1. Department of Biological and Nutritional Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K., Department of Molecular Biology and Biotechnology, Krebs Institute, University of Sheffield, Sheffield S10 2TN, U.K., Institute for Molecular Genetics, Agricultural Biotechnology Centre, P.O. Box 411, Gödöllô H-2101, Hungar, and Department of Biotechnology and Microbial Genetics, Gödöllô University of Agricultural Sciences, Páter Károly u. l., Gödöllô H-2101, Hungary