1. Imperial College Centre for Synthetic Biology, Imperial College London, London, SW7 2AZ, UK
2. Department of Bioengineering, Imperial College London, London, SW7 2AZ, UK
3. Department of Life Sciences, Imperial College London, London, SW7 2AZ, UK
4. School of Life Sciences, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK