1. Department of Chemistry, Seoul National University, Seoul, 151-747, Republic of Korea
2. Center for RNA Research, Institute for Basic Science (IBS), Seoul, 151-747, Republic of Korea
3. School of Biological Sciences, Seoul National University, Seoul 151-747, Republic of Korea
4. Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 305-701, Republic of Korea