at 25°C or 6°C did not consistently affect the intracellular level of cyclic AMP or cyclic GMP in the strains examined. We could detect no change in intracellular cyclic AMP when mycelia of the St.L.74A wild-type strain were irradiated with blue light for periods of up to 3.0 hours at 18°C, or in cyclic AMP and cyclic GMP for irradiation times of up to 1 minute at 6°C. We propose that the plasma membrane of Neurospora crassa is permeable to cyclic nucleotides, and the export of cyclic nucleotides into the growth medium may be a means of regulating intracellular levels. We conclude that three factors that affect carotenogenesis in Neurospora crassa (blue light, temperature, and the white collar mutations) have no appreciable effect on the total measurable intracellular cyclic nucleotides in this organism. There was no extracellular or intracellular cyclic AMP or cyclic GMP in the crisp-1 mutant strain, which suggested either that adenylate cyclase (which is absent in crisp-1) catalyzes the synthesis of both cyclic AMP and cyclic GMP or that the crisp-1 mutation somehow results in a deficiency of two enzymes (adenylate and guanylate cyclase).