1. Department of Biochemistry and Biophysics, Stockholm University, Svante Arrhenius v. 16, SE-106 91 Stockholm,
Sweden
2. Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge
CB2 1EW, U.K.
3. Department
of Molecular Biology, Swedish University of Agricultural Sciences, Uppsala Biomedical Center, Box 590, SE-751 24 Uppsala,
Sweden