Ecological triggers and transcriptional profiling to guide antibiotic discovery 7 Summary This proposal combines microarray-based transcriptional profiling with ecologically relevant small molecule inducers to identify expressed antibiotic and virulence factor gene clusters for small molecule discovery efforts. We recently discovered that L-Pro in insect circulatory fluid induces bioactive small molecule production in insect pathogenic bacteria of the Photorhabdus and Xenorhabdus genera. These Gram-negative Gammaproteobacteria rival Streptomyces, the most studied antibiotic producing genus, in terms of their secondary metabolic potential. Unlike many Streptomyces species, in which we know very little about their ecological niches, Photorhabdus and Xenorhabdus species are at the center of a trilateral symbiosis with nematodes and insects that provides an ecological framework for laboratory investigation. The bacteria persist peacefully in the guts of infective juvenile (IJ) nematodes that hunt insect larvae in the environment. When a worm succeeds in entering its prey's circulatory system, it regurgitates the bacteria, which then produce an assortment of toxins that kill the larva, small molecules that signal for the IJ worms to become reproducing adults, small molecules that counter insect defense mechanisms, and antibiotics to protect their prey from competing bacteria and fungi. By tallying expressed antibiotic and small molecule virulence factor gene clusters using insect regulatory metabolite stimulation signals, we will employ a genetics-driven approach to identify the encoded bioactive products for NMR-based structure elucidation. The approach will immediately connect the new metabolites to their corresponding gene clusters and will also likely lead to novel biosynthetic transformations, as many of the clusters harbor unusual enzymes. To validate the genetics-driven approach, we selected a metabolite up-regulated by L-Pro in our earlier metabolomic profiling studies. This led to a series of new bioactive compounds and an unusual facet of non-ribosomal peptide synthetase (NRPS) enzymology - adenylation domain promiscuity as a conduit for scaffold diversity. Biochemical and site-directed genetic mutation studies will illuminate this phenomenon in connection to downstream tandem condensation domains that may provide a novel fork in the biosynthetic path. These biosynthetic studies will provide the basis for investigating the phenomenon in other medically relevant pathways for structural diversification. The microarray studies, which represent the key training opportunity, will certainly lead to new antibiotic gene cluster targets that will be tracked using similar genetic and differential metabolomic profiling strategies. Finally, to begin probing the generality of metabolite induction in insect pathogens, we will produce a crude natural product library from approximately 200 bacterial and fungal entomopathogens grown with our metabolite inducing conditions. Because insect pathogens must overcome the insect's innate immune system, which shares features with current anticancer targets, we will screen a series of cell lines, including leukemia, breast, and lung cancer cells, in addition to the anticancer target indoleamine 2,3-dioxygenase. In sum, these studies will shed light on whether insect pathogens could be a revitalized source of biomedical small molecules and provide a launch pad for investigating their regulation, biosynthesis, and structure in an independent academic research program.
Insect-pathogenic microbes harbor many as yet unidentified and unusual biosynthetic pathways that encode an assortment of antibiotics to successfully compete against other microbes in their ecological niche and to overcome the insect's innate immune system, which shares many similarities with current anticancer molecular targets. By using bacterial expression analysis on two model insect pathogens and by mimicking the host's physiology, we will not only uncover host recognition signals in bacterial pathogenesis, but also will target the small molecule biosynthetic gene clusters to discover their encoded bioactive products that hold promising biomedical potential. An anticancer screen will be conducted with natural product libraries generated from a panel of bacterial and fungal entomopathogens using metabolite-inducing conditions to identify new anticancer compounds and to begin exploring the generality of insect pathogens as a revitalized source of bioactive small molecules.
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