1. School of Chemistry and Biochemistry; The University of Western Australia; Crawley Western Australia 6009 Australia
2. Department of Biochemistry and Genetics; La Trobe University; Melbourne Victoria Australia
3. European Molecular Biology Laboratories; Hamburg 22761 Germany
4. Harry Perkins Institute of Medical Research, QEII Medical Centre, Nedlands and Centre for Medical Research; The University of Western Australia; Crawley Western Australia 6009 Australia