1. Graduate School of Life Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
2. Faculty of Advanced Life Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
3. Alberta RNA Research and Training Institute, Department of Chemistry and Biochemistry, University of Lethbridge, Lethbridge AB T1K 3M4, Canada
4. Department of Biology, Faculty of Science, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan