RNAi Discussion Group (1)

Organizer: Oliver Hobert, Columbia University

The goal of this group is to provide a forum for scientists in the New York area engaged in research in the biology, biochemistry, and applications of RNAi silencing to meet regularly to discuss advances in this exciting new field and to promote interinstitutional and interdisciplinary dialogue.

Program

5:00–7:30: Presentations

Danesh Moazed, Harvard Medical School, "RNAi-Mediated Assembly of Epigenetic Chromatin Domains."

Victor Ambros, Dartmouth University, "MicroRNA Pathways and the Control of Animal Development."

Antonio Giraldez, New York University School of Medicine, "MicroRNAs Regulate Brain Morphogenesis and Other Aspects of Zebrafish Embryogenesis."

Abstracts

"RNAi-Mediated Assembly of Epigenetic Chromatin Domains."
Danesh Moazed
The RNA interference pathway is required for transcriptional gene silencing in many organisms. Our laboratory has purified a ribonucleoprotein complex, termed RITS (RNA-induced transcriptional silencing), which physically links the RNAi pathway to heterochromatin assembly in fission yeast. RITS contains the conserved siRNA-binding protein, Ago1; a chromodomain histone-binding protein, Chp1; and a novel protein, Tas3. Moreover, RITS contains siRNAs that match the sequence of fission yeast centromeric repeats, which are assembled into heterochromatin, and siRNAs are required for the association of RITS with its target DNA regions. These results suggest that RITS uses siRNAs as guides to initiate heterochromatin assembly at specific chromosome regions. More recently, we have found that RITS associates with another RNAi complex that contains the fission yeast RNA-directed RNA polymerase, Rdp1, termed RDRC (RNA-directed RNA polymerase complex). The association of RITS with RDRC requires the Dicer ribonuclease and the Clr4 histone H3-lysine9 methyltransferase, suggesting that both siRNA and chromatin localization contribute to this interaction. In addition to Rdp1, RDRC contains a conserved RNA helicase and a protein that belongs to the polyA polymerase family of enzymes. We have shown that RDRC has RNA-directed RNA polymerase activity and that this activity is required for heterochromatin assembly at centromeres. Finally, components of RITS and RDRC associate with noncoding centromeric RNAs in an siRNA-directed manner. Our findings suggest that nascent noncoding RNAs provide a platform for the assembly of RNAi complexes at specific chromosome regions.

"MicroRNA Pathways and the Control of Animal Development."
Victor Ambros
We are using genetic approaches to identify the roles for certain evolutionarily conserved microRNAs in development of nematodes (Caenorhabditis elegans) and insects (Drosophila melanogaster). For microRNA genes with orthologs in both flies and worms, we find that loss-of-function phenotypes often result in significantly distinct phenotypes in the two animals. We generally do not observe developmental defects for C. elegans loss-of-function microRNA mutants unless multiple microRNA genes are simultaneously eliminated by mutation. This suggests functional redundancy among worm microRNA genes, particularly within gene families encoding similar microRNAs. The current analysis of the microRNA mutant phenotypes and the relative roles of conserved microRNA genes in worms and flies will be discussed.

"MicroRNAs Regulate Brain Morphogenesis and Other Aspects of Zebrafish Embryogenesis." Antonio Giraldez
MicroRNAs (miRNAs) are small RNAs that regulate gene expression posttranscriptionally. To block all miRNA formation in zebrafish, we generated maternal–zygotic dicer (MZdicer) mutants that disrupt the Dicer RNaseIII and dsRNA-binding domains. Mutant embryos do not process precursor miRNAs into mature miRN