Microbial production of small peptide: pathway engineering and synthetic biology

Small peptides are a group of natural products with low molecular weights and complex structures. The diverse structures of small peptides endow them with broad bioactivities and suggest their potential therapeutic use in the medical field. The remaining challenge is methods to address the main limitations, namely (i) the low amount of available small peptides from natural sources, and (ii) complex processes required for traditional chemical synthesis. Therefore, harnessing microbial cells as workhorse appears to be a promising approach to synthesize these bioactive peptides. As an emerging engineering technology, synthetic biology aims to create standard, well-characterized and controllable synthetic systems for the biosynthesis of natural products. In this review, we describe the recent developments in the microbial production of small peptides. More importantly, synthetic biology approaches are considered for the production of small peptides, with an emphasis on chassis cells, the evolution of biosynthetic pathways, strain improvements and fermentation.

Impact on Multiple Antibiotic Pathways Reveals MtrA as a Master Regulator of Antibiotic Production in Streptomyces spp. and Potentially in Other Actinobacteria

Regulation of antibiotic production by Streptomyces is complex. We report that the response regulator MtrA is a master regulator for antibiotic production in Streptomyces Deletion of MtrA altered production of actinorhodin, undecylprodigiosin, calcium-dependent antibiotic, and the yellow-pigmented type I polyketide and resulted in altered expression of the corresponding gene clusters in S. coelicolor Integrated in vitro and in vivo analyses identified MtrA binding sites upstream of cdaR, actII-orf4, and redZ and between cpkA and cpkD MtrA disruption also led to marked changes in chloramphenicol and jadomycin production and in transcription of their biosynthetic gene clusters (cml and jad, respectively) in S. venezuelae, and MtrA sites were identified within cml and jad MtrA also recognized predicted sites within the avermectin and oligomycin pathways in S. avermitilis and in the validamycin gene cluster of S. hygroscopicus The regulator GlnR competed for several MtrA sites and impacted production of some antibiotics, but its effects were generally less dramatic than those of MtrA. Additional potential MtrA sites were identified in a range of other antibiotic biosynthetic gene clusters in Streptomyces species and other actinobacteria. Overall, our study suggests a universal role for MtrA in antibiotic production in Streptomyces and potentially other actinobacteria.IMPORTANCE In natural environments, the ability to produce antibiotics helps the producing host to compete with surrounding microbes. In Streptomyces, increasing evidence suggests that the regulation of antibiotic production is complex, involving multiple regulatory factors. The regulatory factor MtrA is known to have additional roles beyond controlling development, and using bioassays, transcriptional studies, and DNA-binding assays, our study identified MtrA recognition sequences within multiple antibiotic pathways and indicated that MtrA directly controls the production of multiple antibiotics. Our analyses further suggest that this role of MtrA is evolutionarily conserved in Streptomyces species, as well as in other actinobacterial species, and also suggest that MtrA is a major regulatory factor in antibiotic production and in the survival of actinobacteria in nature.

A Hierarchical Network of Four Regulatory Genes Controlling Production of the Polyene Antibiotic Candicidin in Streptomyces sp. Strain FR-008

The four regulatory genes fscR1 to fscR4 in Streptomyces sp. strain FR-008 form a genetic arrangement that is widely distributed in macrolide-producing bacteria. Our previous work has demonstrated that fscR1 and fscR4 are critical for production of the polyene antibiotic candicidin. In this study, we further characterized the roles of the other two regulatory genes, fscR2 and fscR3, focusing on the relationship between these four regulatory genes. Disruption of a single or multiple regulatory genes did not affect bacterial growth, but transcription of genes in the candicidin biosynthetic gene cluster decreased, and candicidin production was abolished, indicating a critical role for each of the four regulatory genes, including fscR2 and fscR3, in candicidin biosynthesis. We found that fscR1 to fscR4, although differentially expressed throughout the growth phase, displayed similar temporal expression patterns, with an abrupt increase in the early exponential phase, coincident with initial detection of antibiotic production in the same phase. Our data suggest that the four regulatory genes fscR1 to fscR4 have various degrees of control over structural genes in the biosynthetic cluster under the conditions examined. Extensive transcriptional analysis indicated that complex regulation exists between these four regulatory genes, forming a regulatory network, with fscR1 and fscR4 functioning at a lower level. Comprehensive cross-complementation analysis indicates that functional complementation is restricted among the four regulators and unidirectional, with fscR1 complementing the loss of fscR3 or -4 and fscR4 complementing loss of fscR2 Our study provides more insights into the roles of, and the regulatory network formed by, these four regulatory genes controlling production of an important pharmaceutical compound.IMPORTANCE The regulation of antibiotic biosynthesis by Streptomyces species is complex, especially for biosynthetic gene clusters with multiple regulatory genes. The biosynthetic gene cluster for the polyene antibiotic candicidin contains four consecutive regulatory genes, which encode regulatory proteins from different families and which form a subcluster within the larger biosynthetic gene cluster in Streptomyces sp. FR-008. Syntenic arrangements of these regulatory genes are widely distributed in polyene gene clusters, such as the amphotericin and nystatin gene clusters, suggesting a conserved regulatory mechanism controlling production of these clinically important medicines. However, the relationships between these multiple regulatory genes are unknown. In this study, we determined that each of these four regulatory genes is critical for candicidin production. Additionally, using transcriptional analyses, bioassays, high-performance liquid chromatography (HPLC) analysis, and genetic cross-complementation, we showed that FscR1 to FscR4 comprise a hierarchical regulatory network that controls candicidin production and is likely representative of how expression of other polyene biosynthetic gene clusters is controlled.

