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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.tibs.2005.06.008 Byline: Fritz Eckstein Abstract: RNA interference (RNAi) - inhibition of gene expression by small, non-coding RNAs [small interfering RNAs (siRNAs) or microRNAs (miRNAs)] - has changed our view of regulation of expression dramatically. The application of siRNAs for both functional analysis of genes and medication raises several questions. These include the design of the double-stranded oligonucleotides, their preparation and introduction into cells or animals either as chemically synthesized entities or as transcripts from a suitable vector. Delivery of the oligonucleotides, choice of vector, chemical modification to stabilize against nucleases and avoidance of side effects (e.g. stimulation of interferons) are major challenges. Work to identify the multiple targets of miRNAs is still in its infancy, and a clear distinction between siRNAs and miRNAs is difficult in some instances. Moreover, transcriptional silencing by RNAi is poorly understood; it is evident that the siRNA machinery is involved but the details await clarification. Given the multitude of interactions of the small non-coding RNAs revealed so far, we should be prepared to encounter, as yet, undiscovered interactions and mechanisms. Author Affiliation: Max-Planck-Institut fur experimentelle Medizin, Hermann-Rein-Str. 3, 37075 Gottingen, Germany

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2005.04.024 Byline: Marcin Nowotny (1), Sergei A. Gaidamakov (2), Robert J. Crouch (2), Wei Yang (1) Abstract: RNase H belongs to a nucleotidyl-transferase superfamily, which includes transposase, retroviral integrase, Holliday junction resolvase, and RISC nuclease Argonaute. We report the crystal structures of RNase H complexed with an RNA/DNA hybrid and a mechanism for substrate recognition and two-metal-ion-dependent catalysis. RNase H specifically recognizes the A form RNA strand and the B form DNA strand. Structure comparisons lead us to predict the catalytic residues of Argonaute and conclude that two-metal-ion catalysis is a general feature of the superfamily. In nucleases, the two metal ions are asymmetrically coordinated and have distinct roles in activating the nucleophile and stabilizing the transition state. In transposases, they are symmetrically coordinated and exchange roles to alternately activate a water and a 3'-OH for successive strand cleavage and transfer by a ping-pong mechanism. Author Affiliation: (1) Laboratory of Molecular Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892 (2) Laboratory of Molecular Genetics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892 Article History: Received 4 March 2005; Revised 7 April 2005; Accepted 19 April 2005 Article Note: (miscellaneous) Published: June 30, 2005