9617471 Pikaard There is growing realization that gene silencing is probably as important as activation in the carefully orchestrated expression of genes essential for proper development of complex organisms. Relevant phenomena include X-chromosome inactivation, gametic imprinting, provirus inactivation, co-supression and transposable element inactivation. It is not yet clear how biased gene repression is first established in any of these epigenetic phenomena; however, DNA methylation and alterations in chromatin structure are strongly implicated in gene silencing in both animals and plants. Nucleolar dominance is an epigenetic phenomenon which is widespread in both plants and animals. Nuclear dominance is the selective formation of nucleoli on chromosomes inherited from only one parent in an interspecific hybrid or allopolyploid. Nucleoli form at nucleolus organizer regions, chromosomal sites where tandemly arrayed rRNA genes are transcribed by RNA polymerase I. The cytological manifestation of nucleolar dominance is the transcription of only one parental set of rRNA genes. The molecular mechanisms responsible for nucleolar dominance are not clear. This research will test two hypotheses involving biased gene repression mechanisms: First, that one set of rRNA genes is selectively silenced by DNA methylation, and second that nucleolar organizer regions are indirectly controlled as a single locus or chromosomal domain whose chromatin state determines gene activity. These models are not necessarily mutually exclusive at a mechanistic level. This laboratory has developed the biochemical and genetic tools necessary to test these hypotheses in the model plant Arabidopsis. In the allotetraploid Arabidopsis suecica, rRNA genes derived from Arabidopsis arenosa are transcriptionally active whereas rRNA genes derived from Arabidopsis thaliana are repressed. Since the A. arenosa and A. thaliana genomes can be combined by a genetic cross, an arenosa x thaliana genetic cross using an A. thalian a DNA hypomethylation mutant will be used to determine if a causal relationship exists between DNA methylation and nucleolar dominance. A. thaliana cell lines with stable integrated rRNA transgenes will be used to determine if location within a nucleolar organizer region is required for nucleolar dominance. Progress in understanding the mechanisms responsible for nucleolar dominance will provide important new insights into chromosomal controls of gene expression. Gene silencing is probably as important as activation in the carefully orchestrated expression of genes essential for proper development of complex organisms. It is not yet clear how biased gene repression is first established; however, DNA methylation and alterations in chromatin structure are strongly implicated in gene silencing in both animals and plants. Nucleolar dominance is the selective formation of nucleoli on chromosomes inherited from one parent. The manifestation of nucleolar dominance is the expression of only one parental set of rRNA genes. This research will investigate whether nuclear dominance occurs by gene silencing mechanisms involving selective DNA methylation and/or locus-specific chromosomal structure. Progress in understanding the mechanisms responsible for nucleolar dominance will provide important new insights into chromosomal controls of gene expression.
