Transcriptional regulation of the yeast PHO8 promoter in comparison to thecoregulated PHO5 promoter

Expression of the PHO8 and PHO5 genes that encode a nonspecific alkaline an d acid phosphatase, respectively, is regulated in response to the Pi concen tration in the medium by the same transcription factors. Upon induction by phosphate starvation, both promoters undergo characteristic chromatin remod eling, yet the extent of remodeling at the PHO8 promoter is significantly l ower than at PHO5. Despite the coordinate regulation of the two promoters, the PHO8 promoter is almost 10 times weaker than PHO5. Here we show that of two Pho4 binding sites that had been previously mapped at the PHO8 promote r in vitro, only the high affinity one, UASp2, is functional in vivo. Activ ation of the PHO8 promoter is partially Pho2-dependent. However, unlike at PHO5, Pho4 can bind strongly to its binding site in the absence of Pho2 and remodel chromatin in a Pho2-independent manner. Replacement of the inactiv e UASp1 element by the UASp1 element hom the PHO5 promoter results in more extensive chromatin remodeling and a concomitant a-fold increase in promote r activity. In contrast, replacement of the high affinity UASp2 site with t he corresponding site from PHO5 precludes chromatin remodeling completely a nd as a consequence promoter activation, despite efficient binding of Pho4 to this site. Deletion of the promoter region normally covered by nucleosom es -3 and -2 results in a a-fold increase in promoter activity, further sup porting a repressive role of these nucleosomes. These data show that there can be strong binding of Pho4 to a UAS element without any chromatin remode ling and promoter activation. The close correlation between promoter activi ty and the extent of chromatin disruption strongly suggests that the low le vel of PHO8 induction in comparison with PHO5 is partly due to the inabilit y of Pho4 to achieve complete chromatin remodeling at this promoter.