DEPLETION AND REPLENISHMENT OF THE INORGANIC POLYPHOSPHATE POOL IN NEUROSPORA CRASSA

Abstract

Turnover of the inorganic polyphosphate pool of Neurospora crassa was demonstrated in both growing and nongrowing mycelium. In nitrogen-gamma deficient cultures, polyphosphate synthesis and degradation were in balance and no net changes occurred. When mycelium was suspended in a growth medium deficient in phosphate, polyphosphate was degraded to acid-soluble compounds, including adenosine triphosphate (ATP) and orthophosphate; net synthesis of nucleic acids and phospholipids occurred at the expense of polyphosphate. Various attempts were made to demonstrate direct formation of ATP from polyphosphate when oxidative and fermentative ATP generation were blocked. No evidence for this reaction could be obtained, suggesting that the primary product of polyphosphate degradation is not ATP but probably orthophosphate. Addition of phosphate to phosphate-starved mycelium induced rapid replenishment of the polyphosphate pool; polyphosphate accumulation ceased abruptly when the original level had been restored. The finding that ATP accumulation within the mycelium preceded polyphosphate synthesis, together with the rapid turnover of this compound, supports the view that ATP is the metabolic precursor of polyphosphate. The results suggest a cyclic pattern of polyphosphate metabolism. In N. crassa at least, polyphosphate does not appear to function as a reservoir of "high-energy" phosphate, but the polyphosphate cycle may be involved in the dissipation of excess ATP.