Identified a mechanism of SWR1-C recruitment to target genes in Arabidopsis

Researchers of the CBGP (UPM-INIA) from the ¨Molecular Bases of Plant Developmental Phase Transitions¨ group, in collaboration with three more research teams, have revealed a key role for the Arabidopsis homolog of the yeast SANT domain protein Swc4/Eaf2 in the recruitment of the SWR1 complex (SWR1-C) to the chromatin of target genes.

Eukaryotic organisms have a number of histone variants that perform specialized functions. Among them, H2A.Z is essential for cell viability in metazoans and plants and its deposition is mediated by the SWR1-C in regulatory regions of specific loci, where it is assumed to generate chromatin regions with particular structural characteristics that modulate transcriptional activation. But how the SWR1-C is specifically recruited to the chromatin of its target genes remains largely unknown in eukaryotes. In plants, the characterization of Arabidopsis mutations in homologs of the yeast SWR1-C has also revealed a role for H2A.Z exchange in different plant developmental processes. Nevertheless, the exact composition of the plant SWR1-C and the recruitment mechanisms to the chromatin of the target genes awaits to be established.

In this work the authors have provided evidence showing that Arabidopsis SWC4 is a SWR1-C- associated subunit. Interestingly, they have demonstrated that swc4-knock-out alleles are lethal, revealing that similarly to H2A.Z this gene is essential for embryo viability. Besides, SWC4 RNAi knockdown lines displayed pleiotropic phenotypic alterations in vegetative and reproductive traits, including acceleration of flowering time, indicating that SWC4 also controls post-embryonic processes. Transcriptomic analyses and genome-wide H2A.Z localization studies carried out have revealed that SWC4 negatively regulates the expression of, among others, the florigen gene FT and a subset of regulatory key transcription factors, mainly by modulating H2A.Z deposition. All SWC4 homologs bear an N-terminal SANT/Myb_DMAP1 domain that has been implicated in the interaction with DNA, histones and other proteins. All SANT domains have structural similarity to the DNA-binding domain of Myb-related proteins and importantly this report show that SWC4 is able to recognize preferentially AT-rich elements and that these motifs are over-represented in the promoters of a subset of genes where H2A.Z incorporation negatively modulates transcription.

These data hold an attractive model for SWR1-C targeting specific chromatin regions through SWC4 protein and represent the first evidence showing that a given SWC4 homolog binds DNA, unveiling a role for Arabidopsis SWC4 protein in the recruitment of the SWR1-C to target chromatin regions through the recognition of specific AT-rich DNA elements modulating H2A.Z deposition in key regulatory genes.