Epigenetic gene silencing induced by genetic hybridization: When two
species are crossed, genes inherited from one parent are sometimes
preferentially expressed in the hybrid. How are parental alleles
discriminated and selected? We are addressing these questions through
genetic and molecular analyses of nucleolar dominance, an epigenetic
phenomenon in which only one parental set of ribosomal RNA (rRNA) genes is
expressed in an inter-species hybrid. The phenomenon has been known for
~70 years and occurs in plants, insects and animals. We have shown that
inactive rRNA genes are preferentially silenced through changes in
chromatin involving increased DNA (cytosine) methylation and decreased
histone acetylation. Current efforts are focused on defining the
mechanisms that discriminate the genes inherited from each parent and lead
to the establishment of nucleolar dominance. Cytosine hypomethylation
mutants are being tested to explore whether DNA methylation is involved in
establishment as well as enforcement of dominance. rRNA transgenes
integrated at chromosomal sites outside the rRNA gene clusters (NORs) will
help us determine if silencing acts on individual rRNA genes or the
multi-megabase NORs. Other studies will examine NOR-euchromatin junctions
acting as boundaries between transcriptionally active and silent
chromosomal domains. Sequences within these regions may play roles in NOR
silencing.
Control of RNA polymerase I transcription: We are purifying RNA
polymerase I complexes (holoenzymes) from both plants and Xenopus. These
large (~2 Mda) complexes contain all the proteins required for
promoter-dependent rRNA gene transcription. The complexes also contain
protein kinase and chromatin modifying enzymes. Priorities include
identifying and cloning holoenzyme subunits and determining the role of
holoenzyme-associated activities in rRNA gene regulation.