Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.|Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.|Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.|Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.|Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.|Department of Molecular, Cell, and Developmental Biology and Center for Molecular Biology of RNA, University of California Santa Cruz, Santa Cruz, California, USA.