ARGONAUTE4 Control of Locus-Specific siRNA Accumulation and DNA and Histone Methylation

Proteins of the ARGONAUTE family are important in diverse posttranscriptional RNA-mediated gene-silencing systems as well as in transcriptional gene silencing in Drosophila and fission yeast and in programmed DNA elimination in Tetrahymena . We cloned ARGONAUTE4 ( AGO4 ) from a screen for mutants that suppress silencing of the Arabidopsis SUPERMAN ( SUP ) gene. The ago4-1 mutant reactivated silent SUP alleles and decreased CpNpG and asymmetric DNA methylation as well as histone H3 lysine-9 methylation. In addition, ago4-1 blocked histone and DNA methylation and the accumulation of 25-nucleotide small interfering RNAs (siRNAs) that correspond to the retroelement AtSN1 . These results suggest that AGO4 and long siRNAs direct chromatin modifications, including histone methylation and non-CpG DNA methylation.