Regulation of Bottromycin Biosynthesis Involves an Internal Transcriptional Start Site and a Cluster-Situated Modulator

Bottromycin is a ribosomally synthesized and post-translationally modified peptide (RiPP) produced by several streptomycetes, including the plant pathogen Streptomyces scabies. There is significant interest in this molecule as it possesses strong antibacterial activity against clinically relevant multidrug resistant pathogens and is structurally distinct from all other antibiotics. However, studies into its efficacy are hampered by poor yields. An understanding of how bottromycin biosynthesis is regulated could aid the development of strategies to increase titres. Here, we use 5′-tag-RNA-seq to identify the transcriptional organization of the gene cluster, which includes an internal transcriptional start site that precedes btmD, the gene that encodes the bottromycin precursor peptide. We show that the gene cluster does not encode a master regulator that controls pathway expression and instead encodes a regulatory gene, btmL, which functions as a modulator that specifically affects the expression of btmD but not genes up- or downstream of btmD. In order to identify non-cluster associated proteins involved in regulation, proteins were identified that bind to the main promoter of the pathway, which precedes btmC. This study provides insights into how this deceptively complex pathway is regulated in the absence of a pathway specific master regulator, and how it might coordinate with the central metabolism of the cell.

The PolS-PolR Two-Component System Regulates Genes Involved in Poly-P Metabolism and Phosphate Transport in Microlunatus phosphovorus

Microlunatus phosphovorus NM-1 is a polyphosphate (poly-P)-accumulating bacterium that accumulates poly-P under aerobic conditions and degrades poly-P under anaerobic conditions. In this study, the two-component system (TCS) PolS-PolR was identified in NM-1, and the response regulator PolR was found to directly bind to the promoters of genes related to phosphate transport (MLP_RS00235, MLP_RS23035, and MLP_RS24590); poly-P catabolism (MLP_RS12905) and poly-P synthesis (MLP_RS23025). RT-qPCR assays showed that ppgk (MLP_RS12905), ppk (MLP_RS23025), pstS (MLP_RS23035), and pit (MLP_RS24590) were down-regulated during the aerobic-anaerobic shift. The sequence GTTCACnnnnnGTTCaC was identified as a recognition sequence for PolR by MEME analysis and DNase I footprinting. EMSAs and ChIP-qPCR assays indicated that PolR binds to the promoters of pit (MLP_RS00235), ppgk (MLP_RS12905), ppk (MLP_RS23025), pstS (MLP_RS23035) and pit (MLP_RS24590), and ChIP-qPCR further suggested that the binding affinity of PolR was lower under anaerobic conditions than under aerobic conditions in vivo. These findings indicate that the PolS-PolR TCS in M. phosphovorus may be involved in the regulation of poly-P metabolism in response to levels of dissolved oxygen in the environment, and our results provide insights into new approaches for understanding the mechanisms of phosphorus accumulation and release.

