New Glycosylated Polyene Macrolides: Refining the Ore from Genome Mining

Turonicin A, an Antifungal Linear Polyene Polyketide from an Australian Streptomyces sp

Turonicin A (1) was isolated from Streptomyces sp. MST-123921, which was recovered from soil collected on the banks of the Turon River in New South Wales, Australia. Turonicin A (1) is an amphoteric linear polyene polyketide featuring independent pentaene and tetraenone chromophores and is structurally related to linearmycins A-C (2-4). The structure of 1 was determined by detailed spectroscopic analysis and comparison to literature data. Bioinformatic analysis of the linearmycin biosynthetic gene cluster also allowed the previously unresolved absolute stereostructures of 2-4 to be elucidated. Turonicin A (1) exhibited very potent activity against the fungi Candida albicans (MIC 0.0031 μg/mL, 2.7 nM) and Saccharomyces cerevisiae (MIC 0.0008 μg/mL, 0.7 nM), moderate activity against the bacteria Bacillus subtilis (MIC 0.097 μg/mL, 85 nM) and Staphylococcus aureus (MIC 0.39 μg/mL, 340 nM), and no cytotoxicity against human fibroblasts, making it an attractive candidate for further development as a potential next-generation antibiotic scaffold.

Nysfungin Production Improvement by UV Mutagenesis in Streptomyces noursei D-3-14

Streptomyces noursei D-3-14 was taken as a starting strain and treated with UV (15 W, 30 cm) mutagenesis for 40 s for three consecutive rounds. High yielding strains were screened using chemical and biological potency determination, and the components of the fermentation products were detected using HPLC. Finally, the mutant strain Streptomyces noursei 72-22-1 with a chemical potency of 8912 (U/mL) and a biological potency of 5557 (U/mL) was obtained after the genetic stability evaluation. After optimization of the fermentation conditions, the chemical potency and biological potency of Streptomyces noursei 72-22-1 reached 14,082 U/mL and 10579 U/mL, respectively, which is 1.58 and 1.91 times those before optimization. HPLC analysis indicated that the mutant strain 72-22-1 displayed a higher content of polyfungin B. When equimolar nystatin A1, A3, and polyfungin B were tested for their fungicidal activities towards Saccharomyces cerevisiae ATCC 2061, polyfungin B exhibited a better efficacy than nystatin A1 and A3.

BAC cloning and heterologous expression of a giant biosynthetic gene cluster encoding antifungal neotetrafibricin in streptomyces rubrisoli

Polyene natural products including nystatin A1, amphotericin B, ECO-02301, and mediomycin belong to a large family of valuable antifungal polyketide compounds typically produced by soil actinomycetes. A previous study (Park et al., Front. Bioeng. Biotechnol., 2021, 9, 692340) isolated Streptomyces rubrisoli Inha501 with strong antifungal activity and analyzed a large-sized biosynthetic gene cluster (BGC) of a linear polyene compound named Inha-neotetrafibricin (I-NTF) using whole genome sequencing and bioinformatics. In the present study, an entire I-NTF BGC (∼167 kb) was isolated through construction and screening of Streptomyces BAC library. Overexpression of the cloned I-NTF BGC in the wild-type S. rubrisoli Inha501 and its heterologous expression in S. lividans led to 2.6-fold and 2.8-fold increase in I-NTF yields, respectively. The qRT-PCR confirmed that the transcription levels of I-NTF BGC were significantly increased in both homologous and heterologous hosts containing the BAC integration of I-NTF BGC. In addition, the I-NTF aglycone-producing strains were constructed by a target-specific deletion of glycosyltransferase gene present in I-NTF BGC. A comparison of the in vitro biological activities of I-NTF and I-NTF aglycone confirmed that the rhamnose sugar motif of I-NTF plays a critical role in both antifungal and antibacterial activities. These results suggest that the Streptomyces BAC cloning of a large-sized natural product BGC is a valuable approach for natural product titer improvement and biological activity screening of natural product in actinomycetes.

Streptomyces BAC Cloning of a Large-Sized Biosynthetic Gene Cluster of NPP B1, a Potential SARS-CoV-2 RdRp Inhibitor

As valuable antibiotics, microbial natural products have been in use for decades in various fields. Among them are polyene compounds including nystatin, amphotericin, and nystatin-like Pseudonocardia polyenes (NPPs). Polyene macrolides are known to possess various biological effects, such as antifungal and antiviral activities. NPP A1, which is produced by Pseudonocardia autotrophica, contains a unique disaccharide moiety in the tetraene macrolide backbone. NPP B1, with a heptane structure and improved antifungal activity, was then developed via genetic manipulation of the NPP A1 biosynthetic gene cluster (BGC). Here, we generated a Streptomyces artificial chromosomal DNA library to isolate a large-sized NPP B1 BGC. The NPP B1 BGC was successfully isolated from P. autotrophica chromosome through the construction and screening of a bacterial artificial chromosome (BAC) library, even though the isolated 140-kb BAC clone (named pNPPB1s) lacked approximately 8 kb of the right-end portion of the NPP B1 BGC. The additional introduction of the pNPPB1s as well as co-expression of the 32-kb portion including the missing 8 kb led to a 7.3-fold increase in the production level of NPP B1 in P. autotrophica. The qRT-PCR confirmed that the transcription level of NPP B1 BGC was significantly increased in the P. autotrophica strain containing two copies of the NPP B1 BGCs. Interestingly, the NPP B1 exhibited a previously unidentified SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) inhibition activity in vitro. These results suggest that the Streptomyces BAC cloning of a large-sized, natural product BGC is a valuable approach for titer improvement and biological activity screening of natural products in actinomycetes.

Polyene Antibiotics Physical Chemistry and Their Effect on Lipid Membranes; Impacting Biological Processes and Medical Applications

This review examined a collection of studies regarding the molecular properties of some polyene antibiotic molecules as well as their properties in solution and in particular environmental conditions. We also looked into the proposed mechanism of action of polyenes, where membrane properties play a crucial role. Given the interest in polyene antibiotics as therapeutic agents, we looked into alternative ways of reducing their collateral toxicity, including semi-synthesis of derivatives and new formulations. We follow with studies on the role of membrane structure and, finally, recent developments regarding the most important clinical applications of these compounds.