MRC Laboratory of Molecular Biology, Cambridge, UK. sat@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, UK. sat@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, UK. sat@mrc-lmb.cam.ac.uk|MRC Laboratory of Molecular Biology, Cambridge, UK. sat@mrc-lmb.cam.ac.uk