(a) Apo ACP and PCP (T) domains are phosphopantetheinylated by a PPTase to create holo domains. (b) The biosynthesis of the siderophore enterobactin by E. coli requires the activity of the PPTase EntD [A=adenylation, C=condensation, T=thiolation (ACP or PCP), TE=thioesterase]. (c) entD deletion mutants do not grow on low iron due to the absence of enterobactin; however, growth on low iron media can be restored by complementing this deletion with eDNA clones expressing PPTases. Approximately 5,000,000 eDNA clones hosted in an E. coli entD deletion mutant were plated on low iron media for this image.