Genetically encoded tags for tracking a given protein continue to be of great interest in a multitude of in vitro and in vivo contexts. Acyl carrier proteins, both free-standing and as embedded 80 - 100 residue domains, contain a specific serine side chain that undergoes post-translational pantetheinylation from CoASH as donor substrate. We have previously used phage display methods to select a 12 residue fragment that retains recognition for modification by the Escherichia coli phosphopantetheinyltransferase (PPTase) AcpS. In this work, we have used N-15-HSQC based NMR titration experiments of a 12-residue peptide substrate with AcpS to identify six specifically interacting residues (S-3, L-4, D-5, M-6, W-9,and L-11) without the formation of any notable secondary structure. Synthesis of a corresponding octapeptide containing 5 of the 6 interacting residues generated a minimal fragment capable of efficient post-translational phosphopantetheinylation. Genetic insertion of this eight residue coding sequence into the proteins sonic hedgehog and transferrin receptor enabled good in vitro and in vivo PPTase-mediated modification by a series of fluorescent CoAs, leading to a set of fluorescent proteins with a peptide tag minimally perturbant to protein folds.