1. Department of Structural Biology; Weizmann Institute; 76100 Rehovot Israel
2. Max Planck Institute for Molecular Genetics; Ihnestr. 73 14195 Berlin Germany
3. Max Planck Research Unit for Ribosomal Structure; Notkestr. 85 22603 Hamburg Germany
4. Department of Biochemistry and Pharmacology; FU-Berlin; Takustr. 3 14195 Berlin Germany