1. Department of Molecular Biology, Uppsala University, Biomedical Centre, S-751 24 Uppsala, Sweden.
2. VTT Biotechnology and Food Research, FIN-02044 VTT, Espoo, Finland.
3. Department of Biochemistry, Uppsala University, Biomedical Centre, S-751 23 Uppsala, Sweden.
4. Glaxo Institute for Molecular Biology, 1228 Plan-les-Ouates, Geneva, Switzerland.