Yanyan Zhang

Guvermectin, a Streptomyces-derived purine nucleoside compound, exhibits dual bioactivities as a plant growth regulator and an antibacterial agent. While its biosynthetic gene cluster (BGC) is regulated by the cluster-situated activator GvmR and the adjacent repressor GvmR2, the role of distal transcriptional regulators (TRs) in guvermectin biosynthesis remains unexplored. Here, we identified ScnR1, a highly conserved LacI-family TR located far from the guvermectin BGC, which is directly activated by GvmR. Overexpression of scnR1 significantly suppressed guvermectin biosynthesis. Further investigations revealed that ScnR1 competitively binds to the gvmR, gvmA, and O1 promoters (overlapping with the GvmR-binding sites), thereby inhibiting the guvermectin BGC transcription. Moreover, ScnR1 formed a reciprocal feedback loop with the adjacent repressor GvmR2, where each repressor inhibits the other’s expression. These findings reveal a multi-layered regulatory mechanism wherein LacI-family TRs fine-tune guvermectin biosynthesis through competitive DNA binding and reciprocal feedback control. This study offers new perspectives on the hierarchical control of secondary metabolism in Streptomyces and provides valuable theoretical guidance for the engineering of strains with enhanced natural product production.

Streptomyces species are prolific producers of a diverse array of bioactive secondary metabolites (SMs), which constitute approximately two-thirds of natural product-based pharmaceuticals with significant clinical, agricultural, and biotechnological applications. However, the regulatory mechanisms underlying the biosynthesis of these compounds remain poorly understood, impeding the development of high-yielding strains for industrial production. Herein, we investigated the role of the WhiB-like (Wbl) family transcriptional regulator, WhiD_sbh, in modulating SM production in Streptomyces bingchenggensis BC04. Overexpression of whiD_sbh in BC04 significantly suppressed milbemycin production by repressing the transcription of the milbemycin biosynthetic gene cluster via the ActK/R-KelR regulatory cascade. Comparative transcriptomic analysis between the whiD_sbh overexpression strain and BC04 revealed that whiD_sbh overexpression not only altered the expression of multiple other biosynthetic gene clusters but also impacted genes involved in central carbon metabolism, glutathione biosynthesis, and cofactor synthesis. Furthermore, cross-species genetic analysis demonstrated that whiD_sbh overexpression enhanced actinorhodin production in Streptomyces coelicolor and avermectin production in Streptomyces avermitilis, while inhibiting the biosynthesis of nemadectin and guvermectin in Streptomyces cyaneogriseus ssp. noncyanogenus and Streptomyces caniferus, respectively. These results expanded our understanding of the regulatory networks controlling SM biosynthesis in Streptomyces and provided beneficial regulatory targets for developing strategies to optimize SM yield.

Guvermectin, a purine nucleoside natural product produced by the genus Streptomyces, has recently been registered as a new biopesticide to boost rice yield. Despite its economic and agricultural significance, the regulatory mechanisms of guvermectin biosynthesis remain essentially unknown, hindering industrial production and widespread agricultural application. Here, we examined the roles of two LacI family regulators, gvmR and gvmR2, located within and adjacent to the guvermectin biosynthesis cluster, respectively, in guvermectin production in Streptomyces caniferus NEAU6. GvmR activated the expression of the guvermectin cluster by binding to the promoters of gvmR, gvmA, and O1, while GvmR2 repressed the guvermectin cluster via competitive binding to promoters containing GvmR-binding sites, specifically, a 14-bp palindromic sequences: 5′-RTCATWCGYATGAY-3′ (R = G/A, W = A/T, Y = T/C). Moreover, GvmR indirectly activates the expression of gvmR2 while GvmR2 feedback inhibits gvmR transcription, suggesting a functional interaction between the two regulators for coordinating guvermectin production. Overexpression of gvmR via the T7 RNA polymerase-T7 promoter system in the gvmR2 mutant significantly elevated guvermectin production by 125 % (from 631 mg L⁻¹ to 1422 mg L⁻¹), compared to the parental strain NEAU6. This suggested that combinatorial manipulation of gvmR and gvmR2 is useful for improving guvermectin production. These findings enrich our knowledge of the regulatory network for guvermectin biosynthesis, and offer key targets and effective strategies for high-titer guvermectin production.

