Reovirus is an icosahedral, double-stranded (ds) RNA virus that uses viral polymerases packaged within the viral core to transcribe its ten distinct plus-strand RNAs. To localize these polymerases, the structure of the reovirion was refined to a resolution of 7.6 Å by cryo-electron microscopy (cryo-EM) and three-dimensional (3D) image reconstruction. X-ray crystal models of reovirus proteins, including polymerase λ3, were then fitted into the density map. Each copy of λ3 was found anchored to the inner surface of the icosahedral core shell, making major contacts with three molecules of shell protein λ1 and overlapping, but not centering on, a five-fold axis. The overlap explains why only one copy of λ3 is bound per vertex. λ3 is furthermore oriented with its transcript exit channel facing a small channel through the λ1 shell, suggesting how the nascent RNA is passed into the large external cavity of the pentameric capping enzyme complex formed by protein λ2.
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We are especially grateful to S. Harrison and Y. Tao for providing the coordinates of the λ3 crystal structure before publication and also for discussions and comments on the manuscript. We also thank W. Zhang, C. Xiao, R. Ashmore, J. Johnson, A. McGough, R. Bernal, M. Sherman, M. Rossmann, P. Chacón and B. Bahlke for helpful discussions; V. Bowman, A. Simpson, P. Leiman, Y. Tao and J. Zhiu for assistance with figures; K. Reinisch and S. Liemann for providing crystal coordinates and discussion; and L. Szpankowski for digitizing micrographs. Work was supported in part by grants from the US National Institutes of Health to T.S.B. and M.L.N., a shared equipment grant from the US National Science Foundation to T.S.B., a Keck Foundation award to the Purdue Structural Biology group for purchase of the CM300 FEG microscope and a Purdue University reinvestment grant to the Structural Biology group. S.B.W. was additionally supported by the Purdue Biophysics Training Grant and a Purdue Research Foundation Fellowship.
The authors declare no competing financial interests.
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