The Epipeptide Biosynthesis Locus epeXEPAB Is Widely Distributed in Firmicutes and Triggers Intrinsic Cell Envelope Stress

Structural and mechanistic basis for RiPP epimerization by a radical SAM enzyme

D-Amino acid residues, found in countless peptides and natural products including ribosomally synthesized and post-translationally modified peptides (RiPPs), are critical for the bioactivity of several antibiotics and toxins. Recently, radical S-adenosyl-L-methionine (SAM) enzymes have emerged as the only biocatalysts capable of installing direct and irreversible epimerization in RiPPs. However, the mechanism underpinning this biochemical process is ill-understood and the structural basis for this post-translational modification remains unknown. Here we report an atomic-resolution crystal structure of a RiPP-modifying radical SAM enzyme in complex with its substrate properly positioned in the active site. Crystallographic snapshots, size-exclusion chromatography-small-angle x-ray scattering, electron paramagnetic resonance spectroscopy and biochemical analyses reveal how epimerizations are installed in RiPPs and support an unprecedented enzyme mechanism for peptide epimerization. Collectively, our study brings unique perspectives on how radical SAM enzymes interact with RiPPs and catalyze post-translational modifications in natural products.

Direct Detection of the α-Carbon Radical Intermediate Formed by OspD: Mechanistic Insights into Radical S-Adenosyl-l-methionine Peptide Epimerization

OspD is a radical S-adenosyl-l-methionine (SAM) peptide epimerase that converts an isoleucine (Ile) and valine (Val) of the OspA substrate to d-amino acids during biosynthesis of the ribosomally synthesized and post-translationally modified peptide (RiPP) natural product landornamide A. OspD is proposed to carry out this reaction via α-carbon (Cα) H-atom abstraction to form a peptidyl Cα radical that is stereospecifically quenched by hydrogen atom transfer (HAT) from a conserved cysteine (Cys). Here we use site-directed mutagenesis, freeze-quench trapping, isotopic labeling, and electron paramagnetic resonance (EPR) spectroscopy to provide new insights into the OspD catalytic mechanism including the direct observation of the substrate peptide Cα radical intermediate. The putative quenching Cys334 was changed to serine to generate an OspD C334S variant impaired in HAT quenching. The reaction of reduced OspD C334S with SAM and OspA freeze-quenched at 15 s exhibits a doublet EPR signal characteristic of a Cα radical coupled to a single β-H. Using isotopologues of OspA deuterated at either Ile or Val, or both Ile and Val, reveals that the initial Cα radical intermediate forms exclusively on the Ile of OspA. Time-dependent freeze quench coupled with EPR spectroscopy provided evidence for loss of the Ile Cα radical concomitant with gain of a Val Cα radical, directly demonstrating the N-to-C directionality of epimerization by OspD. These results provide direct evidence for the aforementioned OspD-catalyzed peptide epimerization mechanism via a central Cα radical intermediate during RiPP maturation of OspA, a mechanism that may extend to other proteusin peptide epimerases.

Proteases Involved in Leader Peptide Removal during RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) have received much attention in recent years because of their promising bioactivities and the portability of their biosynthetic pathways. Heterologous expression studies of RiPP biosynthetic enzymes identified by genome mining often leave a leader peptide on the final product to prevent toxicity to the host and to allow the attachment of a genetically encoded affinity purification tag. Removal of the leader peptide to produce the mature natural product is then carried out in vitro with either a commercial protease or a protease that fulfills this task in the producing organism. This review covers the advances in characterizing these latter cognate proteases from bacterial RiPPs and their utility as sequence-dependent proteases. The strategies employed for leader peptide removal have been shown to be remarkably diverse. They include one-step removal by a single protease, two-step removal by two dedicated proteases, and endoproteinase activity followed by aminopeptidase activity by the same protease. Similarly, the localization of the proteolytic step varies from cytoplasmic cleavage to leader peptide removal during secretion to extracellular leader peptide removal. Finally, substrate recognition ranges from highly sequence specific with respect to the leader and/or modified core peptide to nonsequence specific mechanisms.

