Nucleosome free DNA- and acetyl histone-binding components recruit SWR1 to yeast promoters for H2A.Z replacement

The histone variant H2A.Z is a genome-wide signature of nucleosomes proximal to eukaryotic regulatory DNA. While the multi-subunit SWR1 chromatin remodeling complex is known to catalyze ATP-dependent deposition of H2A.Z, the mechanism of recruitment to S. cerevisiae promoters has been unclear. A sensitive assay for competitive binding of di-nucleosome substrates revealed that SWR1 preferentially binds long nucleosome-free DNA adjoining core particles, allowing discrimination of gene promoters over gene bodies. We traced the critical DNA binding component of SWR1 to the conserved Swc2/YL1 subunit, whose activity is required for both SWR1 binding and H2A.Z incorporation in vivo. Histone acetylation by NuA4 enhances SWR1 binding, but the interaction with nucleosome-free DNA is the major determinant. ‘Hierarchical cooperation’ between high affinity DNA- and low affinity histone modification-binding factors may reconcile the large disparity in affinities for chromatin substrates, and unify classical control by DNA-binding factors with post-translational histone modifications and ATP-dependent nucleosome mobility. Swr1 TAP IF of various mutants