MOM1 mediates DNA-methylation-independent silencing of repetitive sequences in Arabidopsis.

The heterochromatic regions around centromeres of animal and plant chromosomes are composed of tandem repetitive sequences, interspersed with transposons and transposon derivatives. These sequences are largely transcriptionally silent and highly methylated, and are associated with specifically modified histones. Although embedded in heterochromatin, Arabidopsis 5S ribosomal RNA genes are among the most highly transcribed genes. However, some 5S genes are silenced, and we show here that this silencing can be suppressed by a reduction in CG methylation. Importantly, we show that mutation of MORPHEUS' MOLECULE 1 (MOM1) releases 5S repeat silencing independently of chromatin properties, as illustrated by the absence of detectable alteration of DNA and histone H3 methylation patterns. MOM1 also prevents transcription of 180-bp satellite repeats and 106B dispersed repeats but not of transposons. Our results provide evidence that transcription of densely methylated and highly repetitive heterochromatic sequences is controlled by two distinct epigenetic silencing pathways, one dependent on and the other independent of DNA methylation.