1. Plymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK.
2. School of Biosciences, University of Birmingham, Birmingham, B15 2TT, UK.
3. Department of Molecular Biology, Umeå University, S-901 87 Umeå, Sweden.
4. School of Pharmaceutical Sciences/Institute of Infections & Immunity, University of Nottingham, NG7 2RD, UK.