A Promiscuous rSAM Enzyme Enables Diverse Peptide Cross-linking

Ribosomally produced and post-translationally modified polypeptides (RiPPs) are a diverse group of natural products that are processed by a variety of enzymes to their biologically relevant forms. PapB is a member of the radical S-adenosyl-l-methionine (rSAM) superfamily that introduces thioether cross-links between Cys and Asp residues in the PapA RiPP. We report that PapB has high tolerance for variations in the peptide substrate. Our results demonstrate that branched side chains in the thiol- and carboxylate-containing residues are processed and that lengthening of these groups to homocysteine and homoglutamate does not impair the ability of PapB to form thioether cross-links. Remarkably, the enzyme can even cross-link a peptide substrate where the native Asp carboxylate moiety is replaced with a tetrazole. We show that variations to residues embedded between the thiol- and carboxylate-containing residues are tolerated by PapB, as peptides containing both bulky (e.g., Phe) and charged (e.g., Lys) side chains in both natural L- and unnatural D-forms are efficiently cross-linked. Diastereomeric peptides bearing (2S,3R)- and (2S,3S)-methylaspartate are processed by PapB to form cyclic thioethers with markedly different rates, suggesting the enzymatic hydrogen atom abstraction event for the native Asp-containing substrate is diastereospecific. Finally, we synthesized two diastereomeric peptide substrates bearing E- and Z-configured γ,δ-dehydrohomoglutamate and show that PapB promotes addition of the deoxyadenosyl radical (dAdo•) instead of hydrogen atom abstraction. In the Z-configured γ,δ-dehydrohomoglutamate substrate, a fraction of the dAdo-adduct peptide is thioether cross-linked. In both cases, there is evidence for product inhibition of PapB, as the dAdo-adducts likely mimic the native transition state where dAdo• is poised to abstract a substrate hydrogen atom. Collectively, these findings provide critical insights into the arrangement of reacting species in the active site of the PapB, reveal unusual promiscuity, and highlight the potential of PapB as a tool in the development peptide therapeutics.

The putative role of the epipeptide EpeX in Bacillus subtilis intra-species competition

Bacteria engage in competitive interactions with neighbours that can either be of the same or different species. Multiple mechanisms are deployed to ensure the desired outcome and one tactic commonly implemented is the production of specialised metabolites. The Gram-positive bacterium Bacillus subtilis uses specialized metabolites as part of its intra-species competition determinants to differentiate between kin and non-kin isolates. It is, however, unknown if the collection of specialized metabolites defines competitive fitness when the two isolates start as a close, interwoven community that grows into a densely packed colony biofilm. Moreover, the identity of specialized metabolites that have an active role in defining the outcome of an intra-species interaction has not been revealed. Here, we determine the competition outcomes that manifest when 21 environmental isolates of B. subtilis are individually co-incubated with the model isolate NCIB 3610 in a colony biofilm. We correlated these data with the suite of specialized metabolite biosynthesis clusters encoded by each isolate. We found that the epeXEPAB gene cluster was primarily present in isolates with a strong competitive phenotype. This cluster is responsible for producing the epipeptide EpeX. We demonstrated that EpeX is a competition determinant of B. subtilis in an otherwise isogenic context for NCBI 3610. However, when we competed the NCIB 3610 EpeX-deficient strain against our suite of environmental isolates we found that the impact of EpeX in competition is isolate-specific, as only one of the 21 isolates showed increased survival when EpeX was lacking. Taken together, we have shown that EpeX is a competition determinant used by B. subtilis that impacts intra-species interactions but only in an isolate-specific manner.

