SUMMARY PARAGRAPH for PHO5 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 homeodo