Figure 5.

Chromatin modifications on HSP genes after binding HSF1 and TAC1 in heat shocked cells. Heat shock-induced modification of lysines within tail residues of histones H3 and H4, within nucleosomes (each containing histones H1, H2A, H2B, H3, H4) on chromatin may alter the accessibility of factors to HSP genes, provide binding sites for nucleasome modeling proteins and lead to transcriptional initiation and elongation. Histone acetylases and methyltransferases may be recruited during stress by the binding of HSF1 to HSE on the promoter as well as TAC1 association with downstream residues 3’ to the promoter. In the hsp70 promoter, histone H3 may be targeted by HATs of the SAGA protein complex family and H4 by CREB binding protein (CBP). Histone acetylation may permit binding of the SWI/SNF complex to the promoter and subsequent nucleosome remodeling. In the downstream ORF region, binding of the TAC1 complex after heat shock leads to methylation of histone H3 on lysine 4 (MeK4) and acetylation of H4 on lysine 9 (AcK9). This may facilitate the release of promoter proximal paused PolII in a process that involves interaction of Pol II with P-TEFb, phosphorylation and transcriptional elongation. We depict the activated Pol II complex negotiating a nucleosomes during elongation, a process that may be facilitated by lysine modifications induced by heat shock.