1. School of Biochemistry and Genetics, The Medical School, University of Newcastle; Newcastle-upon-Tyne NE2 4HH UK
2. School of Biological Sciences, University of Manchester; Oxford Road Manchester M13 9PT UK
3. Division of Yeast Genetics, National Institute for Medical Research; The Ridgeway, Mill Hill London NW7 1AA UK
4. Present address: Laboratory of Molecular Biology, NIDDK, 5-218, NIH, 9000 Rocdkville Pike; Bethesda MD 20892 USA