School of Biochemistry and Microbiology, University of Leeds, Leeds, LS2 9JT, UK. bmbagg@bmb.leeds.ac.uk|School of Biochemistry and Microbiology, University of Leeds, Leeds, LS2 9JT, UK. bmbagg@bmb.leeds.ac.uk|School of Biochemistry and Microbiology, University of Leeds, Leeds, LS2 9JT, UK. bmbagg@bmb.leeds.ac.uk