A new paradigm has emerged in biology in which RNA molecules are active participants in regulating, catalyzing and controlling fundamental cellular processes - roles that were reserved for proteins until recently. Two emerging themes are particularly fascinating and have been the research focus in my lab. The first theme involves RNA serving as a guide, an information carrier, to direct the action of proteins on nucleic acid targets. The power in such systems, exemplified by RNAi and CRISPR-Cas, can be harnessed for therapeutics as well as genome engineering applications. CRISPR-Cas defense systems have been identified in 88% of archaeal genomes and 39% of bacterial genomes thus far sequenced, including important human pathogens such as Campylobacter human jejuni, Clostridium botulinum, Escherichia coli, Listeria monocytogenes, Mycobacterium tuberculosis and Yersinia pestis. It has been shown to modulate the horizontal gene transfer and biofilm formation. Our proposed Project 1 is based on the successful structure determination of several important Cas proteins and the successful reconstitution of the Type I-C Cascade complex from B. halodurans. In this proposal, we propose experiments to understand the CRISPR interference mechanism in Type I-C CRISPR- Cas system. We build upon strong preliminary data to (1) characterize the structure-function of the target searching Cascade complex in Type I-C system, (2) characterize the structure-function of the Cascade- interacting protein Cas3, an essential factor in all Type I CRISPR-Cas systems. (3) capture structure snapshots of the Cascade-dsDNA and the Cascade-Cas3 complexes. Our finding will serve to reveal the common theme and mechanistic diversity among different CRISPR-Cas systems. The second central theme in RNA biology involves structured RNAs performing gene regulatory function in cis. The discovery of short cis- acting RNA elements termed riboswitches led to a paradigm shift in the concept of gene regulation. Riboswitches are widespread in prokaryotes, where they are estimated to control as many as 2-4% of all genes in Firmicutes. They almost exclusively function in cis, usually reside in the 5' untranslated regions (5'- UTRs) of the host mRNAs, and regulate gene expression mainly through the means of premature transcription termination or inhibition of translation initiation, although other regulatory mechanisms including the control of mRNA cleavage, stability, and alternative splicing have been demonstrated. We identify the following frontiers in the riboswitch research and align our efforts accordingly: 1. novel ligand sensing strategy utilized riboswitches, the study of which may reveal novel aspects of bacterial physiology (the study of T box riboswitches in Project 2); 2. deeper understanding of the conformational switching mechanism (the yybP-ykoY orphan riboswitches in Project 3); 3. structure-function characterization of orphan riboswitch families (Project 3); and 4. synthetic biology applications in industry, medicine, pharmacy or environmental protection (fluorescent Mn2+ sensor applications in Project 3).

Public Health Relevance

The described structure-function studies of T-box riboswitch and the yybP-ykoY riboswitch systems in bacteria will contribute to the development of strategies to target these RNAs that control essential genes in human pathogens for antibiotics development. The described structure-function studies of CRISPR interference system in bacteria will contribute to the development of strategies to manipulate the CRISPR-Cas operon to control the proliferation and virulence of microbial pathogens. Novel genome editing applications may be derived from the proposed mechanistic studies.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Unknown (R35)
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Special Emphasis Panel (ZGM1-TRN-9 (MR))
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Preusch, Peter
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Cornell University
Schools of Arts and Sciences
United States
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Xiao, Yibei; Luo, Min; Dolan, Adam E et al. (2018) Structure basis for RNA-guided DNA degradation by Cascade and Cas3. Science 361:
Xiao, Yibei; Luo, Min; Hayes, Robert P et al. (2017) Structure Basis for Directional R-loop Formation and Substrate Handover Mechanisms in Type I CRISPR-Cas System. Cell 170:48-60.e11
Xiao, Yibei; Ng, Sherwin; Nam, Ki Hyun et al. (2017) How type II CRISPR-Cas establish immunity through Cas1-Cas2-mediated spacer integration. Nature 550:137-141
Battaglia, Robert A; Price, Ian R; Ke, Ailong (2017) Structural basis for guanidine sensing by the ykkC family of riboswitches. RNA 23:578-585