Transcription of PHO5 is strongly regulated in response to the level of inorganic phosphate (Pi) present in the growth medium. We have identified elements required for PHO5 expression by analyzing small deletions in the PHO5 promoter on chromosome II. The results reveal three functionally different components of the PHO5 promoter: regulatory regions, a "TATA" element, and specific mRNA initiation sites. The regulatory regions contain related 19-base-pair (bp) dyad sequences acting as phosphate-controlled upstream activation sites (UASpS). These UASpS mediate the transcriptional activation of PHO5 observed in low Pi conditions. The unlinked but coordinately regulated PHO11 promoter contains a single copy of an almost identical dyad sequence, suggesting that there is a common regulatory UASp for both genes. A TATA element is absolutely required for detectable PHO5 transcription. Specific purine-pyrimidine motifs (RRYRR) (R = purine and Y = pyrimidine) serve as PHO5 mRNA initiation sites, but only if they lie 55-110 bp downstream of a functional TATA element. Such an "initiation window" is not found in higher eukaryotes and implies mechanistic differences in the transcription machineries between yeast and higher eukaryotes.
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