Preserved polyadenylated ribonucleic acid in dormant conidia of Neurospora crassa and new RNA synthesis during spore germination

Molecular hybridization with cloned cDNAs established that transcripts for specific proteins were present in the RNA of dormant conidiospores of Neurospora crassa. Among these were transcripts for the proteolipid subunit of the mitochondrial ATPase-ATP synthase; a larger proportion of the proteolipid transcripts was present in the nonpolyadenylated rather than the polyadenylated RNA fraction (relative to the pattern of germinated spores). The mRNA encoding the Neurospora M_r 83,000 heat-shock protein was also preserved in the dormant spores; however, the transcript for the M_r 30,000 heat-shock protein was not detectable. The polyadenylated RNA from dormant conidia was translatable in vitro; specific immunoprecipitation and electrophoresis assays showed that this preserved RNA included transcripts for nucleus-encoded subunit peptides of the mitochondrial enzymes cytochrome c oxidase and the ATPase-ATP synthase. As determined by molecular hybridization, conidia began to accumulate RNA for the ATPase proteolipid subunit immediately upon their suspension in incubation medium, with a doubling of the quantity of these specific transcripts by 15 min. Total RNA content of the spores dropped sharply during the first 30 min of germination, but by 45 min the amount of total RNA began to increase. Two inhibitors of RNA synthesis, lomofungin and proflavine-SO₄, inhibited spore germination by about 90%, in addition to inhibiting incorporation of [³H]uridine into RNA. This inhibition of spore germination by proflavine-SO₄ required that the spores be exposed to the drug during the first 60 min after inoculation into nutrient medium; if the inhibitor was added after 60 min of incubation, the spores germinated with normal kinetics. Dormant conidia, therefore, contain a population of preserved mRNA, but the translation of this preserved mRNA is not sufficient for spore germination; new gene transcription appears to be required for the resumption of protein synthesis and germ tube emergence.