1. Department of Biology, Graduate School of Science, Osaka University; Toyonaka Osaka 560-0043 Japan
2. RIKEN Harima Institute, SPring-8, 1-1-1 Kouto, Mikazuki; Sayo-gun Hyogo 679-5148 Japan
3. Present address: Cellular and Molecular Biology Laboratory, RIKEN; 2-1 Hirosawa Wako Saitama 351-0198 Japan
4. Genomic Sciences Center (RIKEN), 2-1 Hirosawa; Wako Saitama 351-0198 Japan