H3K4 methylation is an epigenetic modification associated with gene activation. The COMPASS-like complex contains three core subunits and associates with methyltransferases such as Arabidopsis trithorax 1 and 2 (ATX1 and ATX2) or ATX-related 3 (ATXR3). These methyltransferases methylate H3K4 one- to three-fold, thus generating mono-, di- or trimethylated H3K4. In animals, several mechanisms have been described as to how COMPASS complexes are guided to the correct genomic loci, involving either proteins that recognize DNA elements, or non-coding RNA. For plants, it remains unknown as to how the COMPASS-like complex is recruited to targets on the DNA.
The researchers found that the histone demethylase JMJ28 interacts with RbBP5-like (RBL), one of the core components of the COMPASS-like complex, via its C-terminal region. JMJ28 then binds specific DNA sequences through its WRC domain. ChIP-Seq experiments confirmed that JMJ28 bound to euchromatic DNA regions enriched in activating H3K4me3 and H3K4me2 marks and depleted of repressive H3K9me2 marks. ATX1 and ATX2 were co-purified with JMJ28 and DNA targets between ATX2 and JMJ28 overlap, suggesting that JMJ28 interacts with a COMPASS-like complex associated with ATX1/2 methyltransferases. Interestingly, in the jmj28 mutant background, only specific COMPASS target genes are depleted in H3K4me3 and therefore have a reduced expression level, while other target genes are not affected. The observation that jmj28 mutants display increased resistance to two fungal pathogens suggests that the JMJ28-specific COMPASS targets may be involved in immune suppression.
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