RESUMEN
Heritable, but reversible, changes in transposable element activity were first observed in maize by Barbara McClintock in the 1950s. More recently, transposon silencing has been associated with DNA methylation, histone H3 lysine-9 methylation (H3mK9), and RNA interference (RNAi). Using a genetic approach, we have investigated the role of these modifications in the epigenetic regulation and inheritance of six Arabidopsis transposons. Silencing of most of the transposons is relieved in DNA methyltransferase (met1), chromatin remodeling ATPase (ddm1), and histone modification (sil1) mutants. In contrast, only a small subset of the transposons require the H3mK9 methyltransferase KRYPTONITE, the RNAi gene ARGONAUTE1, and the CXG methyltransferase CHROMOMETHYLASE3. In crosses to wild-type plants, epigenetic inheritance of active transposons varied from mutant to mutant, indicating these genes differ in their ability to silence transposons. According to their pattern of transposon regulation, the mutants can be divided into two groups, which suggests that there are distinct, but interacting, complexes or pathways involved in transposon silencing. Furthermore, different transposons tend to be susceptible to different forms of epigenetic regulation.
Asunto(s)
Arabidopsis/genética , Elementos Transponibles de ADN , Regulación de la Expresión Génica de las Plantas , Genes de Plantas , Histonas/metabolismo , ARN Interferente Pequeño/genética , Adenosina Trifosfatasas/genética , Proteínas de Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Proteínas Argonautas , Cromatina/metabolismo , Inmunoprecipitación de Cromatina , Biología Computacional , ADN (Citosina-5-)-Metiltransferasas/genética , Metilación de ADN , Cartilla de ADN/química , Proteínas de Unión al ADN/metabolismo , ADN-Citosina Metilasas , Silenciador del Gen , N-Metiltransferasa de Histona-Lisina/genética , Modelos Genéticos , Mutación , Sistemas de Lectura Abierta , Reacción en Cadena de la Polimerasa , Interferencia de ARN , Factores de Transcripción/metabolismo , Transcripción GenéticaRESUMEN
The Arabidopsis gene DDM1 is required to maintain DNA methylation levels and is responsible for transposon and transgene silencing. However, rather than encoding a DNA methyltransferase, DDM1 has similarity to the SWI/SNF family of adenosine triphosphate-dependent chromatin remodeling genes, suggesting an indirect role in DNA methylation. Here we show that DDM1 is also required to maintain histone H3 methylation patterns. In wild-type heterochromatin, transposons and silent genes are associated with histone H3 methylated at lysine 9, whereas known genes are preferentially associated with methylated lysine 4. In ddm1 heterochromatin, DNA methylation is lost, and methylation of lysine 9 is largely replaced by methylation of lysine 4. Because DNA methylation has recently been shown to depend on histone H3 lysine 9 methylation, our results suggest that transposon methylation may be guided by histone H3 methylation in plant genomes. This would account for the epigenetic inheritance of hypomethylated DNA once histone H3 methylation patterns are altered.