S Darby, K Chulliyallipalil performed the experiments. S Darby, M Przyjalgowski, P McGowan built the system. S Darby, K Chulliyallipalil, S Jeffers, A Giltinan, N Smith and RD Sleator analyzed the data. S Darby and RD Sleator wrote the manuscript. RD Sleator, N Smith and L Lewis supervised the project. All authors reviewed the manuscript.