This research was funded by an ERC Starting Grant (Strp-differentiation 280304). Our thanks to Beatriz Gutiérrez Magán (Universidad de Oviedo, Dpto. Biología Funcional, Área de Microbiología) for laboratory assistance, Paul Dyson and Meirwyn Evans (Swansea University) for providing the disrupted cosmids, Carlos Barreiro and Mar Calonge (INBIOTEC, Leon) for the mass spectrometry and amino acid analyses, Maggie Smith (University of York) for providing the pMS82 plasmid, and Nature Publishing Group Language Editing service for proof-reading the text. B.R., P.Y., M.T.L.G., N.G.Q. and E.B. performed the experiments; B.R., P.Y., E.B., F.M. and A.M. assisted with data analysis; all authors contributed to the critical discussion of the manuscript; B.R., F.M. and A.M. conceived, designed the work and wrote the manuscript. All authors read and approved the final manuscript.