Mutations in the Arabidopsis H3K4me2/3 demethylase JMJ14 suppress posttranscriptional gene silencing by decreasing transgene transcription.

Posttranscriptional gene silencing (PTGS) mediated by sense transgenes (S-PTGS) results in RNA degradation and DNA methylation of the transcribed region. Through a forward genetic screen, a mutant defective in the Histone3 Lysine4 di/trimethyl (H3K4me2/3) demethylase Jumonji-C (JmjC) domain-containing protein14 (JMJ14) was identified. This mutant reactivates various transgenes silenced by S-PTGS and shows reduced Histone3 Lysine9 Lysine14 acetylation (H3K9K14Ac) levels, reduced polymerase II occupancy, reduced transgene transcription, and increased DNA methylation in the promoter region, consistent with the hypothesis that high levels of transcription are required to trigger S-PTGS. The jmj14 mutation also reduces the expression of transgenes that do not trigger S-PTGS. Moreover, expression of transgenes that undergo S-PTGS in a wild-type background is reduced in jmj14 sgs3 double mutants compared with PTGS-deficient sgs3 mutants, indicating that JMJ14 is required for high levels of transcription in a PTGS-independent manner. Whereas endogenous loci regulated by JMJ14 exhibit increased H3K4me2 and H3K4me3 levels in the jmj14 mutant, transgene loci exhibit unchanged H3K4me2 and decreased H3K4me3 levels. Because jmj14 mutations impair PTGS of transgenes expressed under various plant or viral promoters, we hypothesize that JMJ14 demethylation activity is prevented by antagonistic epigenetic marks specifically imposed at transgene loci. Removing JMJ14 likely allows other H3K4 demethylases encoded by the Arabidopsis thaliana genome to act on transgenes and reduce transcription levels, thus preventing the triggering of S-PTGS.

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