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Cryo-EM shows the polymerase structures and a nonspooled genome within a dsRNA virus

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Science  18 Sep 2015:
Vol. 349, Issue 6254, pp. 1347-1350
DOI: 10.1126/science.aaa4938

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Caught in the act of copying

The genomes of double-stranded RNA (dsRNA) viruses consist of about a dozen dsRNA segments enclosed by a protein coat. Inside the host cell, the coat remains intact, and the dsRNAs have to replicate within the coat. Liu and Cheng used cryo–electron microscopy of cypovirus particles to catch the dsRNAs in the act of being copied. The structures revealed that the RNA formed a liquid-crystalline array on which viral enzymes carry out multiple rounds of transcription to replicate the viral genome.

Science, this issue p. 1347

Abstract

Double-stranded RNA (dsRNA) viruses possess a segmented dsRNA genome and a number of RNA-dependent RNA polymerases (RdRps) enclosed in a capsid. Until now, the precise structures of genomes and RdRps within the capsids have been unknown. Here we report the structures of RdRps and associated RNAs within nontranscribing and transcribing cypoviruses (NCPV and TCPV, respectively), using a combination of cryo–electron microscopy (cryo-EM) and a symmetry-mismatch reconstruction method. The RdRps and associated RNAs appear to exhibit a pseudo-D3 symmetric organization in both NCPV and TCPV. However, the molecular interactions between RdRps and the genomic RNA were found to differ in these states. Our work provides insight into the mechanisms of the replication and transcription in dsRNA viruses and paves a way for structural determination of lower-symmetry complexes enclosed in higher-symmetry structures.

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