1. Laboratorium voor Moleculaire Celbiologie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Flanders, Belgium
2. Department of Microbiology and Biochemistry, University of the Orange Free State, PO Box 339, Bloemfontein 9300, South Africa
3. Department of Cell and Molecular Biology/Microbiology, Göteborg University, Box 462, SE-405 30 Göteborg, Sweden