A model industrial workhorse: Bacillus subtilis strain 168 and its genome after a quarter of a century

The vast majority of genomic sequences are automatically annotated using various software programs. The accuracy of these annotations depends heavily on the very few manual annotation efforts that combine verified experimental data with genomic sequences from model organisms. Here, we summarize the updated functional annotation of Bacillus subtilis strain 168, a quarter century after its genome sequence was first made public. Since the last such effort 5 years ago, 1168 genetic functions have been updated, allowing the construction of a new metabolic model of this organism of environmental and industrial interest. The emphasis in this review is on new metabolic insights, the role of metals in metabolism and macromolecule biosynthesis, functions involved in biofilm formation, features controlling cell growth, and finally, protein agents that allow class discrimination, thus allowing maintenance management, and accuracy of all cell processes. New 'genomic objects' and an extensive updated literature review have been included for the sequence, now available at the International Nucleotide Sequence Database Collaboration (INSDC: AccNum AL009126.4).

Interspecies interaction reduces selection for antibiotic resistance in Escherichia coli

Evolution of microbial traits depends on the interaction of a species with its environment as well as with other coinhabiting species. However, our understanding of the evolution of specific microbial traits, such as antibiotic resistance in complex environments is limited. Here, we determine the role of interspecies interactions on the dynamics of nitrofurantoin (NIT) resistance selection among Escherichia coli. We created a synthetic two-species community comprised of two variants of E. coli (NIT susceptible and resistant) and Bacillus subtilis in minimal media with glucose as the sole carbon source. We show that the presence of B. subtilis significantly slows down the selection for the resistant E. coli mutant when NIT is present and that this slowdown is not due to competition for resources. Instead, the dampening of NIT resistance enrichment is largely mediated by extracellular compounds produced by B. subtilis with the peptide YydF playing a significant role. Our results not only demonstrate the impact of interspecies interactions on the evolution of microbial traits but also show the importance of using synthetic microbial systems in unravelling relevant interactions and mechanisms affecting the evolution of antibiotic resistance. This finding implies that interspecies interactions should be considered to better understand and predict resistance evolution in the clinic as well as in nature.

Mechanism of Action of Ribosomally Synthesized and Post-Translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a natural product class that has undergone significant expansion due to the rapid growth in genome sequencing data and recognition that they are made by biosynthetic pathways that share many characteristic features. Their mode of actions cover a wide range of biological processes and include binding to membranes, receptors, enzymes, lipids, RNA, and metals as well as use as cofactors and signaling molecules. This review covers the currently known modes of action (MOA) of RiPPs. In turn, the mechanisms by which these molecules interact with their natural targets provide a rich set of molecular paradigms that can be used for the design or evolution of new or improved activities given the relative ease of engineering RiPPs. In this review, coverage is limited to RiPPs originating from bacteria.

Leveraging Substrate Promiscuity of a Radical S-Adenosyl-L-methionine RiPP Maturase toward Intramolecular Peptide Cross-Linking Applications

Radical S-adenosyl-l-methionine (RS) enzymes operate on a variety of substrates and catalyze a wide range of complex radical-mediated transformations. Radical non-α-carbon thioether peptides (ranthipeptides) are a class of ribosomally synthesized and post-translationally modified peptides (RiPPs). The RS enzyme PapB catalyzes the formation of thioether cross-links between Cys/Asp (or Cys/Glu) residues located in six Cys-X3-Asp/Glu motifs. In this report, using a minimal substrate that contains a single cross-link motif, we explore the substrate scope of the PapB and show that the enzyme is highly promiscuous and will accept a variety of Cys-X n -Asp sequences where n = 0-6. Moreover, we show that the enzyme will introduce in-line and nested thioether cross-links independently in peptide sequences that contain two motifs derived from the wild-type sequence. Additionally, the enzyme accepts peptides that contain d-amino acids at either the Cys or the Asp position. These observations are leveraged to produce a thioether cyclized analogue of the FDA-approved therapeutic agent octreotide, with a Cys-Glu cross-link replacing the disulfide that is found in the drug. These findings highlight the remarkable substrate tolerance of PapB and show the utility of RS RiPP maturases in biotechnological applications.

