1. Center for Bioinformatics Saar, Bldg. 36.1, Saarland University, 66123 Saarbrücken, Germany, Wilhelm Schickard Institute for Computer Science, University of Tübingen, Sand 14, 72076 Tübingen, Germany, Department of Information Technologies, Faculty of Physics, Astronomy and Applied Informatics, Jagiellonian University, Reymonta 4, PL 30-059 Kraków, Poland, and Max-Planck-Institut für Informatik, Stuhlsatzenhausweg 85, 66123 Saarbrücken, Germany