Methionine-Containing Rhabdopeptide/Xenortide-like Peptides from Heterologous Expression of the Biosynthetic Gene Cluster kj12ABC in Escherichia coli

Engineering of Specific Single-Module Nonribosomal Peptide Synthetases of the RXP Type for the Production of Defined Peptides

Rhabdopeptide/xenortide-like peptide (RXP) nonribosomal peptide synthetases (NRPSs) derived from entomophathogenic Xenorhabdus and Photorhabdus bacteria often produce libraries of different peptides varying in amino acid composition, number and degree of methylation, which mainly is a result of promiscuous docking domains (DDs) mediating protein-protein interactions between the different NRPS subunits. In this study, we present two specific RXP-NRPS systems with rather specific DDs that were used as platforms to generate a series of defined RXPs via the exchange of adenylation/methyltransferase (A-MT) domains in the systems followed by heterologous expression in Escherichia coli. Additionally, these results suggest that NRPS subunit interaction is not only exclusively dependent on DDs but at least partially also on A domains.

Uncovering Nematicidal Natural Products from Xenorhabdus Bacteria

Parasitic nematodes infect different species of animals and plants. Root-knot nematodes are members of the genus Meloidogyne, which is distributed worldwide and parasitizes numerous plants, including vegetables, fruits, and crops. To reduce the global burden of nematode infections, only a few chemical therapeutic classes are currently available. The majority of nematicides are prohibited due to their harmful effects on the environment and public health. This study was intended to identify new nematicidal natural products (NPs) from the bacterial genus Xenorhabdus, which exists in symbiosis with Steinernema nematodes. Cell-free culture supernatants of Xenorhabdus bacteria were used for nematicidal bioassay, and high mortality rates for Caenorhabditis elegans and Meloidogyne javanica were observed. Promoter exchange mutants of biosynthetic gene clusters encoding nonribosomal peptide synthetases (NRPS) or NRPS-polyketide synthase hybrids in Xenorhabdus bacteria carrying additionally a hfq deletion produce a single NP class, which have been tested for their bioactivity. Among the NPs tested, fabclavines, rhabdopeptides, and xenocoumacins were highly toxic to nematodes and resulted in mortalities of 95.3, 74.6, and 72.6% to C. elegans and 82.0, 90.0, and 85.3% to M. javanica, respectively. The findings of such nematicidal NPs can provide templates for uncovering effective and environmentally safe alternatives to commercially available nematicides.

CRAGE enables rapid activation of biosynthetic gene clusters in undomesticated bacteria

It is generally believed that exchange of secondary metabolite biosynthetic gene clusters (BGCs) among closely related bacteria is an important driver of BGC evolution and diversification. Applying this idea may help researchers efficiently connect many BGCs to their products and characterize the products' roles in various environments. However, existing genetic tools support only a small fraction of these efforts. Here, we present the development of chassis-independent recombinase-assisted genome engineering (CRAGE), which enables single-step integration of large, complex BGC constructs directly into the chromosomes of diverse bacteria with high accuracy and efficiency. To demonstrate the efficacy of CRAGE, we expressed three known and six previously identified but experimentally elusive non-ribosomal peptide synthetase (NRPS) and NRPS-polyketide synthase (PKS) hybrid BGCs from Photorhabdus luminescens in 25 diverse γ-Proteobacteria species. Successful activation of six BGCs identified 22 products for which diversity and yield were greater when the BGCs were expressed in strains closely related to the native strain than when they were expressed in either native or more distantly related strains. Activation of these BGCs demonstrates the feasibility of exploiting their underlying catalytic activity and plasticity, and provides evidence that systematic approaches based on CRAGE will be useful for discovering and identifying previously uncharacterized metabolites.