1. Helmholtz Institute for RNA-based Infection Research (HIRI), Josef-Schneider-Straβe 2/D15, Würzburg 97080, Germany
2. Medical Faculty, Julius-Maximilians University of Würzburg, Würzburg 97080, Germany
3. Division of Virology, Department of Pathology, University of Cambridge, Tennis Court Road, Cambridge CB2 1QP, U.K.