1. Department of Molecular Genetics; Weizmann Institute of Science; Rehovot 7610001 Israel
2. Universitätsmedizin Göttingen Institut für Molekularbiologie Humboldtallee 23; D-37073 Göttingen Germany
3. MRC Laboratory of Molecular Biology; Cambridge UK
4. Max-Planck Institute for Biophysical Chemistry; Göttingen Germany