Identification and characterization of a type II peptidyl carrier protein from the bleomycin producer Streptomyces verticillus ATCC 15003

Abstract

Nonribosomal peptide synthetases (NRPSs) catalyze the assembly of a structurally diverse group of peptides by the multiple-carrier thiotemplate mechanism. All NRPSs known to date are exclusively type I modular enzymes that consist of domains, such as adenylation (A), peptidyl carrier protein (PCP) and condensation (C) domains, for individual enzyme activities. Although several A and PCP domains have been demonstrated to function independently, aminoacylation in trans has been successful only between PCPs and their cognate A domains. We have identified within the bleomycin-biosynthesis gene cluster from Streptomyces verticillus ATCC15003 the blmI gene that encodes a discrete PCP protein. We overexpressed the blmI gene in Escherichia coli, purified the BlmI protein, and demonstrated that apo-BlmI can be efficiently modified into holo-BlmI either in vivo or in vitro by PCP-specific 4'-phosphopantetheine transferases (PPTases). Unlike the PCP domains in type I NRPSs, BlmI lacks its cognate A domain and can be aminoacylated by Val-A, an A domain from a completely unrelated type I NRPS. BlmI represents the first characterized type II PCP. The BlmI type II PCP, like the PCP domains of type I NRPSs, can be 4'-phospho-pantetheinylated by PCP-specific PPTases but is biochemically distinct in that it can be aminoacylated by an A domain from a completely unrelated type I NRPS. Our results provide for the first time the genetic and biochemical evidence to support the existence of a type II NRPS, which might be useful in the combinatorial manipulation of NRPS proteins to generate novel peptides.