Cell cycle-dependent binding of yeast heat shock factor to nucleosomes

In the nucleus, transcription factors must contend with the presence of chr omatin in order to gain access to their cognate regulatory sequences. As mo st 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 inv estigated the mechanism by which heat shock factor (HSF) binds to target nu cleosomal heat shock elements (HSEs), using as our model a dinucleosomal he at shock promoter (hsp82-Delta HSE1). We find that activated HSF cannot bin d a stable, sequence-positioned nucleosome in G(1)-arrested cells. It can d o so readily, however, following release from G(1) arrest or after the impo sition of either an early S- or late G(2)-phase arrest. Surprisingly, despi te the S-phase requirement, HSF nucleosomal binding activity is restored in the absence of hsp82 replication. These results contrast with the prevaili ng paradigm for activator-nucleosome interactions and implicate a nonreplic ative, S-phase-specific event as a prerequisite for HSF binding to nucleoso mal sites in vivo.