The Swi2/Snf2 Bromodomain Is Required for the Displacement of SAGA and the Octamer Transfer of SAGA-acetylated Nucleosomes

Salma Awad,Philippe Prochasson,Ahmed H. Hassan

doi:10.1074/jbc.m602851200

Abstract

The SWI/SNF and SAGA chromatin-modifying complexes contain bromodomains that help anchor these complexes to acetylated promoter nucleosomes. To study the importance of bromodomains in these complexes, we have compared the chromatin-remodeling and octamer-transfer activity of the SWI/SNF complex to a mutant complex that lacks the Swi2/Snf2 bromodomain. Here we show that the SWI/SNF complex can remodel or transfer SAGA-acetylated nucleosomes more efficiently than the Swi2/Snf2 bromodomain-deleted complex. These results demonstrate that the Swi2/Snf2 bromodomain is important for the remodeling as well as for the octamer-transfer activity of the complex on H3-acetylated nucleosomes. Moreover, we show that, although the wild-type SWI/SNF complex displaces SAGA that is bound to acetylated nucleosomes, the bromodomain mutant SWI/SNF complex is less efficient in SAGA displacement. Thus, the Swi2/Snf2 bromodomain is required for the full functional activity of SWI/SNF on acetylated nucleosomes and is important for the displacement of SAGA from acetylated promoter nucleosomes.

Highlights

Accessibility of transcriptional factors to promoters in eukaryotes often requires modification of chromatin structure, which is accomplished by the action of multisubunit chromatin-modifying complexes
The Swi2/Snf2 Bromodomain Is Not Required for Activity of the SWI/ SNF Complex on Unmodified Nucleosomes—To directly test whether the bromodomain within the Swi2/Snf2 subunit of the SWI/SNF complex contributes to the functional activity of the complex on nucleosomes, we purified wild-type SWI/SNF as well as SWI/SNF from a strain lacking the Swi2/Snf2 bromodomain and tested them in chromatin-remodeling and octamer-transfer assays
The results show that removing the SAGA complex after it has acetylated the template has resulted in a moderate increase in the activity of the SWI/SNF complexes (Fig. 5B, compare lanes 3 and 4 to lanes 1 and 2), suggesting that SAGA when stabilized on acetylated templates can block binding and activity of the SWI/SNF complex to some extent

Summary

Introduction

Accessibility of transcriptional factors to promoters in eukaryotes often requires modification of chromatin structure, which is accomplished by the action of multisubunit chromatin-modifying complexes. 10 ng of the 32P-labeled GUB, naked DNA template, which was used as an octamer acceptor in this assay, was incubated with ϳ10 ng of donor short oligonucleosomes (SONs) in the presence of wild-type or ⌬bromodomain mutant SWI/ SNF and ATP for 2 h at 30 °C in the same binding buffer as before.

Results

Conclusion