title: Two means of transcriptional reactivation within heterochromatin.
abstract: DNA methylation levels and specific histone modifications of chromatin in interphase nuclei are taken as an indicator of transcriptional activity or silencing. Arabidopsis mutants impaired in maintenance of transcriptional gene silencing (TGS) alleviate TGS with or without affecting DNA methylation. Mutant ddm1, representing the first type, lacks a chromatin remodeling factor that regulates histone and DNA methylation. Mutant mom1, representing the second type, is affected in a different but still unknown silencing mechanism. Both classes of mutation have been studied mainly for their effects on specific loci. Here, we describe the cytological analysis of chromatin in ddm1 and mom1 mutants. The ddm1 mutation causes a striking decondensation of centromeric heterochromatin, a re-distribution of the remaining methylation of DNA, and a drastic change in the pattern of histone modification. A complex transgenic locus, which underwent stable inactivation and became heterochromatin-like, follows similar structural alterations. In contrast, nuclear organization in mom1 appears unaltered, demonstrating an involvement of MOM1 in transcriptional regulation within a heterochromatic environment.
authors.affiliation: Friedrich Miescher Institute for Biomedical Research, PO Box 2543, CH-4002 Basel, Switzerland. aprobst@fmi.ch
authors.forename: Aline V
authors.initials: AV
authors.lastname: Probst
authors.forename: Paul F
authors.initials: PF
authors.lastname: Fransz
authors.forename: Jerzy
authors.initials: J
authors.lastname: Paszkowski
authors.forename: Ortrun
authors.initials: O
authors.lastname: Mittelsten Scheid
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000502:physiology; D029681:Arabidopsis Proteins; D002467:Cell Nucleus / Q000235:genetics / Q000502:physiology; D019175:DNA Methylation; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D020868:Gene Silencing / Q000502:physiology; D006570:Heterochromatin / Q000235:genetics* / Q000378:metabolism; D006657:Histones / Q000378:metabolism; D017404:In Situ Hybridization, Fluorescence; D009154:Mutation; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D015533:Transcriptional Activation / Q000235:genetics* / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; C118957:DDM1 protein, Arabidopsis; D004268:DNA-Binding Proteins; D006570:Heterochromatin; D006657:Histones; D014157:Transcription Factors