Fragiadakis George S, Tzamarias Dimitris, Alexandraki Despina
Institute of Molecular Biology and Biotechnology-FORTH, University of Crete, Vassilika Vouton, Heraklion, Crete, Greece.
EMBO J. 2004 Jan 28;23(2):333-42. doi: 10.1038/sj.emboj.7600043. Epub 2004 Jan 22.
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.
我们发现Nhp6a/b酵母HMG盒染色质相关结构因子和Ssn6（Cyc8）共抑制因子是FRE2基因至关重要的转录共激活因子。编码质膜铁还原酶的FRE2由铁响应性、DNA结合转录激活因子Aft1诱导产生。我们已经表明，Nhp6直接与Aft1的N端半段相互作用，包括DNA结合区域，以促进Aft1在FRE2上游激活序列（UAS）处的结合。Ssn6也直接与Aft1的N端半段相互作用，并且仅在Aft1和Nhp6同时存在时才被募集到FRE2启动子上。Nhp6/Ssn6在Aft1介导的转录中的作用是FRE2启动子背景特异性的，并且这两种调节因子都是激活依赖性染色质重塑所必需的。我们的结果首次提供了体内生化证据，证明非序列特异性HMG盒蛋白促进酵母基因特异性反式激活因子募集到其DNA靶位点，以及Nhp6介导的Ssn6启动子募集。Ssn6仅在诱导时才在FRE2启动子上具有明确的共激活作用。因此，响应铁可用性的转录激活涉及Aft1铁响应性DNA结合因子与全局调节因子（如Nhp6和Ssn6）之间的多种蛋白质相互作用。