The CagRS Two-Component System Regulates Clavulanic Acid Metabolism via Multiple Pathways in Streptomyces clavuligerus F613-1

Streptomyces clavuligerus F613-1 produces a clinically important β-lactamase inhibitor, clavulanic acid (CA). Although the biosynthesis pathway of CA has essentially been elucidated, the global regulatory mechanisms of CA biosynthesis remain unclear. The paired genes cagS and cagR, which are annotated, respectively, as orf22 and orf23 in S. clavuligerus ATCC 27064, encode a bacterial two-component regulatory system (TCS) and were found next to the CA biosynthetic gene cluster of S. clavuligerus F613-1. To further elucidate the regulatory mechanism of CA biosynthesis, the CagRS TCS was deleted from S. clavuligerus F613-1. Deletion of cagRS resulted in decreased production of CA, but the strain phenotype was not otherwise affected. Both transcriptome and ChIP-seq data revealed that, in addition to CA biosynthesis, the CagRS TCS mainly regulates genes involved in primary metabolism, such as glyceraldehyde 3-phosphate (G3P) metabolism and arginine biosynthesis. Notably, both G3P and arginine are precursors of CA. Electrophoretic mobility shift assays demonstrated that the response regulator CagR could bind to the intergenic regions of argG, argC, oat1, oat2, ceaS1, and claR in vitro, suggesting that CagR can directly regulate genes involved in arginine and CA biosynthesis. This study indicated that CagRS is a pleiotropic regulator that can directly affect the biosynthesis of CA and indirectly affect CA production by regulating the metabolism of arginine and G3P. Our findings provide new insights into the regulation of CA biosynthetic pathways and provide an innovative approach for future metabolic engineering efforts for CA production in S. clavuligerus.

Novel Two-Component System MacRS Is a Pleiotropic Regulator That Controls Multiple Morphogenic Membrane Protein Genes in Streptomyces coelicolor

As with most annotated two-component systems (TCSs) of Streptomyces coelicolor, the function of TCS SCO2120/2121 was unknown. Based on our findings, we have designated this TCS MacRS, for morphogenesis and actinorhodin regulator/sensor. Our study indicated that either single or double mutation of MacRS largely blocked production of actinorhodin but enhanced formation of aerial mycelium. Chromatin immunoprecipitation (ChIP) sequencing, using an S. coelicolor strain expressing MacR-Flag fusion protein, identified in vivo targets of MacR, and DNase I footprinting of these targets revealed a consensus sequence for MacR binding, TGAGTACnnGTACTCA, containing two 7-bp inverted repeats. A genome-wide search revealed sites identical or highly similar to this consensus sequence upstream of six genes encoding putative membrane proteins or lipoproteins. These predicted sites were confirmed as MacR binding sites by DNase I footprinting and electrophoretic mobility shift assays in vitro and by ChIP-quantitative PCR in vivo, and transcriptional analyses demonstrated that MacR significantly impacts expression of these target genes. Disruption of three of these genes, sco6728, sco4924, and sco4011, markedly accelerated aerial mycelium formation, indicating that their gene products are novel morphogenic factors. Two-hybrid assays indicated that these three proteins, which we have named morphogenic membrane protein A (MmpA; SCO6728), MmpB (SCO4924), and MmpC (SCO4011), interact with one another and with the putative membrane protein and MacR target SCO4225. Notably, SAV6081/82 and SVEN1780/81, homologs of MacRS TCS from S. avermitilis and S. venezuelae, respectively, can substitute for MacRS, indicating functional conservation. Our findings reveal a role for MacRS in cellular morphogenesis and secondary metabolism in Streptomyces IMPORTANCE TCSs help bacteria adapt to environmental stresses by altering gene expression. However, the roles and corresponding regulatory mechanisms of most TCSs in the Streptomyces model strain S. coelicolor are unknown. We investigated the previously uncharacterized MacRS TCS and identified the core DNA recognition sequence, two seven-nucleotide inverted repeats, for the DNA-binding protein MacR. We further found that MacR directly controls a group of membrane proteins, including MmpA-C, which are novel morphogenic factors that delay formation of aerial mycelium. We also discovered that these membrane proteins interact with one another and that other Streptomyces species have conserved MacRS homologs. Our findings suggest a conserved role for MacRS in morphogenesis and/or other membrane-associated activities. Additionally, our study showed that MacRS impacts, albeit indirectly, the production of the signature metabolite actinorhodin, further suggesting that MacRS and its homologs function as novel pleiotropic regulatory systems in Streptomyces.

