Transcriptional activators function in vivo via binding sites that may be packaged into chromatin. Here we show that whereas the transcriptional activator GAL4 is strongly able to perturb chromatin structure via a nucleosomal binding site in yeast, GCN4 does so poorly. Correspondingly, GCN4 requires assistance from an accessory protein, RAP1, for activation of the HIS4 promoter, whereas GAL4 does not. The requirement for RAP1 for GCN4-mediated HIS4 activation is dictated by the DNA-binding domain of GCN4 and not the activation domain, suggesting that RAP1 assists GCN4 in gaining access to its binding site. Consistent with this, overexpression of GCN4 partially alleviates the requirement for RAP1, whereas HIS4 activation via a weak GAL4 binding site requires RAP1. RAP1 is extremely effective at interfering with positioning of a nucleosome containing its binding site, consistent with a role in opening chromatin at the HIS4 promoter. Furthermore, increasing the spacing between binding sites for RAP1 and GCN4 by 5 or 10 bp does not impair HIS4 activation, indicating that cooperative protein-protein interactions are not involved in transcriptional facilitation by RAP1. We conclude that an important role of RAP1 is to assist activator binding by opening chromatin.

Transcriptional activators are believed to work in part by recruiting general transcription factors, such as TATA-binding protein (TBP) and the RNA polymerase II holoenzyme. Activation domains also contribute to remodeling of chromatin in vivo. To determine whether these two activities represent distinct functions of activation domains, we have examined transcriptional activation and chromatin remodeling accompanying artificial recruitment of TBP in yeast (Saccharomyces cerevisiae). We measured transcription of reporter genes with defined chromatin structure by artificial recruitment of TBP and found that a reporter gene whose TATA element was relatively accessible could be activated by artificially recruited TBP, whereas two promoters, GAL10 and CHA1, that have accessible activator binding sites, but nucleosomal TATA elements, could not. A third reporter gene containing the HIS4 promoter could be activated by GAL4-TBP only when a RAP1 binding site was present, although RAP1 alone could not activate the reporter, suggesting that RAP1 was needed to open the chromatin structure to allow activation. Consistent with this interpretation, artificially recruited TBP was unable to perturb nucleosome positioning via a nucleosomal binding site, in contrast to a true activator such as GAL4, or to perturb the TATA-containing nucleosome at the CHA1 promoter. Finally, we show that activation of the GAL10 promoter by GAL4, which requires chromatin remodeling, can occur even in swi gcn5 yeast, implying that remodeling pathways independent of GCN5, the SWI-SNF complex, and TFIID can operate during transcriptional activation in vivo.Transcriptional activators are thought to stimulate transcription of TATA-containing promoters in part by recruiting TFIID, a multiprotein complex consisting of the TATA-binding protein (TBP) and TBP-associated factors (TAFs), to the TATA box (25,60). Several in vitro and in vivo studies support this model. For example, transcription initiated at a mutated TATA element by induced synthesis of TBP with altered specificity was enhanced in both rate and extent in the presence of an activator that could bind upstream of the relevant promoter, consistent with activator-mediated recruitment (40). Recruitment has also been inferred from the results of "activator bypass" experiments in which artificial recruitment of TBP to promoter sites near the TATA element resulted in transcriptional activation, implying that TBP recruitment is a rate-limiting step in transcriptional activation in vivo (10,39,79). Most convincingly, chromatin immunoprecipitation experiments revealed TBP to be physically associated with promoters of numerous genes under activated, but not nonactivated, conditions, implying that recruitment accompanies activation (43,46).A potential obstacle to recruitment of TBP in vivo is posed by chromatin. Binding of TBP to nucleosomal TATA elements is greatly impeded in vitro (23, 32). Transcription is also strongly repressed by chromatin, but this repression can be largely alleviated by binding of act...

Repressor activator protein 1 (RAP1) assists GCN4-mediated HIS4 activation by overcoming some repressive aspect of chromatin structure to facilitate GCN4 binding. RAP1 also participates in other nuclear processes, and discrete domains of RAP1 have been shown to have specific properties including DNA binding, DNA bending, transcriptional activation, and silencing and telomere functions. To investigate whether specific domains of RAP1 are required to "open" chromatin and help GCN4 to activate the HIS4 gene, we examined the abilities of different truncated RAP1 proteins to perturb positioned nucleosomes via a nucleosomal RAP1 site in a yeast episome in vivo, and we tested HIS4 activation in yeast strains harboring truncated RAP1 mutants. We found that neither the DNA bending domain nor the putative activation domain of RAP1 is required for its ability to perturb the chromatin structure of a plasmid containing a RAP1 site. Similarly, neither the putative activation domain nor the N-terminal DNA-bending domain was required for GCN4-mediated activation of HIS4. We also used a rap1 ts mutant to show that continuous occupancy of the HIS4 promoter by RAP1 is required for GCN4-mediated gene activation. Repressor activator protein 1 (RAP1)1 is an essential protein in yeast. Binding sites for RAP1 have been found in promoters, silencers, and telomeres, and correspondingly, RAP1 participates in gene transcription, silencing, and telomere maintenance (1). RAP1 has been implicated in transcriptional activation of many genes including the mating-type genes MAT␣1 and MAT␣2, ribosomal protein genes, and glycolytic genes. RAP1 also contributes to a meiotic recombination at the HIS4 locus (2), and a cluster of RAP1 binding sites from the TEF1 promoter has recently been shown to function as a boundary element that can prevent the spread of silent chromatin (3).The ability of RAP1 to play such disparate roles in yeast depends in part on its ability to interact with a variety of other proteins. In many cases, the sites of interaction have been mapped, and a domain structure for RAP1 has been constructed based on these and other findings (Fig.