Pantothenate is the precursor of the essential cofactor coenzyme A (CoA). Pantothenate kinase (CoaA) catalyzes the first and regulatory step in the CoA biosynthetic pathway. The pantothenate analogs N-pentylpantothenamide and N-heptylpantothenamide possess antibiotic activity against Escherichia coli. Both compounds are substrates for E. coli CoaA and competitively inhibit the phosphorylation of pantothenate. The phosphorylated pantothenamides are further converted to CoA analogs, which were previously predicted to act as inhibitors of CoA-dependent enzymes. Here we show that the mechanism for the toxicity of the pantothenamides is due to the inhibition of fatty acid biosynthesis through the formation and accumulation of the inactive acyl carrier protein (ACP), which was easily observed as a faster migrating protein using conformationally sensitive gel electrophoresis. E. coli treated with the pantothenamides lost the ability to incorporate [1-14 C]acetate to its membrane lipids, indicative of the inhibition of fatty acid synthesis. Cellular CoA was maintained at the level sufficient for bacterial protein synthesis. Electrospray ionization time-of-flight mass spectrometry confirmed that the inactive ACP was the product of the transfer of the inactive phosphopantothenamide moiety of the CoA analog to apo-ACP, forming the ACP analog that lacks the sulfhydryl group for the attachment of acyl chains for fatty acid synthesis. Inactive ACP accumulated in pantothenamide-treated cells because of the active hydrolysis of regular ACP and the slow turnover of the inactive prosthetic group. Thus, the pantothenamides are pro-antibiotics that inhibit fatty acid synthesis and bacterial growth because of the covalent modification of ACP.CoA is the major acyl group carrier in living systems and is synthesized by a universal series of reactions beginning with the vitamin (B 5 ) pantothenate (1). All of the genes and enzymes involved in the CoA biosynthetic pathway have been identified in Escherichia coli (Fig. 1). The first step in the pathway is catalyzed by the key rate-controlling pantothenate kinase (CoaA) 1 (ATP:D-pantothenate 4Ј-phosphotransferase; EC 2.7.1.33). Cysteine is next added to the phosphopantothenate by 4Ј-phosphopantothenoylcysteine synthase and rapidly decarboxylated to phosphopantetheine. These two steps are carried out by a bifunctional polypeptide, phosphopantothenoylcysteine synthetase decarboxylase (denoted CoaBC, formally Dfp) (2). The last two steps are carried out by two separate enzymes, namely phosphopantetheine adenylyltransferase (CoaD) (3, 4) followed by the addition of the 3Ј-ribose phosphate by dephospho-CoA kinase (CoaE) (5). E. coli is capable of de novo pantothenate biosynthesis (1) or can import pantothenate from the medium via a sodium-dependent active transport process (6-8). CoA is also required for the synthesis of ACP, the acyl group carrier in bacterial fatty acid synthesis. The phosphopantetheine moiety of CoA is transferred to serine 36 of apo-ACP by [ACP]synthase (AcpS) (9, 10), and ...