Abstract: The PHO regulatory pathway is involved in the acquisition of phosphate (P(i)) in the yeast Saccharomyces cerevisiae. When extracellular P(i) concentrations are low, several genes are transcriptionally induced by this pathway, which includes the Pho4 transcriptional activator, the Pho80-Pho85 cyclin-CDK pair, and the Pho81 CDK inhibitor. In an attempt to identify all the components regulated by this system, a whole-genome DNA microarray analysis was employed, and 22 PHO-regulated genes were identified. The promoter regions of 21 of these genes contained at least one copy of a sequence that matched the Pho4 recognition site. Eight of these genes, PHM1-PHM8, had no previously defined function in phosphate metabolism. The amino acid sequences of PHM1 (YFL004w), PHM2 (YPL019c), PHM3 (YJL012c), and PHM4 (YER072w) are 32-56% identical. The phm3 and phm4 single mutants and the phm1 phm2 double mutant were each severely deficient in accumulation of inorganic polyphosphate (polyP) and P(i). The phenotype of the phm5 mutant suggests that PHM5 (YDR452w) is essential for normal catabolism of polyP in the yeast vacuole. Taken together, the results reveal important new features of a genetic system that plays a critical role in P(i) acquisition and polyP metabolism in yeast.