1. Max-Planck Research Unit, Enzymology of Protein Folding, Kurt-Mothes-Str. 3, D-06120 Halle/Saale, Germany, Institute of Biology II, Albert-Ludwigs-University Freiburg, Schänzlestr. 1, D-79104 Freiburg i. Br., Germany, and Hans-Knöll-Institute of Natural Products Research e.V., Beutenbergstr. 11, D-07745 Jena, Germany