Characterization of subtilosin gene in wild type Bacillus spp. and possible physiological role

In a designed study to screen for antimicrobial exhibiting bacteria using molecular aspects, Bacillus species were considered to investigate antibiotic biosynthesis genes. 28 bacterial strains and 3 induced mutants were screened for the presence of subtilosin gene (sbo) and subtilosin through PCR and Mass spectrometry respectively. Sbo gene was detected in 16 out of 28 Bacillus strains. The results from gene sequences deliberated by multiple sequence alignments revealed high-level homology to the sequences of the sbo-alb gene locus of B. subtilis 168 and the other limited reported strains. Hence, this report provided additional strains to support the idea of subtilosin gene predominance amongst Bacillus strains isolated from environment and to find different species containing homologous genes, furthermore the utilization of its conserved region as a means of identifying Bacillus spp. that produce subtilosin. This is the first report to confirm the detection of subtilosin production from B. amyloliquefaciens.

Characterization of subtilosin gene in wild type Bacillus spp. and possible physiological role

In a designed study to screen for antimicrobial exhibiting bacteria using molecular aspects, Bacillus species were considered to investigate antibiotic biosynthesis genes. 28 bacterial strains and 3 induced mutants were screened for the presence of subtilosin gene (sbo) and subtilosin through PCR and Mass spectrometry respectively. Sbo gene was detected in 16 out of 28 Bacillus strains. The results from gene sequences deliberated by multiple sequence alignments revealed high-level homology to the sequences of the sbo-alb gene locus of B. subtilis 168 and the other limited reported strains. Hence, this report provided additional strains to support the idea of subtilosin gene predominance amongst Bacillus strains isolated from environment and to find different species containing homologous genes, furthermore the utilization of its conserved region as a means of identifying Bacillus spp. that produce subtilosin. This is the first report to confirm the detection of subtilosin production from B. amyloliquefaciens.

Macrocyclization and Backbone Modification in RiPP Biosynthesis

The past decade has seen impressive advances in understanding the biosynthesis of ribosomally synthesized and posttranslationally modified peptides (RiPPs). One of the most common modifications found in these natural products is macrocyclization, a strategy also used by medicinal chemists to improve metabolic stability and target affinity and specificity. Another tool of the peptide chemist, modification of the amides in a peptide backbone, has also been observed in RiPPs. This review discusses the molecular mechanisms of biosynthesis of a subset of macrocyclic RiPP families, chosen because of the unusual biochemistry involved: the five classes of lanthipeptides (thioether cyclization by Michael-type addition), sactipeptides and ranthipeptides (thioether cyclization by radical chemistry), thiopeptides (cyclization by [4+2] cycloaddition), and streptide (cyclization by radical C-C bond formation). In addition, the mechanisms of backbone amide methylation, backbone epimerization, and backbone thioamide formation are discussed, as well as an unusual route to small molecules by posttranslational modification.

Radical SAM Enzymes and Ribosomally-Synthesized and Post-translationally Modified Peptides: A Growing Importance in the Microbiomes

To face the current antibiotic resistance crisis, novel strategies are urgently required. Indeed, in the last 30 years, despite considerable efforts involving notably high-throughput screening and combinatorial libraries, only few antibiotics have been launched to the market. Natural products have markedly contributed to the discovery of novel antibiotics, chemistry and drug leads, with more than half anti-infective and anticancer drugs approved by the FDA being of natural origin or inspired by natural products. Among them, thanks to their modular structure and simple biosynthetic logic, ribosomally synthesized and posttranslationally modified peptides (RiPPs) are promising scaffolds. In addition, recent studies have highlighted the pivotal role of RiPPs in the human microbiota which remains an untapped source of natural products. In this review, we report on recent developments in radical SAM enzymology and how these unique biocatalysts have been shown to install complex and sometimes unprecedented posttranslational modifications in RiPPs with a special focus on microbiome derived enzymes.