Changes in nucleotide pools during conidial germination in Neurospora crassa

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Abstract
The acid-soluble nucleotide pools of wild type and several adenine auxotrophs of N. crassa were studied immediately prior to and during conidial germination in the presence of adenine. A 2- to 4-fold increase in most nucleotide pools occurred after 6 h of germination at 33.degree. C indicating a general increase in nucleotide pools during this developmental period. The largest components of the acid-soluble nucleotide pools were uracil-containing nucleotide-sugars, which are precursors of chitin and glucan, the major constituents of cell walls. On removal of adenine, the UDP-sugar pools decreased significantly, in adenine auxotrophs, while the pools of UTP increased significantly. ATP levels increased .apprx. 2-fold in the first 6 h of germination. After 1 h without exogenous adenine, ATP dropped 2-fold or more in adenine auxotrophs but not in the wild type. There was a net decrease in all adenine nucleotide pools during adenine starvation and a much greater decrease occurred in adenine auxotrophs than in the wild type. The adenylate energy charge remained stable despite major changes in the adenylate pools. Accumulation of intermediates occurred in germinated conidia from purine auxotrophs blocked at various steps in the purine pathway. IMP accumulated in ungerminated and in starved conidia of the adenine-8 (ad-8) strain. Ungerminated conidia of the ad-5 strain contained a large pool of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) while the ad-1 strain had smaller amounts of AICAR, but significantly more than any other strain tested. The AICAR pools disappeared from ad-5 and ad-1 in the presence of 50 mg histidine/100 ml. Similarly the IMP pools in ad-8 decreased markedly in the presence of histidine, indicating that the contribution from the histidine biosynthetic pathway to purine nucleotide formation is significant.