Crosstalk regulation is widespread in Streptomyces species. Elucidating the influence of a specific regulator on target biosynthetic gene clusters (BGCs) and cell metabolism is crucial for strain improvement through regulatory protein engineering. PteF and PteR are two regulators that control the biosynthesis of filipin, which competes for building blocks with avermectins in Streptomyces avermitilis. However, little is known about the effects of PteF and PteR on avermectin biosynthesis. In this study, we investigated their impact on avermectin biosynthesis and global cell metabolism. The deletion of pteF resulted in a 55.49% avermectin titer improvement, which was 23.08% higher than that observed from pteR deletion, suggesting that PteF plays a more significant role in regulating avermectin biosynthesis, while PteF hardly influences the transcription level of genes in avermectin and other polyketide BGCs. Transcriptome data revealed that PteF exhibited a global regulatory effect. Avermectin production enhancement could be attributed to the repression of the tricarboxylic acid cycle and fatty acid biosynthetic pathway, as well as the enhancement of pathways supplying acyl-CoA precursors. These findings provide new insights into the role of PteF on avermectin biosynthesis and cell metabolism, offering important clues for designing and building efficient metabolic pathways to develop high-yield avermectin-producing strains.

Guvermectin, produced by Streptomyces bacteria, is a purine nucleoside natural product recently registered as a new biopesticide to boost rice yield. Despite its importance, the regulatory network governing guvermectin biosynthesis remains largely unknown, severely impeding industrial-scale production and widespread application in rice production. Here, we investigated the diverse regulatory mechanisms employed by the cluster-situated regulatory gene, gvmR , in controlling guvermectin production from the perspective of widespread disturbance of gene expression at genome scale. GvmR activates the expression of guvermectin cluster by binding to the gvmR , gvmA and O1 promoters. Additionally, GvmR binds to the promoter of scnR1 , a previously unidentified and highly conserved regulator in Streptomyces . scnR1 overexpression significantly suppressed guvermectin production by regulating the guvermectin cluster through binding to the same promoters as GvmR. Transcriptomic analysis revealed that GvmR extensively influences the expression of numerous genes located outside the guvermectin cluster, including the precursor supply (purine biosynthesis) and energy synthesis (oxidative phosphorylation) pathway genes, as well as 252 transcriptional regulatory genes. By genetic screening from 48 of these 252 regulatory genes, we identified additional five highly conserved genes that impact guvermectin production, suggesting a functional interplay between GvmR and highly conserved regulators in coordinating guvermectin production. These findings enrich our knowledge of the regulatory network governing guvermectin biosynthesis and offer a broadly applicable approach for investigating the molecular regulation of natural product biosynthesis and their high-titer production.

Milbemycins are a group of macrolide pesticides with great potential in the agricultural field owing to their high insecticidal activity and environmental compatibility. Milbemycin A3 and A4 with high bioactivities are the main components of milbemycin-derived products, which require a component ratio A4:A3 of 2.3- to 4.0-fold. Streptomyces bingchenggensis BC04 is a promising milbemycin producer, whereas the component ratio of its products (A4:A3 of 9.0-fold) could not meet the requirement for industrial production. To address this issue, we reconstructed the precursor biosynthetic pathways to fine tune the supply of different acyl-coenzyme A precursors required for milbemycin biosynthesis. Based on an analysis of the intracellular acyl-coenzyme A precursors, we reconstructed stepwise heterogeneous biosynthetic pathways of extender units for milbemycin biosynthesis. Then, we coordinated the supply of milbemycin biosynthetic starter units with temporal promoters. Thanks to these manipulations, we obtained an engineered strain with 39.5% milbemycin titer improvement to 3417.88 mg/L and a qualified component ratio A4:A3 of 3.3-fold. This work demonstrated that coordinating the precursor supply is a simple and effective approach to optimize the component ratio of A4:A3 in milbemycin fermentation products. Moreover, this strategy might also be useful to construct high-yield strains with optimized component ratios of fermentation products in other Streptomyces.

