1. Permanent address: School of Biological and Molecular Sciences, Oxford Brookes University, Headington, Oxford OX3 OBP, U.K.
2. School of Pure and Applied Biology, University of Wales College of Cardiff, P.O. Box 911, Cardiff CF1 3US, U.K.
3. School of Molecular and Medical Biosciences & University of Wales College of Cardiff, P.O. Box 911, Cardiff CF1 3US, U.K.