1. Laboratorio Biotecnologie Unità Tecnica BIORAD, ENEA C.R. Casaccia, 00123 Roma, Italy
2. Department of Biochemistry, University of Cambridge, Building O, Downing Site, Cambridge CB2 1QW, United Kingdom
3. Department of Biological Chemistry, Rothamsted Research, Harpenden, Herts, AL5 2JQ, United Kingdom