PHM6 and PHM7 genes are essential for phosphate surplus in the cells of Saccharomyces cerevisiae

The cells of Saccharomyces cerevisiae are capable for phosphate surplus: the increased uptake of phosphate (Pi) and accumulation of inorganic polyphosphate (polyP) occur when the cells after Pi limitation were cultivated in a medium supplemented with Pi. We demonstrated that single knockout mutations in the PHO84, PHO87, and PHO89 genes encoding plasma membrane phosphate transporters suppressed the Pi uptake and polyP accumulation under phosphate surplus at nitrogen starvation. The knockout strains in the PHM6 and PHM7 genes encoding unannotated PHO-proteins showed decreased polyP accumulation under Pi surplus both at nitrogen starvation and in complete YPD medium. This is due to the suppression of Pi uptake in the cells of these mutant strains. We speculate that Pi transporters of plasma membrane, and Phm6 and Phm7 proteins function in concert providing increased Pi uptake at phosphate surplus conditions.

Purification and properties of recombinant exopolyphosphatase PPN1 and effects of its overexpression on polyphosphate in Saccharomyces cerevisiae

Inorganic polyphosphate performs many regulatory functions in living cells. The yeast exopolyphosphatase PPN1 is an enzyme with multiple cellular localization and probably variable functions. The Saccharomyces cerevisiae strain with overexpressed PPN1 was constructed for large-scale production of the enzyme and for studying the effect of overproduction on polyphosphate metabolism. The ΔPPN1 strain was transformed by the vector containing this gene under a strong constitutive promoter of glycerol aldehyde-triphosphate dehydrogenase of S. cerevisiae. Exopolyphosphatase activity in the transformant increased 28- and 11-fold compared to the ΔPPN1 and parent strains, respectively. The content of acid-soluble polyphosphate decreased ∼6-fold and the content of acid-insoluble polyphosphate decreased ∼2.5-fold in the cells of the transformant compared to the ΔPPN1 strain. The recombinant enzyme was purified. The substrate specificity, cation requirement, and inhibition by heparin were found to be similar to native PPN1. The molecular mass of a subunit (∼33 kD) and the amino acid sequence of the recombinant enzyme were the same as in mature PPN1. The recombinant enzyme was localized mainly in the cytoplasm (40%) and vacuoles (20%). The overproducer strain had no growths defects under phosphate deficiency or phosphate excess. In contrast to the parent strains accumulating polyphosphate, the transformant accumulated orthophosphate under phosphate surplus.