1. Institute of Molecular Biology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, Department of Biomolecular Sciences, Microspectroscopy Centre, Wageningen Agricultural University, Dreyenlaan 3, 6703 HA Wageningen, Department of Organic Chemistry, University of Padua, 35121 Padua, Italy, EMBL, Meyerhofstrasse 1, 69117 Heidelberg, Germany, and Eukaryotic Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, Kerklaan 30, 9751 NN Haren, The Netherlands