Characterization of a LAL-type regulator NemR in nemadectin biosynthesis and its application for increasing nemadectin production in Streptomyces cyaneogriseus

Nemadectin, a macrocyclic lactone antibiotic, is produced by Streptomyces cyaneogriseus ssp. noncyanogenus. A methoxime derivative of nemadectin, moxdectin, has been widely used to control insect and helminth in animal health. Despite the importance of nemadectin, little attention has been paid to the regulation of nemadectin biosynthesis, which has hindered efforts to improve nemadectin production via genetic manipulation of regulatory genes. Here, we characterize the function of nemR, the cluster-situated regulatory gene encoding a LAL-family transcriptional regulator, in the nemadectin biosynthesis gene cluster of S. cyaneogriseus ssp. noncyanogenus NMWT1. NemR is shown to be essential for nemadectin production and found to directly activate the transcription of nemA1-1/A1-2/A2, nemC and nemA4/A3/E/D operons, but indirectly activate that of nemG and nemF. A highly conserved sequence 5'-TGGGGTGKATAGGGGGTA-3' (K=T/G) is verified to be essential for NemR binding. Moreover, four novel targets of NemR, including genes encoding an SsgA-like protein (TU94_12730), a methylmalonyl-CoA mutase (TU94_19950), a thioesterase of oligomycin biosynthesis (TU94_22425) and a MFS family transporter (TU94_24835) are identified. Overexpression of nemR significantly increased nemadectin production by 79.9%, in comparison with NMWT1, suggesting that nemR plays an important role in the nemadectin biosynthesis.

A Master Switch in Thiopeptide Biosynthesis

Although considerable knowledge of the biosynthetic machinery of thiopeptide antibiotics has been accumulated, the regulation of their production remains unclear. In this issue of Cell Chemical Biology, Li et al. (2018) have now characterized a key transcription factor and suggest its feedback regulation by biosynthesis intermediates and the final product.

Deletion of MtrA Inhibits Cellular Development of Streptomyces coelicolor and Alters Expression of Developmental Regulatory Genes

The developmental life cycle of Streptomyces species includes aerial hyphae formation and spore maturation, two distinct developmental processes that are controlled, respectively, by two families of developmental regulatory genes, bld and whi. In this study, we show that the response regulator MtrA (SCO3013) is critical for normal development of aerial hyphae in S. coelicolor and related species. ΔmtrA, a deletion mutant of the response regulator gene mtrA, exhibited the bald phenotype typical of bld mutants defective in aerial mycelium formation, with formation either much delayed or absent depending on the culture medium. Transcriptional analysis indicated that MtrA activates multiple genes involved in formation of aerial mycelium, including chp, rdl, and ram genes, as well as developmental regulatory genes of the bld and whi families. However, the major regulatory gene bldD showed enhanced expression in ΔmtrA, suggesting it is repressed by MtrA. electrophoretic mobility shift assays indicated that MtrA binds upstream of several genes with altered expression in ΔmtrA, including bldD and whiI, and sequences similar to the consensus binding sequence for MtrA of another actinomycete, Mycobacterium tuberculosis, were found in the bound sites. A loosely conserved recognition sequence containing two short, direct repeats was identified for MtrA of S. coelicolor and was validated using mutational analysis. MtrA homologs are widely distributed among Streptomyces species, and as with S. coelicolor, deletion of the mtrA homologs sve_2757 from S. venezuelae and sli_3357 from S. lividans resulted in conditional bald morphology. Our study suggests a critical and conserved role for MtrA in Streptomyces development.

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

With advances in sequencing technology, uncharacterized proteins and domains of unknown function (DUFs) are rapidly accumulating in sequence databases and offer an opportunity to discover new protein chemistry and reaction mechanisms. The focus of this review, the formerly enigmatic YcaO superfamily (DUF181), has been found to catalyze a unique phosphorylation of a ribosomal peptide backbone amide upon attack by different nucleophiles. Established nucleophiles are the side chains of Cys, Ser, and Thr which gives rise to azoline/azole biosynthesis in ribosomally synthesized and posttranslationally modified peptide (RiPP) natural products. However, much remains unknown about the potential for YcaO proteins to collaborate with other nucleophiles. Recent work suggests potential in forming thioamides, macroamidines, and possibly additional post-translational modifications. This review covers all knowledge through mid-2016 regarding the biosynthetic gene clusters (BGCs), natural products, functions, mechanisms, and applications of YcaO proteins and outlines likely future research directions for this protein superfamily.

