ARABIDOPSIS TRITHORAX-RELATED7 Is Required for Methylation of Lysine 4 of Histone H3 and for Transcriptional Activation of FLOWERING LOCUS C    

Abstract

In the winter-annual accessions of Arabidopsis thaliana, presence of an active allele of FRIGIDA (FRI) elevates expression of FLOWERING LOCUS C (FLC), a repressor of flowering, and thus confers a vernalization requirement. FLC activation by FRI involves methylation of Lys 4 of histone H3 (H3K4) at FLC chromatin. Many multicellular organisms that have been examined contain two classes of H3K4 methylases, a yeast (Saccharomyces cerevisiae) Set1 class and a class related to Drosophila melanogaster Trithorax. In this work, we demonstrate that ARABIDOPSIS TRITHORAX-RELATED7 (ATXR7), a putative Set1 class H3K4 methylase, is required for proper FLC expression. The atxr7 mutation partially suppresses the delayed flowering of a FRI-containing line. The rapid flowering of atxr7 is associated with reduced FLC expression and is accompanied by decreased H3K4 methylation and increased H3K27 methylation at FLC. Thus, ATXR7 is required for the proper levels of these histone modifications that set the level of FLC expression to create a vernalization requirement in winter-annual accessions. Previously, it has been reported that lesions in ATX1, which encodes a Trithorax class H3K4 methylase, partially suppress the delayed flowering of winter-annual Arabidopsis. We show that the flowering phenotype of atx1 atxr7 double mutants is additive relative to those of single mutants. Therefore, both classes of H3K4 methylases appear to be required for proper regulation of FLC expression.

SET DOMAIN GROUP2 is the major histone H3 lysine 4 trimethyltransferase in Arabidopsis

“…These findings suggest that histone methylation may play broad and important roles in regulating transcription. Consistent with this notion, several Arabidopsis histone methyltransferase (HMTase) mutants have been found to exhibit a myriad of developmental abnormalities (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). Furthermore, transcriptome profiling studies have identified numerous genes that are misregulated in HMTase mutants (19,27,28).…”

“…This largely reflects the fact that the major H3K4me3 HMTase has yet to be identified. Mutants in three of the seven class III HMTase genes (ATX1/SDG27, ATX2/SDG30, ATXR7/SDG25) display locus-specific defects in H3K4me; however, the global levels of H3K4me3 in these mutants remain similar to that of wild type (25,26,28,35).…”

Physical coupling of activation and derepression activities to maintain an active transcriptional state at FLC

“…Local chromatin states are critical for quantitatively controlling FLC expression (9). Active FLC expression requires functional FRIGIDA (FRI) and a set of histone modifying Trithorax homologs, Complex Proteins Associated with Set1 (COMPASS), RNA polymerase II Associated Factor 1 complex (PAF1C), and SET DOMAIN GROUP 8 (SDG8), the major H3K36 methyltransferase (10)(11)(12)(13)(14)(15)(16)(17). Repression of FLC requires either the autonomous pathway, which coordinately influences transcriptional initiation and elongation with associated chromatin modulation, or vernalization, the cold-induced Polycomb silencing (18)(19)(20)(21).…”

Impact of nucleosome dynamics and histone modifications on cell proliferation during Arabidopsis development

“…ATXR7 is a Set1-related H3K4 methyltransferase that acts in combination with ATX1 to regulate various processes, for example, flowering (Tamada et al, 2009).…”