Improved production of antifungal angucycline Sch47554 by manipulating three regulatory genes inStreptomycessp. SCC‐2136

Transcriptional regulators of secondary metabolite biosynthesis in Streptomyces

In the post-genome era, great progress has been made in metabolic engineering using recombinant DNA technology to enhance the production of high-value products by Streptomyces. With the development of microbial genome sequencing techniques and bioinformatic tools, a growing number of secondary metabolite (SM) biosynthetic gene clusters in Streptomyces and their biosynthetic logics have been uncovered and elucidated. In order to increase our knowledge about transcriptional regulators in SM of Streptomyces, this review firstly makes a comprehensive summary of the characterized factors involved in enhancing SM production and awakening SM biosynthesis. Future perspectives on transcriptional regulator engineering for new SM biosynthesis by Streptomyces are also provided.

TetR and OmpR family regulators in natural product biosynthesis and resistance

This article provides a comprehensive review and sequence-structure analysis of transcription regulator (TR) families, TetR and OmpR/PhoB, involved in specialized secondary metabolite (SSM) biosynthesis and resistance. Transcription regulation is a fundamental process, playing a crucial role in orchestrating gene expression to confer a survival advantage in response to frequent environmental stress conditions. This process, coupled with signal sensing, enables bacteria to respond to a diverse range of intra and extracellular signals. Thus, major bacterial signaling systems use a receptor domain to sense chemical stimuli along with an output domain responsible for transcription regulation through DNA-binding. Sensory and output domains on a single polypeptide chain (one component system, OCS) allow response to stimuli by allostery, that is, DNA-binding affinity modulation upon signal presence/absence. On the other hand, two component systems (TCSs) allow cross-talk between the sensory and output domains as they are disjoint and transmit information by phosphorelay to mount a response. In both cases, however, TRs play a central role. Biosynthesis of SSMs, which includes antibiotics, is heavily regulated by TRs as it diverts the cell's resources towards the production of these expendable compounds, which also have clinical applications. These TRs have evolved to relay information across specific signals and target genes, thus providing a rich source of unique mechanisms to explore towards addressing the rapid escalation in antimicrobial resistance (AMR). Here, we focus on the TetR and OmpR family TRs, which belong to OCS and TCS, respectively. These TR families are well-known examples of regulators in secondary metabolism and are ubiquitous across different bacteria, as they also participate in a myriad of cellular processes apart from SSM biosynthesis and resistance. As a result, these families exhibit higher sequence divergence, which is also evident from our bioinformatic analysis of 158 389 and 77 437 sequences from TetR and OmpR family TRs, respectively. The analysis of both sequence and structure allowed us to identify novel motifs in addition to the known motifs responsible for TR function and its structural integrity. Understanding the diverse mechanisms employed by these TRs is essential for unraveling the biosynthesis of SSMs. This can also help exploit their regulatory role in biosynthesis for significant pharmaceutical, agricultural, and industrial applications.

Identification of a novel glucuronyltransferase from Streptomyces chromofuscus ATCC 49982 for natural product glucuronidation

Glycosylation is an effective way to increase the polarity of natural products. UDP-glucuronyltransferases (UGTs) are commonly observed and extensively studied in phase II drug metabolism. However, UGTs in microorganisms are not well studied, which hampered the utilization of this type of enzyme in microbial glucuronidation of natural products. Screening of five actinomycete strains showed that Streptomyces chromofuscus ATCC 49982 can convert diverse plant polyphenols into more polar products, which were characterized as various glucuronides based on their spectral data. Analysis of the genome of this strain revealed a putative glucuronidation gene cluster that contains a UGT gene (gcaC) and two UDP-glucuronic acid biosynthetic genes (gcaB and gcaD). The gcaC gene was cloned and heterologously expressed in Escherichia coli BL21(DE3). Incubation of the purified enzyme with resveratrol and UDP-glucuronic acid led to the production of resveratrol-4'-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide, allowing GcaC to be characterized as a flexible UGT. The optimal in vitro reaction pH and temperature for GcaC are 7.5 and 30 °C, respectively. Its activity can be stimulated by Ca²⁺, Mg²⁺, and Mn²⁺, whereas Zn²⁺, Cu²⁺, and Fe²⁺ showed inhibitory effects. Furthermore, GcaC has a broad substrate specificity, which can glucuronidate various substrates besides resveratrol, including quercetin, ferulic acid, vanillic acid, curcumin, vanillin, chrysin, zearalenone, and apigenin. The titers of resveratrol-4'-O-β-D-glucuronide and resveratrol-3-O-β-D-glucuronide in E. coli-GcaC were 78.381 ± 0.366 mg/L and 14.991 ± 0.248 mg/L from 114.125 mg/L resveratrol within 3 h. Therefore, this work provides an effective way to produce glucuronides of resveratrol and other health-benefitting natural products. KEY POINTS: • A novel versatile microbial UDP-glucuronyltransferase was discovered and characterized from Streptomyces chromofuscus ATCC 49982. • The UDP-glucuronyltransferase was expressed in Escherichia coli and can convert resveratrol into two glucuronides both in vitro and in vivo. • The UDP-glucuronyltransferase has a highly flexible substrate specificity and is an effective tool to prepare mono- or diglucuronides of bioactive molecules.

The Biocontrol and Plant Growth-Promoting Properties of Streptomyces alfalfae XN-04 Revealed by Functional and Genomic Analysis

Fusarium wilt of cotton, caused by the pathogenic fungal Fusarium oxysporum f. sp. vasinfectum (Fov), is a devastating disease of cotton, dramatically affecting cotton production and quality. With the increase of pathogen resistance, controlling Fusarium wilt disease has become a significant challenge. Biocontrol agents (BCAs) can be used as an additional solution to traditional crop breeding and chemical control. In this study, an actinomycete with high inhibitory activity against Fov was isolated from rhizosphere soil and identified as Streptomyces alfalfae based on phylogenetic analyses. Next, an integrative approach combining genome mining and metabolites detection was applied to decipher the significant biocontrol and plant growth-promoting properties of XN-04. Bioinformatic analysis and bioassays revealed that the antagonistic activity of XN-04 against Fov was associated with the production of various extracellular hydrolytic enzymes and diffusible antifungal metabolites. Genome analysis revealed that XN-04 harbors 34 secondary metabolite biosynthesis gene clusters. The ability of XN-04 to promote plant growth was correlated with an extensive set of genes involved in indoleacetic acid biosynthesis, 1-aminocyclopropane-1-carboxylic acid deaminase activity, phosphate solubilization, and iron metabolism. Colonization experiments indicated that EGFP-labeled XN-04 had accumulated on the maturation zones of cotton roots. These results suggest that S. alfalfae XN-04 could be a multifunctional BCA and biofertilizer used in agriculture.