1. Department of Physics, Northeastern University, Boston, MA 02115, USA, 2Department of Chemistry and Biochemistry, Center for RNA Biology, and Center for Retrovirus Research, The Ohio State University, Columbus, OH 43210, USA, 3AIDS and Cancer Virus Program, Leidos Biomedical Research, Inc., Frederick National Laboratory for Cancer Research, Frederick, MD 21702, USA and 4Department of Biochemistry