1. Drug Discovery, Lek Pharmaceuticals d.d., Verovskova 57, SI-1526 Ljubljana, Slovenia, National Institute of Chemistry, POB 660, Hajdrihova 19, 1001 Ljubljana, Slovenia, Antibiotic Research Institute, Sandoz, A-1235 Wien, Austria, and Department of Biochemistry and Molecular Biology, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia