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Significance Pyridoxal phosphate (PLP)-dependent enzymes rarely react with oxygen, but an emerging group of oxygen-, PLP-dependent enzymes oxidize l -arginine. Two types of oxidases are known: hydroxylases and desaturases. We demonstrate that arginine desaturases have a minor hydroxylase activity and then show through X-ray crystallographic, mutagenesis, spectroscopic, and computational studies that their mechanism involves two rounds of single-electron transfer to oxygen and superoxide rebound, ultimately giving a conjugated hydroperoxyl intermediate. Water can attack to give a hydroxylated product and release H 2 O 2 , but with water absent, the intermediate can be deprotonated, instead giving a desaturated product and H 2 O 2 . Our work outlines the unique mechanism and evolutionary history of these enzymes and sets the stage toward engineering these enzymes to catalyze oxidative reactions.

Microbiome-associated bacteria can have diverse effects on health of their hosts, yet the genetic and molecular bases of these effects have largely remained elusive. This work demonstrates that a novel bacterial locus can modulate systemic plant immunity. Additionally, this work demonstrates that growth-promoting strains may have unanticipated consequences for plant immunity, and this is critical to consider when the plant microbiome is being engineered for agronomic improvement., Plant root-associated microbes promote plant growth and elicit induced systemic resistance (ISR) to foliar pathogens. In an attempt to find novel growth-promoting and ISR-inducing strains, we previously identified strains of root-associated Pseudomonas spp. that promote plant growth but unexpectedly elicited induced systemic susceptibility (ISS) rather than ISR to foliar pathogens. Here, we demonstrate that the ISS-inducing phenotype is common among root-associated Pseudomonas spp. Using comparative genomics, we identified a single Pseudomonas fluorescens locus that is unique to ISS strains. We generated a clean deletion of the 11-gene ISS locus and found that it is necessary for the ISS phenotype. Although the functions of the predicted genes in the locus are not apparent based on similarity to genes of known function, the ISS locus is present in diverse bacteria, and a subset of the genes were previously implicated in pathogenesis in animals. Collectively, these data show that a single bacterial locus contributes to modulation of systemic plant immunity.

Plant root-associated microbes promote plant growth and induce systemic resistance (ISR) to foliar pathogens. In an attempt to find novel growth-promoting and ISR-inducing strains, we previously identified strains of root-associated Pseudomonas spp. that promote plant growth but unexpectedly induced systemic susceptibility (ISS) rather than ISR to foliar pathogens. Here we demonstrate that the ISS-inducing phenotype is common among root-associated Pseudomonas spp. Using comparative genomics, we identified a single P. fluorescens locus that is unique to ISS strains. We generated a clean deletion of the 11-gene ISS locus and found that it is necessary for the ISS phenotype. Although the functions of the predicted genes in the locus are not apparent based on similarity to genes of known function, the ISS locus is present in diverse bacteria and a subset of the genes have previously been implicated in pathogenesis in animals. Collectively these data show that a single bacterial locus contributes to modulation of systemic plant immunity.ImportanceMicrobiome-associated bacteria can have diverse effects on health of their hosts, yet the genetic and molecular basis of these effects have largely remained elusive. This work demonstrates that a novel bacterial locus can modulate systemic plant immunity. Additionally, this work demonstrates that growth promoting strains may have unanticipated consequences on plant immunity and this is critical to consider when engineering the plant microbiome for agronomic improvement.

Correction for ‘An engineered biosynthetic–synthetic platform for production of halogenated indolmycin antibiotics’ by Elesha R. Hoffarth et al., Chem. Sci., 2021, 12, 8817–8821, DOI: 10.1039/D0SC05843B.

Pyrazomycin is a rare C-nucleoside antibiotic with a naturally occurring pyrazole ring, whose biosynthetic origin has remained obscure for decades. In this study, we report the identification of the gene cluster responsible for pyrazomycin biosynthesis in Streptomyces candidus NRRL 3601, revealing that StrR-family regulator PyrR is the cluster-situated transcriptional activator governing pyrazomycin biosynthesis. Furthermore, our results from in vivo reconstitution and stable-isotope feeding experiments support that PyrN is a new nitrogen-nitrogen bond forming enzyme linking the ε-NH2 nitrogen of l-N6-OH-lysine and α-NH2 nitrogen of l-glutamate. This study lays the foundation for further genetic and biochemical characterization of pyrazomycin pathway enzymes constructing the characteristic pyrazole ring.

Two molecules of the amino acid L-tryptophan are the biosynthetic precursors to a class of natural products named the “bisindoles.” Hundreds of these bisindole molecules have been isolated from natural sources, and many of these molecules have potent medicinal properties. Recent studies have clarified the biosynthetic construction of six bisindole molecules, revealing novel enzymatic mechanisms and leading to combinatorial synthesis of new bisindole compounds. Collectively, these results provide a vantage point for understanding how much of the diversity of the bisindole class is generated from a small number of diverging pathways from L-tryptophan, as well as enabling identification of bisindoles that are likely derived via completely distinct biosynthetic pathways., Howard Hughes Predoctoral Fellowship, National Institutes of Health (U.S.) (grant GM65337)

Tryptophan, the most chemically complex and the least abundant of the 20 common proteinogenic amino acids, is a biosynthetic precursor to a large number of complex microbial natural products. Many of these molecules are promising scaffolds for drug discovery and development. The chemical features of tryptophan, including its ability to undergo enzymatic modifications at almost every atom in its structure and its propensity to undergo spontaneous, non-enzyme catalyzed chemistry, make it a unique biological precursor for the generation of chemical complexity. Here, we review the pathways that enable incorporation of tryptophan into complex metabolites in bacteria, with a focus on recently discovered, unusual metabolic transformations.