streptogramin biosynthesis streptogramin B biosynthesis antibiotic_resistance ARO:3000389 Type B streptogramins are cyclic hexa- or hepta-depsipeptides produced by NRPSs in Streptomyces. The NRPS contains 6 or 7 modules arranged on one or more genes. SnbA catalyzes the activation of the first residue, 3-hydroxypicolynic acid. SnbC then activates and incorporates threonine and aminobutyric acid. Lastly, SnbDE activates and incoporates the last four amino acids. The thioesterase domain is responsible for peptide cyclization, and it is located at the end of the assembly line.