Mechanisms of HDA6-mediated rRNA gene silencing: suppression of intergenic Pol II transcription and differential effects on maintenance versus siRNA-directed cytosine methylation.
The Arabidopsis histone deacetylase HDA6 is required to silence transgenes, transposons, and ribosomal RNA (rRNA) genes subjected to nucleolar dominance in genetic hybrids. In nonhybrid Arabidopsis thaliana, we show that a class of 45S rRNA gene variants that is normally inactivated during development fails to be silenced in hda6 mutants. In these mutants, symmetric cytosine methylation at CG and CHG motifs is reduced, and spurious RNA polymerase II (Pol II) transcription occurs throughout the intergenic spacers. The resulting sense and antisense spacer transcripts facilitate a massive overproduction of siRNAs that, in turn, direct de novo cytosine methylation of corresponding gene sequences. However, the resulting de novo DNA methylation fails to suppress Pol I or Pol II transcription in the absence of HDA6 activity; instead, euchromatic histone modifications typical of active genes accumulate. Collectively, the data reveal a futile cycle of unregulated transcription, siRNA production, and siRNA-directed DNA methylation in the absence of HDA6-mediated histone deacetylation. We propose that spurious Pol II transcription throughout the intergenic spacers in hda6 mutants, combined with losses of histone deacetylase activity and/or maintenance DNA methylation, eliminates repressive chromatin modifications needed for developmental rRNA gene dosage control.
Biology Department, Washington University, St. Louis, Missouri 63130, USA.
Keith W
KW
Earley
Frédéric
F
Pontvianne
Andrzej T
AT
Wierzbicki
Todd
T
Blevins
Sarah
S
Tucker
Pedro
P
Costa-Nunes
Olga
O
Pontes
Craig S
CS
Pikaard
D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D003596:Cytosine / Q000378:metabolism*; D004257:DNA Polymerase II / Q000378:metabolism*; D021901:DNA, Intergenic / Q000378:metabolism; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D020868:Gene Silencing; D020459:Genes, rRNA / Q000235:genetics*; D006655:Histone Deacetylases / Q000235:genetics / Q000378:metabolism*; D006657:Histones / Q000378:metabolism; D008745:Methylation; D009154:Mutation; D034741:RNA, Small Interfering / Q000378:metabolism*
D029681:Arabidopsis Proteins; D021901:DNA, Intergenic; D006657:Histones; D034741:RNA, Small Interfering; D003596:Cytosine; D004257:DNA Polymerase II; D004259:DNA-Directed DNA Polymerase; C432452:AT5G63110 protein, Arabidopsis; D006655:Histone Deacetylases