PHO5 encodes the major phosphate-regulated secreted acid phosphatase in budding yeast (5, 1). It is highly expressed under low-phosphate conditions and repressed when phosphate is abundant. There are two similarly regulated phosphatases, Pho10p and Pho11p, that supply less than 10% of secreted acid phosphatase activity when cells are starved for phosphate (6). One other acid phosphatase, Pho3p, is regulated in the opposite manner, being induced under high-phosphate conditions (7). The acid phosphatases are secreted from the cell and are predominantly found in the periplasmic space (7). Phosphatases and phosphate transporters such as Pho84p help maintain the appropriate level of bioavailable phosphate in the cell.

PHO5 regulation has been extensively researched (5). Under high-phosphate conditions, PHO5 is inactive and its promoter is protected by four positioned nucleosomes (6, 7). When phosphate is depleted, Pho4p, a basic helix-loop-helix transcription factor, and Pho2p, a homeodomain protein, act cooperatively to bind the PHO5 promoter and activate PHO5 transcription (2, 1). Pho4p activity is regulated by the Pho80p(cyclin)/Pho85p(cyclin-dependent kinase) complex, which is in turn regulated by Pho81p, a cyclin-dependent kinase inhibitor (8, 9).

Last updated: 1999-09-01