The stress response in yeast cells is regulated by at least two types of transcriptional activators: heat shock factor (HSF) and the partially redundant Msn2 and Msn4 (Msn2/4) activators (7
). The HSF system is highly conserved in its overall composition and function from yeast to humans (61
). HSF binds to the major groove of DNA in heat shock promoter elements (HSEs), which are also conserved from yeast to humans (61
). The activity of HSF is regulated via several distinct pathways. These include a monomer-trimer transition (45
), phosphorylation, and other posttranslational modifications (30
), as well as repression by molecular chaperones interacting with HSF, thus blocking their own production (45
) and forming a self-regulatory loop.
The Msn2/4 system is more specific to Saccharomyces cerevisiae
. The Msn2 and Msn4 factors recognize and bind stress response element (STRE) sequences (44
) in promoter regions of a large array of genes partially overlapping the HSF-regulated array (3
). The Msn2 factor seems to have a more pronounced role, since mutants lacking only Msn2 have an already distinct decrease in STRE-regulated transcription; however, only MSN2
double deletions exhibit pleiotropic stress sensitivity (19
). Under stress, Msn2/4 accumulate in the nucleus within a few minutes (24
). The Msn2/4 factors are regulated by efficient and oscillatory nuclear transport (32
), hyperphosphorylation upon stress (21
), and degradation associated with transcription initiation (37
). While the HSF system is actively involved in chromatin remodeling events at gene promoters, the role of the Msn2/4 system in these processes is poorly understood.
Chromatin remodeling varies in intensity and intermediate states between genes. The most evident and intense examples of chromatin remodeling are the changes taking place at the promoters of certain heat shock genes during heat induction. Chromatin remodeling at the HSP12
, and SSA4
) surpasses in rate and intensity such canonical models as the PHO5
promoters. At the HSP82
promoter, a noticeable two- to fourfold nucleosome diminishment is observed in the first seconds of heat shock which by 8 to 16 min can reach 15- to 20-fold (14
). The kinetic profiles and the intensities of nucleosome displacement vary considerably even between the highly related and coregulated heat shock genes, indicating the possibility of different chromatin remodeling mechanisms. The involvement of the SWI/SNF complex in chromatin remodeling at the HSP82
promoter has been reported previously (65
), but the deletion of the SNF2 activity has not been shown to affect nucleosome displacement. Therefore, the function of the SWI/SNF complex at heat shock gene promoters remains unclear.
The SWI/SNF complex is one of the major ATP-dependent chromatin remodeling protein complexes in eukaryotic cells directly affecting expression of a large number of genes. Consequently, there has been a longstanding interest in investigations of the mechanisms of recruitment and action of this complex. The SWI/SNF complex is composed of 12 subunits (40
), with the Swi2/Snf2 subunit serving as an ATPase, required for providing energy for disruption and/or mobilization of nucleosomes (8
). The mechanistic aspects of SWI/SNF action in vivo are not well defined. Some early data suggest a nontargeted mode of action determining the accessibility of promoter cis
elements for transcription activators (11
), while more recent studies argue for the targeted recruitment of ATP-dependent chromatin remodeling complexes by activation domains of activators causing nucleosome translocations (4
). The prevailing point of view is that due to the low abundance of the SWI/SNF complex in the cell and its interaction with nucleosomal DNA without sequence specificity, the SWI/SNF complex needs to be targeted prior to stimulating local nucleosome sliding or displacement in trans
In the present study, we compared events taking place at three different heat shock gene promoters during the time course of heat induction and found that the involvement of the SWI/SNF complex in chromatin remodeling at these promoters differs drastically. This involvement is exhibited in the effects caused by the inactivation of the SNF2 subunit, which either eliminates (HSP12) or delays (HSP82 and SSA4) chromatin remodeling at the studied heat shock gene promoters. While the kinetic profiles of nucleosome displacement are promoter specific, the rate of reoccupation appears to be the same for all promoters analyzed and is delayed to the same degree in the ΔSNF2 strain. Additionally we show that Msn2/4 activators, which dominate the HSP12 promoter, are likely under the control of targeted degradation associated with transcription initiation complex assembly. This regulatory mechanism is likely responsible for the attenuation of transcription of Msn2/4-regulated genes during the loss of induction stimuli.