1. Interfakultäres Institut für Biochemie Universität Tübingen Auf der Morgenstelle 34 72076 Tübingen Germany
2. Current address: Department of Infection Immunity & Cardiovascular Disease University of Sheffield, The Medical School Beech Hill Rd Sheffield S10 2RX UK
3. Biocenter Johannes Gutenberg University Mainz 55128 Mainz Germany
4. Institute of Molecular Biology Mainz 55128 Mainz Germany
5. Current address: Department of Biosystems Science and Engineering Basel ETH Zurich Mattenstrasse 26 4058 Basel Switzerland