This project was supported by a NSF CAREER Award MCB-1149427 (to S.G.-T.), by startup funds from the College of Pharmacy at the University of Kentucky (to S.G.-T.). G.V. was supported by a fellowship from project PEST08-17 from FICYT (Government of Asturias) and J.F. was supported by a fellowship from project AGL2010-20622 from MICINN(Government of Spain). We thank Dr. Michael G. Thomas(University of Wisconsin-Madison, WI) for the gift of the BL21(DE3)ybdZ::aac(3)IV bacterial strain. We thank Dr. Oleg V.Tsodikov for critical reading of the manuscript.