In the nucleus, transcription factors must contend with the presence of chromatin in order to gain access to their cognate regulatory sequences. As most nuclear DNA is assembled into nucleosomes, activators must either invade a stable, preassembled nucleosome or preempt the formation of nucleosomes on newly replicated DNA, which is transiently free of histones. We have investigated the mechanism by which heat shock factor (HSF) binds to target nucleosomal heat shock elements (HSEs), using as our model a dinucleosomal heat shock promoter (hsp82-⌬HSE1). We find that activated HSF cannot bind a stable, sequence-positioned nucleosome in G 1 -arrested cells. It can do so readily, however, following release from G 1 arrest or after the imposition of either an early S-or late G 2 -phase arrest. Surprisingly, despite the S-phase requirement, HSF nucleosomal binding activity is restored in the absence of hsp82 replication. These results contrast with the prevailing paradigm for activator-nucleosome interactions and implicate a nonreplicative, S-phase-specific event as a prerequisite for HSF binding to nucleosomal sites in vivo.In the eukaryotic cell, nuclear DNA is packaged into a compact structure known as chromatin, a complex of DNA and histone proteins. The packaging of DNA into chromatin not only serves to confine the genome within the boundaries of the nucleus but also regulates the transcriptional activation of genes. The presence of nucleosomes, the individual subunits of chromatin, can inhibit the binding of sequence-specific activators to upstream elements, as well as impede access of the general transcription machinery to the core promoter (reviewed in reference 35). As a consequence, nucleosomes inhibit transcription, both in vitro (34,43,72) and in vivo (27,29,61). Thus, an important function of activators is to overcome nucleosomal repression, either by blocking nucleosome formation over promoters, thereby presetting genes for activation, or by remodeling preassembled nucleosomes as the initial step in transcriptional activation.The mechanisms by which activators recognize and bind their cognate sites within chromatin are varied. Certain factors, like NF-1 and GCN4, can access their sites only within nuclease-hypersensitive, nucleosome-rearranged regions (5,15,66,75). Others, such as PHO4, bind to accessible regions but, once DNA bound, actively remodel neighboring nucleosomes (1,19). Still others are capable of invading a stable nucleosome and binding to target sites wrapped around the nucleosome core. Glucocorticoid receptor, for example, invades a sequence-positioned nucleosome within the mouse mammary tumor virus promoter following exposure to hormone (5). Receptor binding to its cognate sequence leads to a dramatic reconfiguring of the underlying nucleosome (66) and occurs in the absence of replication (55). The ability of activators to invade a preassembled nucleosome may require the involvement of ATP-dependent chromatin remodeling complexes and/or histone acetyltransferases (reviewed in references 36...

In the nucleus, transcription factors must contend with the presence of chromatin in order to gain access to their cognate regulatory sequences. As most nuclear DNA is assembled into nucleosomes, activators must either invade a stable, preassembled nucleosome or preempt the formation of nucleosomes on newly replicated DNA, which is transiently free of histones. We have investigated the mechanism by which heat shock factor (HSF) binds to target nucleosomal heat shock elements (HSEs), using as our model a dinucleosomal heat shock promoter (hsp82-⌬HSE1). We find that activated HSF cannot bind a stable, sequence-positioned nucleosome in G 1 -arrested cells. It can do so readily, however, following release from G 1 arrest or after the imposition of either an early S-or late G 2 -phase arrest. Surprisingly, despite the S-phase requirement, HSF nucleosomal binding activity is restored in the absence of hsp82 replication. These results contrast with the prevailing paradigm for activator-nucleosome interactions and implicate a nonreplicative, S-phase-specific event as a prerequisite for HSF binding to nucleosomal sites in vivo.