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Type III CRISPR-Cas systems employ multiprotein effector complexes bound to small CRISPR RNAs (crRNAs) to detect foreign RNA transcripts and elicit a complex immune response that leads to the destruction of invading RNA and DNA. Type III systems are among the most widespread in nature, and emerging interest in harnessing these systems for biotechnology applications highlights the need for detailed structural analyses of representatives from diverse organisms. We performed cryo-EM reconstructions of the Type III-A Cas10-Csm effector complex fromS.epidermidisbound to an intact, cognate target RNA and identified two oligomeric states, a 276 kDa complex and a 318 kDa complex. 3.1 Å density for the well-ordered 276 kDa complex allowed construction of atomic models for the Csm2, Csm3, Csm4 and Csm5 subunits within the complex along with the crRNA and target RNA. We also collected small-angle X-ray scattering data which was consistent with the 276 kDa Cas10-Csm architecture we identified. Detailed comparisons between theS.epidermidisCas10-Csm structure and the well-resolved bacterial(S.thermophilus) and archaeal (T.onnurineus) Cas10-Csm structures reveal differences in how the complexes interact with target RNA and crRNA which are likely to have functional ramifications. These structural comparisons shed light on the unique features of Type III-A systems from diverse organisms and will assist in improving biotechnologies derived from Type III-A effector complexes. 

With NSF MRI support, we have recently purchased a dual source single crystal diffractometer equipped with a high resoln. detector.  The purpose of this presentation is to publicize the new instrument and we seek users in the Southeast to maximize the pos. impact of this instrument on research efforts in the Southeastern US.  Users at primarily undergraduate institutions and historically black colleges and universities are particularly encouraged to use this new resource.  In general, an X-ray diffractometer allows accurate and precise measurements of the full three-dimensional structure of a mol., including bond distances and angles, and provides accurate information about the spatial arrangement of a mol. relative to neighboring mols.  The studies described here impact many areas, including org. and inorg. chem., materials chem. and biochem.  This instrument is an integral part of teaching as well as research and research training of graduate and undergraduate students in chem. and biochem. at this institution and at partner institutions.  This poster will describe some examples of how the new diffractometer will enhance research in inorg. chem., materials chem. (via diffuse scattering), and biochem.