Thioesterase domains often occur integrated in or associated with peptide synthetaseswhich are involved in the non-ribosomal synthesis of peptide antibiotics.Thioesterases are required for the addition of the last amino acid to the peptideantibiotic, thereby forming a cyclic antibiotic. Next to the operons encoding theseenzymes, in almost all cases, are genes that encode proteins that have similarity tothe type II fatty acid thioesterases of vertebrates.