Nhp6 facilitates Aft1 binding and Ssn6 recruitment, both essential for FRE2 transcriptional activation

We found Nhp6a/b yeast HMG-box chromatin-associated architectural factors and Ssn6 (Cyc8) corepressor to be crucial transcriptional coactivators of FRE2 gene. FRE2 encoding a plasma membrane ferric reductase is induced by the iron-responsive, DNA-binding, transcriptional activator Aft1. We have shown that Nhp6 interacts directly with the Aft1 N-half, including the DNA-binding region, to facilitate Aft1 binding at FRE2 UAS. Ssn6 also interacts directly with the Aft1 N-half and is recruited on FRE2 promoter only in the presence of both Aft1 and Nhp6. This Nhp6/Ssn6 role in Aft1-mediated transcription is FRE2 promoter context specific, and both regulators are required for activation-dependent chromatin remodeling. Our results provide the first in vivo biochemical evidence for nonsequence-specific HMG-box protein-facilitated recruitment of a yeast gene-specific transactivator to its DNA target site and for Nhp6-mediated Ssn6 promoter recruitment. Ssn6 has an explicitly coactivating role on FRE2 promoter only upon induction. Therefore, transcriptional activation in response to iron availability involves multiple protein interactions between the Aft1 iron-responsive DNA-binding factor and global regulators such as Nhp6 and Ssn6.