ABST:Epigenetic silencing of RNA polymerase I transcription[ G&D

[Graham Dellaire]

Epigenetic silencing of RNA polymerase I transcription: a role for DNA 
methylation and histone modification in nucleolar dominance

Z. Jeffrey Chen and Craig S. Pikaard

Biology Department, Washington University, St. Louis, Missouri 63130 USA

Nucleolar dominance is an epigenetic phenomenon that describes nucleolus 
formation around rRNA genes inherited from only one progenitor of an 
interspecific hybrid or allopolyploid. The phenomenon is widespread,
occurring 
in plants, insects, amphibians, and mammals, yet its molecular basis
remains 
unclear. We have demonstrated nucleolar dominance in three allotetraploids
of 
the plant genus Brassica. In Brassica napus, accurately initiated pre-rRNA 
transcripts from one progenitor, Brassica rapa are detected readily,
whereas 
transcripts from the ~3000 rRNA genes inherited from the other progenitor, 
Brassica oleracea, are undetectable. Nuclear run-on confirmed that
dominance is 
controlled at the level of transcription. Growth of B. napus seedlings on 
5-aza-2'-deoxycytidine to inhibit cytosine methylation caused the normally 
silent, under-dominant B. oleracea rRNA genes to become expressed to high 
levels. The histone deacetylase inhibitors sodium butyrate and trichostatin
A 
also de-epressed silent rRNA genes. These results reveal an enforcement 
mechanism for nucleolar dominance in which DNA methylation and histone 
modifications combine to regulate rRNA gene loci spanning tens of megabase
pairs
of DNA.



Z. Jeffrey Chen
Biology Department
Box 1137, One Brookings Drive
Washington University
St. Louis, MO 63130
Phone: (314) 935-8529, fax: -4432
chen_j at biodec.wustl.edu