1. Theodor-Boveri-Institut für Biowissenschaften, Physiologische Chemie I, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Department of Biochemistry, Agricultural University of Wageningen, NL-6703 HA Wageningen, The Netherlands, and Dipartimento di Fisiologia e Biochimica Generali, Università degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy