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

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 approximately 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 derepressed 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.