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From Proteopedia
Structural basis of transcriptional gene silencing mediated by Arabidopsis MOM1
Structural highlights
Function[MOM1_ARATH] Required for the heritable maintenance of transcriptional gene silencing (TGS) in heterochromatin, and particularly in the intermediate bivalent heterochromatin, characterized by an unsual methylation pattern consisting in hypermethylated DNA and histone H3 'Lys-4' methylation (H3K4me) together with depletion of histone H3 'Lys-9' methylation (H3K9me), in a chromatin methylation-independent manner, in a non-CG methylation context. May play a role in the RNA polymerase IV/V (Pol-IV/V)-mediated RNA-directed DNA methylation (RdDM) leading to TGS (also called siRNA-mediated TGS pathway), probably by modulating small interfering RNA (siRNA) accumulation. Especially involved in the gene silencing of the transcriptionally silent information region (TSI), 5S ribosomal RNA genes (localized in the pericentromeric heterochromatin of chromosomes 3, 4, and 5) and of 180-bp satellite repeats and 106B long terminal repeat (LTR)-like repeats of the chromocenters. Prevents the aberrant mRNA transcriptional read-through.[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Publication Abstract from PubMedShifts between epigenetic states of transcriptional activity are typically correlated with changes in epigenetic marks. However, exceptions to this rule suggest the existence of additional, as yet uncharacterized, layers of epigenetic regulation. MOM1, a protein of 2,001 amino acids that acts as a transcriptional silencer, represents such an exception. Here we define the 82 amino acid domain called CMM2 (Conserved MOM1 Motif 2) as a minimal MOM1 fragment capable of transcriptional regulation. As determined by X-ray crystallography, this motif folds into an unusual hendecad-based coiled-coil. Structure-based mutagenesis followed by transgenic complementation tests in plants demonstrate that CMM2 and its dimerization are effective for transcriptional suppression at chromosomal loci co-regulated by MOM1 and the siRNA pathway but not at loci controlled by MOM1 in an siRNA-independent fashion. These results reveal a surprising separation of epigenetic activities that enable the single, large MOM1 protein to coordinate cooperating mechanisms of epigenetic regulation. Structural Basis of Transcriptional Gene Silencing Mediated by Arabidopsis MOM1.,Nishimura T, Molinard G, Petty TJ, Broger L, Gabus C, Halazonetis TD, Thore S, Paszkowski J PLoS Genet. 2012 Feb;8(2):e1002484. Epub 2012 Feb 9. PMID:22346760[12] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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