1. Department of Metabolic Biology, John Innes Centre, Norwich NR4 7UH, UK
2. School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK
3. Department of Biological Chemistry, John Innes Centre, Norwich NR4 7UH, UK
4. Institute of Biological, Environmental and Rural Sciences, Aberystwyth University, Aberystwyth SY23 3FL, UK