RNA interference (RNAi) takes place in the nuclei of human cells, US scientists report in Nature Structural and Molecular Biology this week. Their results suggest new roles for the RNAi machinery in post-transcriptional gene expression in the nucleus.

Co-author Tariq M. Rana said that until now, researchers thought that human-activated RNA-induced silencing complexes (RISCs) localized and functioned only in the cytoplasm where mature mRNA is translated. Specific RNA cleavage induced by a RISC in the nucleus had not previously been documented in vertebrate cells, he added.

To investigate RNAi in the nucleus, Rana and colleagues at the University of Massachusetts Medical School designed a 21-nt siRNA that targeted 7SK snRNA—an abundant RNA with a highly defined structure that specifically localizes to the nucleus. Introducing the siRNA into cells resulted in a 94% decrease in snRNA levels, they found.

"We activated the RISC complex by adding RNA to cells," Rana told...

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