Discovery and characterization of BlsE, a radical S-adenosyl-L-methionine decarboxylase involved in the blasticidin S biosynthetic pathway

Glucuronyl C4 dehydrogenation by the radical SAM enzyme BlsE involved in blasticidin S biosynthesis

Here we report functional investigation of the radical S-adenosylmethionine enzyme BlsE by using cytosylglucuronamide (CGM), which is the amide analog of cytosylglucuronic acid (CGA), an intermediate involved in blasticidin S biosynthesis. We showed that, instead of decarboxylation of CGA reported previously, BlsE catalyzes C4'-dehydrogenation of CGM, and the resulting ketone is acted on by an aminotransferase BlsH to install the C4'-amino group, which uses L-Asp as the amino donor.

Radical S-Adenosyl Methionine Enzyme BlsE Catalyzes a Radical-Mediated 1,2-Diol Dehydration during the Biosynthesis of Blasticidin S

The biosynthesis of blasticidin S has drawn attention due to the participation of the radical S-adenosyl methionine (SAM) enzyme BlsE. The original assignment of BlsE as a radical-mediated, redox-neutral decarboxylase is unusual because this reaction appears to serve no biosynthetic purpose and would need to be reversed by a subsequent carboxylation step. Furthermore, with the exception of BlsE, all other radical SAM decarboxylases reported to date are oxidative in nature. Careful analysis of the BlsE reaction, however, demonstrates that BlsE is not a decarboxylase but instead a lyase that catalyzes the dehydration of cytosylglucuronic acid (CGA) to form cytosyl-4'-keto-3'-deoxy-d-glucuronic acid, which can rapidly decarboxylate nonenzymatically in vitro. Analysis of substrate isotopologs, fluorinated analogues, as well as computational models based on X-ray crystal structures of the BlsE·SAM (2.09 Å) and BlsE·SAM·CGA (2.62 Å) complexes suggests that BlsE catalysis likely proceeds via direct elimination of water from the CGA C4' α-hydroxyalkyl radical as opposed to 1,2-migration of the C3'-hydroxyl prior to dehydration. Biosynthetic and mechanistic implications of the revised assignment of BlsE are discussed.

Identification and characterization of enzymes involved in the biosynthesis of pyrimidine nucleoside antibiotics

Covering: up to September 2020 Hundreds of nucleoside-based natural products have been isolated from various microorganisms, several of which have been utilized in agriculture as pesticides and herbicides, in medicine as therapeutics for cancer and infectious disease, and as molecular probes to study biological processes. Natural products consisting of structural modifications of each of the canonical nucleosides have been discovered, ranging from simple modifications such as single-step alkylations or acylations to highly elaborate modifications that dramatically alter the nucleoside scaffold and require multiple enzyme-catalyzed reactions. A vast amount of genomic information has been uncovered the past two decades, which has subsequently allowed the first opportunity to interrogate the chemically intriguing enzymatic transformations for the latter type of modifications. This review highlights (i) the discovery and potential applications of structurally complex pyrimidine nucleoside antibiotics for which genetic information is known, (ii) the established reactions that convert the canonical pyrimidine into a new nucleoside scaffold, and (iii) the important tailoring reactions that impart further structural complexity to these molecules.

Following the electrons: peculiarities in the catalytic cycles of radical SAM enzymes

Radical SAM enzymes use S-adenosyl-l-methionine as an oxidant to initiate radical-mediated transformations that would otherwise not be possible with Lewis acid/base chemistry alone. These reactions are either redox neutral or oxidative leading to certain expectations regarding the role of SAM as either a reusable cofactor or the ultimate electron acceptor during each turnover. However, these expectations are frequently not realized resulting in fundamental questions regarding the redox handling and movement of electrons associated with these biological catalysts. Herein we provide a focused perspective on several of these questions and associated hypotheses with an emphasis on recently discovered radical SAM enzymes.

