벼에서 histone demethylase로 작용하는 jumonji C domain 단백질에 관한 연구

Abstract

Histone methylation is important in the epigenetic regulation of gene expression. Reversible methylation/demethylation of several histone lysine residues is mediated by distinct histone methyltransferases and histone demethylases. Jumonji proteins are involved in histone demethylations. Plant jumonji homologues have important functions in epigenetic processes, gene expression and development, and are essential in the interplay between histone modifications and DNA methylation. There are 21 and 20 JmjC domain-containing proteins in Arabidopsis and rice, respectively. They are classified into five groups, where each group demethylates a specific substrate. In this study we focused on the KDM5/JARID1 group proteins in rice: OsJMJ703, OsJMJ704 and OsJMJ708. Human KDM5/JARID1 group proteins are active histone demethylases using H3K4me3, H3K4me2 and H3K4me1 as substrates. Mutations of OsJMJ703 displayed pleiotropic phenotypes. To determine whether OsJMJ703 is an active demethylase, in vivo and in vitro enzymatic activity assays were performed. Those analyses showed that OsJMJ703 has histone H3K4 demethylase activity. Using a combination of ChIP-seq and RNA-seq analyses, the non-LTR (long terminal repeat) retrotransposable elements Karma and LINE1 were identified as direct targets of OsJMJ703. Transposable elements (TEs) are ubiquitous in plant genomes and often account for significant fractions of the nuclear DNA. Despite their wide distribution and abundance, very few TEs have been found to be transpositional, indicating that TEs activities may be tightly controlled by the host genome to minimize the potentially mutagenic effects associated with active transposition. Consistent with this notion, a growing body of evidence suggests that epigenetic silencing pathways such as DNA methylation, RNA interference and H3K9me2 function collectively to repress TEs activity at the transcriptional and posttranscriptional levels. Here we show that the rice protein OsJMJ703 is an active H3K4-specific demethylase required for TEs silencing. OsJMJ704 shares considerable homology with OsJMJ703 and contains exactly the same domain architecture as OsJMJ703. Interestingly, a null mutant in OsJMJ704 did not show any visible phenotype. This indicates that OsJMJ704 is a duplicated form of OsJMJ703 and play redundant roles. OsJMJ708 mutants displayed phenotypes of dwarf, short day-specific late flowering and brittle culm. OsJMJ708 showed diurnal rhythms and acts up stream of OsGI as a flowering activator. Understanding functional and biochemical roles of rice Jmj C proteins will open new insight into chromatin regulations by histone demethylases.