Inactivation of PPX1 and PPN1 genes encoding exopolyphosphatases of Saccharomyces cerevisiae does not prevent utilization of polyphosphates as phosphate reserve.

Cytosol polyphosphates (polyPs) are the main phosphate (P(i)) reserve in the yeast Saccharomyces cerevisiae. In this work, the participation of cytosol polyPs and exopolyphosphatases in maintenance of P(i) homeostasis under P(i) deficit in the cultivation medium has been studied in different strains of S. cerevisiae. The growth of yeast strains with inactivated genes PPX1 and PPN1 encoding the yeast exopolyphosphatases and a strain with double mutations in these genes in a P(i)-deficient medium is not disturbed. All the studied strains are able to maintain relatively constant P(i) levels in the cytosol. In P(i)-deficient medium, polyP hydrolysis in the cytosol of the parent and PPN1-deficient strains seems to be performed by exopolyphosphatase Ppx1 and proceeds without any change of the spectrum of polyP chain lengths. In the PPX1-deficient strain, long-chain polyPs are depleted first, and only then short-chain polyPs are hydrolyzed. In the double PPX1 and PPN1 mutant having low exopolyphosphatase activity, polyP hydrolysis in the cytosol starts with a notable delay, and about 20% of short-chain polyPs still remain after the polyP hydrolysis in other strains has almost been completed. This fact suggests that S. cerevisiae possesses a system, which makes it possible to compensate for inactivation of the PPX1 and PPN1 genes encoding exopolyphosphatases of the yeast cells.

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