Abstract
Heterochromatin is an inert structure in the genome and composed of mainly remnants of transposons and repetitive elements. In Arabidopsis, heterchromatin regions are present at around centromeres (pericentromeric regions) and a region on the short arm of chromosome 4 (heterochromatin knob). Here, by using a genome tiling array, we found that a subset of heterochromatin loci was silenced by the action of Morpheus' molecule 1 (MOM1), an epigeneic regulator for transcriptional gene silencing. Most of the up-regulated loci in the mom1 mutant carried sequences related to transposable elements and those showing similarity to a gypsy-type retrotransposon family (Athila) were predominantly activated in mom1. A locus that was unrelated to transposons but flanked by short tandem repeats was also up-regulated in mom1. The results suggest the presence of an unknown level of regulatory network maintaining the silent state of repetitive sequences in the genome.