Flowering is a crucial stage for plant reproductive success; therefore, the regulation of plant flowering has been widely researched. Although multiple well-defined endogenous and exogenous flowering regulators have been reported, new ones are constantly being discovered. Here, we confirm that a novel plant growth regulator guvermectin (GV) induces early flowering in Arabidopsis. Interestingly, our genetic experiments newly demonstrated that WRKY41 and its homolog WRKY53 were involved in GV-accelerated flowering as positive flowering regulators. Overexpression of WRKY41 or WRKY53 resulted in an early flowering phenotype compared to the wild type (WT). In contrast, the w41/w53 double mutants showed a delay in GV-accelerated flowering. Gene expression analysis showed that flowering regulatory genes SOC1 and LFY were upregulated in GV-treated WT, 35S:WRKY41, and 35S:WRKY53 plants, but both declined in w41/w53 mutants with or without GV treatment. Meanwhile, biochemical assays confirmed that SOC1 and LFY were both direct targets of WRKY41 and WRKY53. Furthermore, the early flowering phenotype of 35S:WRKY41 lines was abolished in the soc1 or lfy background. Together, our results suggest that GV plays a function in promoting flowering, which was co-mediated by WRKY41 and WRKY53 acting as new flowering regulators by directly activating the transcription of SOC1 and LFY in Arabidopsis.

Streptomyces can produce a wealth of pharmaceutically valuable antibiotics and other bioactive compounds. Production of most antibiotics is generally low due to the rigorously controlled regulatory networks, in which global/pleiotropic and cluster-situated regulatory proteins coordinate with various intra- and extracellular signals. Thus, mining new antibiotic regulatory proteins, particularly the ones that are widespread, is essential for understanding the regulation of antibiotic biosynthesis. Here, in the biopesticide milbemycin producing strain Streptomyces bingchenggensis, a novel global/pleiotropic regulatory protein, SspH, a single domain protein containing only the HATPase domain, was identified as being involved in controlling antibiotic biosynthesis. The sspH overexpression inhibited milbemycin production by repressing the expression of milbemycin biosynthetic genes. The sspH overexpression also differentially influenced the expression of various antibiotic biosynthetic core genes. Site-directed mutagenesis revealed that the HATPase domain was essential for SspH’s function, and mutation of the conserved amino acid residues N54A and D84A led to the loss of SspH function. Moreover, cross-overexpression experiments showed that SspH and its orthologs, SCO1241 from Streptomyces coelicolor and SAVERM_07097 from Streptomyces avermitilis, shared identical functionality, and all exerted a positive effect on actinorhodin production but a negative effect on avermectin production, indicating that SspH-mediated differential control of antibiotic biosynthesis may be widespread in Streptomyces. This study extended our understanding of the regulatory network of antibiotic biosynthesis and provided effective targets for future antibiotic discovery and overproduction.

Streptomyces can produce numerous antibiotics and many other bioactive compounds. Recently, the molecular mechanisms of transcriptional regulators in control of antibiotic production by influencing the expression of biosynthetic gene clusters (BGCs) have been extensively studied. However, for regulators that affect both antibiotic production and cell growth, the way to influence antibiotic production may be diverse, but related studies are limited. Here, based on time-course transcriptome analysis, a four-component system, SbrH1-R, consisting of the two-component system SbrKR (SBI_03479/3478) and two hypothetical proteins SbrH1 (SBI_03481) and SbrH2 (SBI_03480) potentially related with the biosynthesis of milbemycins was identified in Streptomyces bingchenggensis BC-101-4. Deletion of sbrH1-R resulted in weakened cell growth but a 110% increase of milbemycin production compared with that in BC-101-4. Comparative transcriptome analyses of the sbrH1-R mutant and BC-101-4 revealed that SbrH1-R not only indirectly represses milbemycin BGC expression, but also inhibits milbemycin production by modulating expression levels of genes related to precursor supply and antibiotic efflux. Further genetic experiments identified several new targets, including five precursor supply-associated reactions/pathways (e.g., the reaction from pyruvate to acetyl-CoA, the reaction from acetyl-CoA to citrate, the fatty acid β-oxidation process, and the branched chain amino acid and phenylalanine acid degradation pathways) and a milbemycin exporter system (MilEX2) that can be engineered for milbemycin overproduction. These results shed new light on the understanding of regulation of milbemycin biosynthesis and provide useful targets for future metabolic engineering of the native host to improve milbemycin production.