Draft genome sequence, disease-resistance genes, and phenotype of a Paenibacillus terrae strain (NK3-4) with the potential to control plant diseases

Paenibacillus terrae NK3-4 is a plant growth-promoting rhizobacterium that may be useful for controlling plant diseases. We conducted a genomic analysis and identified the genes mediating antimicrobial functions. Additionally, an extracellular antifungal protein component was isolated and identified. The draft genome sequence was assembled into 54 contigs, with 5 458 568 bp and a G+C content of 47%. Moreover, 4 690 015 bp encoded 5090 proteins, 7 rRNAs, and 54 tRNAs. Forty-four genes involved in antimicrobial functions were detected. They mainly encode 19 non-ribosomal peptide synthetases (NRPSs); one polyketide synthase/NRPSs hybrid enzyme; four Zn-dependent metalloproteases; three antilisterial bacteriocin subtilosin biosynthesis proteins (AlbA); four serine proteases; five pectate lyases; three beta-glucanases; and four 1,4-beta-xylanases. These include four novel NRPSs that have not been found in any species of Paenibacillus. Furthermore, five proteins exhibiting antifungal activity were identified from the antifungal extracellular protein component based on MS/MS and the strain NK3-4 predicted protein library. On the basis of these features, we propose that strain NK3-4 represents a promising biocontrol agent for protecting plant from diseases. The draft genome sequence described herein may provide the genetic basis for the characterization of the molecular mechanisms underlying the biocontrol functions. It may also facilitate the development of rational strategies for improving the strain.

A mechanistic study of the non-oxidative decarboxylation catalyzed by the radical S-adenosyl-l-methionine enzyme BlsE involved in blasticidin S biosynthesis

BlsE-catalyzed non-oxidative decarboxylation is initiated by a hydrogen abstraction from a sugar carbon of the substrate cytosylglucuronic acid (CGA).

Natural and engineered biosynthesis of nucleoside antibiotics in Actinomycetes

Nucleoside antibiotics constitute an important family of microbial natural products bearing diverse bioactivities and unusual structural features. Their biosynthetic logics are unique with involvement of complex multi-enzymatic reactions leading to the intricate molecules from simple building blocks. Understanding how nature builds this family of antibiotics in post-genomic era sets the stage for rational enhancement of their production, and also paves the way for targeted persuasion of the cell factories to make artificial designer nucleoside drugs and leads via synthetic biology approaches. In this review, we discuss the recent progress and perspectives on the natural and engineered biosynthesis of nucleoside antibiotics.

Molecular architectures and functions of radical enzymes and their (re)activating proteins

Certain proteins utilize the high reactivity of radicals for catalysing chemically challenging reactions. These proteins contain or form a radical and therefore named 'radical enzymes'. Radicals are introduced by enzymes themselves or by (re)activating proteins called (re)activases. The X-ray structures of radical enzymes and their (re)activases revealed some structural features of these molecular apparatuses which solved common enigmas of radical enzymes—i.e. how the enzymes form or introduce radicals at the active sites, how they use the high reactivity of radicals for catalysis, how they suppress undesired side reactions of highly reactive radicals and how they are (re)activated when inactivated by extinction of radicals. This review highlights molecular architectures of radical B12 enzymes, radical SAM enzymes, tyrosyl radical enzymes, glycyl radical enzymes and their (re)activating proteins that support their functions. For generalization, comparisons of the recently reported structures of radical enzymes with those of canonical radical enzymes are summarized here.

Biosynthesis of the β-methylarginine residue of peptidyl nucleoside arginomycin in Streptomyces arginensis NRRL 15941

The peptidyl nucleoside arginomycin is active against Gram-positive bacteria and fungi but displays much lower toxicity to mice than its analog blasticidin S. It features a rare amino acid, β-methylarginine, which is attached to the deoxyhexose moiety via a 4'-aminoacyl bond. We here report cloning of the complete biosynthetic gene cluster for arginomycin from Streptomyces arginensis NRRL 15941. Among the 14 putative essential open reading frames, argM, encoding an aspartate aminotransferase (AAT), and adjacent argN, encoding an S-adenosyl methionine (SAM)-dependent methyltransferase, are coupled to catalyze arginine and yield β-methylarginine in Escherichia coli. Purified ArgM can transfer the α-amino group of l-arginine to α-ketoglutaric acid to give glutamate and thereby converts l-arginine to 5-guanidino-2-oxopentanoic acid, which is methylated at the C-3 position by ArgN to form 5-guanidino-3-methyl-2-oxopentanoic acid. Iteratively, ArgM specifically catalyzes transamination from the donor l-aspartate to the resulting 5-guanidino-3-methyl-2-oxopentanoic acid, generating β-methylarginine. The complete and concise biosynthetic pathway for the rare and bioactive amino acid revealed by this study may pave the way for the production of β-methylarginine either by enzymatic conversion or by engineered living cells.