1. European Molecular Biology Laboratory, c/o DESY, Notkestrasse 85, D-22603 Hamburg, Germany, Institute of Crystallography, Russian Academy of Sciences, Leninski pr. 59, 119333 Moscow, Russia, Chemistry Department, Lomonosov Moscow State University, 119992 Moscow, Russia, Institute of Industrial microorganisms, Dorojnii pr. 1, 117545 Moscow, Russia, and Institute of Molecular Genetics, Kurchatov square, 123182 Moscow, Russia