Production of the antibiotic FR-008/candicidin in Streptomyces sp. FR-008 is co-regulated by two regulators, FscRI and FscRIV, from different transcription factor families

In Streptomyces sp. FR-008, the biosynthetic gene cluster of the polyene antibiotic FR-008, also known as candicidin, consists of 21 genes, including four regulatory genes, fscRI-fscRIV. Our bioinformatics analyses indicate that FscRI has an N-terminal PAS domain, whereas the other three regulators have N-terminal AAA domains and are members of the LAL (large ATP-binding regulators of the LuxR type) family. Deletion of fscRI abolished the production of FR-008, with production restored in the complemented strain, supporting a critical role for FscRI in FR-008 biosynthesis. Consistent with these findings, transcription of genes involved in the biosynthesis and efflux of FR-008 was greatly downregulated in a ΔfscRI mutant. Interestingly, the regulatory gene fscRIV was also downregulated in the ΔfscRI mutant. Production of FR-008 was reduced, but not abrogated, in an fscRIV deletion mutant, and although structural genes were downregulated in ΔfscRIV, the changes were much less dramatic than in ΔfscRI, suggesting a stronger regulatory role for FscRI. Remarkably, transcription of fscRI was also decreased in ΔfscRIV. Expression of fscRI restored antibiotic production in a ΔfscRIV mutant, but not vice versa. Putative binding sequences for FscRI were identified upstream of fscRIV and the three structural genes fscA, fscB and fscD, which encode large modular polyketide synthases. Our findings suggest that fscRI and fscRIV are interregulatory, whereas expression of fscRII and fscRIII appears to be independent of fscRI and fscRIV. This study demonstrates that the regulation of polyene antibiotic synthesis can involve mutually regulated transcriptional activators that belong to different families.

EspR, a regulator of the ESX-1 secretion system in Mycobacterium tuberculosis, is directly regulated by the two-component systems MprAB and PhoPR

The regulatory mechanisms that control the ESX-1 secretion system, a key player in the pathogenesis of Mycobacterium tuberculosis, have not been fully elucidated. However, factors that regulate the ESX-1 substrate EspA usually affect ESX-1 function. Previous studies showed that espA is directly regulated by the nucleoid-associated protein EspR and the two-component system (TCS) MprAB. The PhoPR TCS also activates espA, but the direct target of PhoP was unknown. In this report, we reveal that EspR is directly regulated by MprA and PhoP-Rv, but not by PhoP-Ra. PhoP-Rv and MprA binding sites in the espR promoter were determined by gel-shift and DNase I footprinting assays, which identified a PhoP-protected region centred approximately 205 bp before the espR start codon and that encompasses MprA Region-1, one of two MprA-protected regions. MprA Region-2 is located approximately 60 bp downstream of MprA Region-1 and overlaps a known EspR binding site. Nucleotides essential for the binding of PhoP and/or MprA were identified through site-directed DNA mutagenesis. Our studies also indicate that MprA Region-2, but not MprA Region-1/PhoP region, is required for the full expression of espR. Recombinant strains carrying mutations at MprA Region-2 exhibited lower transcription levels for espR, espA and espD, and had reduced EspR and EspA levels in cell lysates. These findings indicate that EspR may mediate the regulatory effect of PhoPR and MprAB, and provide more insight into the mechanisms underlying ESX-1 control.

Nucleophilic 1,4-additions for natural product discovery

Natural products remain an important source of drug candidates, but the difficulties inherent to traditional isolation, coupled with unacceptably high rates of compound rediscovery, limit the pace of natural product detection. Here we describe a reactivity-based screening method to rapidly identify exported bacterial metabolites that contain dehydrated amino acids (i.e., carbonyl- or imine-activated alkenes), a common motif in several classes of natural products. Our strategy entails the use of a commercially available thiol, dithiothreitol, for the covalent labeling of activated alkenes by nucleophilic 1,4-addition. Modification is easily discerned by comparing mass spectra of reacted and unreacted cell surface extracts. When combined with bioinformatic analysis of putative natural product gene clusters, targeted screening and isolation can be performed on a prioritized list of strains. Moreover, known compounds are easily dereplicated, effectively eliminating superfluous isolation and characterization. As a proof of principle, this labeling method was used to identify known natural products belonging to the thiopeptide, lanthipeptide, and linaridin classes. Further, upon screening a panel of only 23 actinomycetes, we discovered and characterized a novel thiopeptide antibiotic, cyclothiazomycin C.