[1] Univ Calif Santa Cruz, Dept Mol Cell & Dev Biol, Santa Cruz, CA 95064 USA
[2] Univ Calif Santa Cruz, Ctr Mol Biol RNA, Santa Cruz, CA 95064 USA
[3] Univ Illinois, Howard Hughes Med Inst, Urbana, IL 61801 USA
[4] Univ Illinois, Dept Phys, Urbana, IL 61801 USA