Disruption of the plant gene MOM releases transcriptional silencing of methylated genes

Epigenetic modifications change transcription patterns in multicellular org anisms to achieve tissue-specific gene expression and inactivate alien DNA such as transposons or transgenes(1,2). In plants and animals, DNA methylat ion is involved in heritability and flexibility of epigenetic states(3), al though its function is far from clear. We have isolated an Arabidopsis gene , MOM, whose product is required for the maintenance of transcriptional gen e silencing. Mutation of this gene or depletion of its transcript by expres sion of antisense RNA reactivates transcription from several previously sil ent, heavily methylated loci. Despite this, the dense methylation at these reactivated loci is maintained even after nine generations, indicating that transcriptional activity and methylation pattern are inherited independent ly. The predicted MOM gene product is a nuclear protein of 2,001 amino acid s containing a region similar to part of the ATPase region of the SWI2/SNF2 family, members of which are involved in chromatin remodelling(4). MOM is the first known molecular component that is essential for transcriptional g ene silencing and does not affect methylation pattern. Thus, it may act dow nstream of methylation in epigenetic regulation, or be part of a new pathwa y that does not require methylation marks.