Distinctive structural basis for DNA recognition by the fission yeast Zn₂Cys₆ transcription factor Pho7 and its role in phosphate homeostasis

Pho7, a member of the Zn₂Cys₆ family of fungal transcription factors, is the key transcriptional activator underlying fission yeast phosphate homeostasis, a physiological response to phosphate starvation in which the pho1, pho84 and tgp1 genes are upregulated. Here, we delineated aminimized 61-amino-acid Pho7 DNA-binding domain (DBD) and determined the 1.7 °A crystal structure of the DBD at its target site in the tgp1 promoter. Two distinctive features of the Pho7 DBD are: It binds DNA as a monomer, unlike most other fungal zinc-cluster factors that bind as homodimers; and it makes extensive interactions with its asymmetric target sequence over a 14-bp footprint that entails hydrogen bonding to 13 individual bases within, and remote from, the CGG triplet typically recognized by other Zn₂Cys₆ DBDs. Base pair substitutions at Pho7 sites in the tgp1 and pho1 promoters highlight the importance of the 5'-CGG triplet for Pho7 binding in vitro and Pho7-dependent gene expression in vivo. We identify several DBD amino acids at which alanine substitution effaced or attenuated the pho1 phosphate starvation response and concordantly reduced Pho7 binding to a pho1 promoter site.