1. Imperial College Centre for Synthetic Biology, London SW7 2AZ, U.K.
2. Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
3. Department of Bioengineering, Imperial College London, London SW7 2AZ, U.K.
4. Department of Biosystems Science and Engineering, ETH Zurich, CH-4058 Basel, Switzerland
5. Department of Life Sciences, Imperial College London, London SW7 2AZ, U.K.