Structural and functional analyses of nucleosomes containing histone variant H2A.Z have drawn a lot of interest over the past few years. Important work in budding yeast has shown that H2A.Z (Htz1)-containing nucleosomes are specifically located on the promoter regions of genes, creating a specific chromatin structure that is poised for disassembly during transcription activation. The SWR1 complex is responsible for incorporation of Htz1 into nucleosomes through ATP-dependent exchange of canonical H2A-H2B dimers for Htz1-H2B dimers. Interestingly, the yeast SWR1 complex is functionally linked to the NuA4 acetyltransferase complex in vivo. NuA4 and SWR1 are physically associated in higher eukaryotes as they are homologous to the TIP60/p400 complex, which encompasses both histone acetyltransferase (Tip60) and histone exchange (p400/Domino) activities. Here we present work investigating the impact of NuA4-dependent acetylation on SWR1-driven incorporation of H2A.Z into chromatin. Using in vitro histone exchange assays with native chromatin, we demonstrate that prior chromatin acetylation by NuA4 greatly stimulates the exchange of H2A for H2A.Z. Interestingly, we find that acetylation of H2A or H4 N-terminal tails by NuA4 can independently stimulate SWR1 activity. Accordingly, we demonstrate that mutations of H4 or H2A N-terminal lysine residues have similar effects on H2A.Z incorporation in vivo, and cells carrying mutations in both tails are nonviable. Finally, depletion experiments indicate that the bromodomain-containing protein Bdf1 is important for NuA4-dependent stimulation of SWR1. These results provide important mechanistic insight into the functional cross-talk between chromatin acetylation and ATP-dependent exchange of histone H2A variants.Genetic information within the eukaryotic cell nucleus is organized in a highly conserved structural polymer, chromatin, which supports and controls crucial functions of the genome. Chromatin remodeling and post-translational modifications of histones are critical processes regulating genome expression and maintenance by controlling access to DNA and signaling local regions for specific molecular interactions (1, 2). Incorporation of different histone variants in specific chromosomal regions is also known to be associated with important biological processes controlling gene expression and genome integrity (3).A specific histone variant, H2A.Z, has been the focus of intense research over the past few years (4 -6). Delineation of its impact on nucleosome structure and stability and on gene expression has turned into a long-heated debate involving apparently contradicting experimental data. However, recent findings led to an emerging model that could partly reconcile these contradictions (7). Genome-wide analyses of H2A.Z localization in eukaryotes indicate that it is preferentially found on gene promoter/regulatory regions within nucleosomes flanking more accessible DNA sequences (8 -14). Importantly, H2A.Z is found within the promoters of inactive or weakly transcribed ge...

The NuA4 histone acetyltransferase complex is required for gene regulation, cell cycle progression, and DNA repair. Dissection of the 13-subunit complex reveals that the Eaf7 subunit bridges Eaf5 with Eaf3, a H3K36me3-binding chromodomain protein, and this Eaf5/7/3 trimer is anchored to NuA4 through Eaf5. This trimeric subcomplex represents a functional module, and a large portion exists in a native form outside the NuA4 complex. Gene-specific and genome-wide location analyses indicate that Eaf5/7/3 correlates with transcription activity and is enriched over the coding region. In agreement with a role in transcription elongation, the Eaf5/7/3 trimer interacts with phosphorylated RNA polymerase II and helps its progression. Loss of Eaf5/7/3 partially suppresses intragenic cryptic transcription arising in set2 mutants, supporting a role in nucleosome destabilization. On the other hand, loss of the trimer leads to an increase of replicationindependent histone exchange over the coding region of transcribed genes. Taken together, these results lead to a model where Eaf5/7/3 associates with elongating polymerase to promote the disruption of nucleosomes in its path, but also their refolding in its wake.

The ribosomal protein RACK1 is required for microRNA function in both C. elegans and humansRACK1, a constituent of the ribosomal 40S subunit, is required for the association of miRISC with translating ribosomes. This suggests that RACK1 contributes to recruit miRISC to the site of translation and supports a post-initiation mode of miRNA-mediated gene repression.