1. Department of Medical Biochemistry and Microbiology, Uppsala Biomedicinska Centrum, Husarg.3, S-75237 Uppsala, Sweden
2. Department of Biochemistry and Biophysics, Arrhenius Laboratories, Stockholm University, Svante Arrhenius v. 16C, Stockholm S-10691, Sweden
3. Department of Molecular Microbiology, John Innes Centre, Norwich NR47 UH, UK