Specialized or secondary metabolites are small molecules of biological origin, often showing potent biological activities with applications in agriculture, engineering and medicine. Usually, the biosynthesis of these natural products is governed by sets of co-regulated and physically clustered genes known as biosynthetic gene clusters (BGCs). To share information about BGCs in a standardized and machine-readable way, the Minimum Information about a Biosynthetic Gene cluster (MIBiG) data standard and repository was initiated in 2015. Since its conception, MIBiG has been regularly updated to expand data coverage and remain up to date with innovations in natural product research. Here, we describe MIBiG version 4.0, an extensive update to the data repository and the underlying data standard. In a massive community annotation effort, 267 contributors performed 8304 edits, creating 557 new entries and modifying 590 existing entries, resulting in a new total of 3059 curated entries in MIBiG. Particular attention was paid to ensuring high data quality, with automated data validation using a newly developed custom submission portal prototype, paired with a novel peer-reviewing model. MIBiG 4.0 also takes steps towards a rolling release model and a broader involvement of the scientific community. MIBiG 4.0 is accessible online at https://mibig.secondarymetabolites.org/. [GRAPHICS] .

The biarylitides are a recently discovered class of RiPP natural products that are fascinating both from the small size of the core peptides as well as the diversity of peptide crosslinking exhibited by the cytochrome P450 enzymes found in these systems. In this review, we address the discovery and biosynthetic diversity of these systems and discuss the methods and challenges of analysing the structures of these constrained cyclic peptides. We also discuss the structures of the P450 enzymes involved in these pathways and address the potential for alternate catalytic outcomes and activities as seen most recently with the inclusion of biarylitide related enzymes within rufomycin biosynthesis.

Peptide natural products possess a fascinating array of complex structures and diverse functions. Central to this is a repertoire of modified amino acid building blocks, which stem from fundamentally different biosynthesis pathways for peptides of nonribosomal and ribosomal origins. Given these origins, integration of nonribosomal and ribosomal pathways have previously been thought unlikely. Now, we demonstrate that ribosomal biosynthesis generates a key noncanonical 3-nitrotyrosine building block for the nonribosomal synthesis of rufomycin. In this pathway, a biarylitide-type ribosomal peptide is nitrated by a modified cytochrome P450 crosslinking enzyme, with the nitrated residue liberated by the actions of a dedicated protease found within the rufomycin gene cluster before being incorporated into rufomycin by the rufomycin nonribosomal peptide synthetase. This resolves the enigmatic origins of 3-nitrotyrosine within rufomycin biosynthesis and demonstrates unexpected integration of ribosomal peptide synthesis as a mechanism for the generation of noncanonical building blocks within nonribosomal synthesis pathways. ### Competing Interest Statement The authors have declared no competing interest.

The structure of the sidechain crosslinked Tyr-Leu-Trp peptide produced by the biarylitide crosslinking cytochrome P450(Blt) from Micromonospora sp. MW-13 has been reanalysed by a series of NMR, computational and isotope labelling experiments and shown to contain a C-N rather than a C-O bond. Additional in vivo experiments using such a modified peptide show there is a general tolerance of biarylitide crosslinking P450 enzymes for histidine to tryptophan mutations within their minimal peptide substrate sequences despite the lack of such residues noted in natural biarylitide gene clusters. This work further highlights the impressive ability of P450s from biarylitide biosynthesis pathways to act as biocatalysts for the formation of a range of sidechain crosslinked tripeptides.

Peptide side chain cross-linking is an important feature of many natural products, with an increasing number of examples catalyzed by cytochrome P450s being reported from ribosomal biosynthesis pathways in addition to well-known examples from nonribosomal peptide antibiotics. Despite the dramatic recent increase in the number of enzymes and reactions catalyzed, substrate bound structures of such P450s have proven elusive to date. Here, we report the structural characterization of the biarylitide cross-linking enzyme P450(Blt) in complex with its pentapeptide substrate MRYLH. This structure, in combination with computational and biochemical experiments, shows the importance of key I-helix residues in this P450 in coordinating to the histidine residue of the substrate and further that this appears to be central to the specificity of this enzyme for generating a C-N link between the tyrosine and histidine residues in the MRYLH substrate. The structure of the P450(Blt)-MRYLH complex provides the first insight into how peptide substrates can be accommodated within P450s and offers insights into how other examples of related P450s can accept the varied substrates that have recently been identified using bioinformatic methods.

Darobactins represent a class of ribosomally synthesized and post-translationally modified peptide (RiPP) antibiotics featuring a rare bicyclic structure. They target the Bam-complex of Gram-negative bacteria and exhibit in vivo activity against drug-resistant pathogens. First isolated from Photorhabdus species, the corresponding biosynthetic gene clusters (BGCs) are widespread among γ-proteobacteria, including the genera Vibrio, Yersinia, and Pseudoalteromonas (P.). While the organization of the BGC core is highly conserved, a small subset of Pseudoalteromonas carries an extended BGC with additional genes. Here, we report the identification of brominated and dehydrated darobactin derivatives from P. luteoviolacea strains. The marine derivatives are active against multidrug-resistant (MDR) Gram-negative bacteria and showed solubility and plasma protein binding ability different from darobactin A, rendering it more active than darobactin A. The halogenation reaction is catalyzed by DarH, a new class of flavin-dependent halogenases with a novel fold.

New polyketides, termed veramycins, were isolated along with their known congeners NFAT-133 and TM-123. Total synthesis from a central building block was accomplished, the BGC identified and a biosynthetic pathway for this molecule class proposed.

We report our investigation of the utility of peptide crosslinking cytochrome P450 enzymes from biarylitide biosynthesis to generate a range of cyclic tripeptides from simple synthons. The crosslinked tripeptides produced by this P450 include both tyrosine-histidine (A− N −B) and tyrosine-tryptophan (A− O −B) crosslinked tripeptides, the latter a rare example of a phenolic crosslink to an indole moiety. Tripeptides are easily isolated following proteolytic removal of the leader peptide and can incorporate a wide range of amino acids in the residue inside the crosslinked tripeptide. Given the utility of peptide crosslinks in important natural products and the synthetic challenge that these can represent, P450 enzymes have the potential to play roles as important tools in the generation of high-value cyclic tripeptides for incorporation in synthesis, which can be yet further diversified using selective chemical techniques through specific handles contained within these tripeptides.