e-Poly-L-lysine dispersity is controlled by a highly unusual nonribosomal peptide synthetase

e-Poly-L-lysine (e-PL) consists of 25–35 L-lysine residues in isopeptide linkages and is one of only two amino acid homopolymers known in nature. Elucidating the biosynthetic mechanism of e-PL should open new avenues for creating novel classes of biopolymers. Here we report the purification of an e-PL synthetase (Pls; 130 kDa) and the cloning of its gene from an e-PL–producing strain of Streptomyces albulus. Pls was found to be a membrane protein with adenylation and thiolation domains characteristic of the nonribosomal peptide synthetases (NRPSs). It had no traditional condensation or thioesterase domain; instead, it had six transmembrane domains surrounding three tandem soluble domains. These tandem domains iteratively catalyzed L-lysine polymerization using free L-lysine polymer (or monomer in the initial reaction) as acceptor and Pls-bound L-lysine as donor, directly yielding chains of diverse length. Thus, Pls is a new single-module NRPS having an amino acid ligase–like catalytic activity for peptide bond formation.