Mechanism of action of granaticin: inhibition of ribosomal RNA maturation and cell cycle specificity

The Analysis of the Glycosyltransferase Gene Function From a Novel Granaticin Producer, Streptomyces Vilmorinianum. YP1

Glycosylation is common among the synthesis of natural product and imparts the bioactivity for natural product. As for granaticin, a natural product with great bioactivity, glycosylation is an unusual sugar attachment and remains enigmatic. Orf14 in the gra cluster is the predicted glycosyltransferase but without being identified. Recently, we isolated and identified a novel granaticin producer Streptomyces vilmorinianum YP1. Orf14 gene in gra cluster of YP1 is knocked out and complemented. The instrumental analysis of the blue product synthesized by orf14-deficient mutant exhibits the none-granaticin detection and deglycosylated intermediates accumulation. The bioactivity and stability test suggests the weaker or none antibacterial activity and cytotoxicity of this blue product with greater ultraviolet stability and thermostability than granaticin and derivatives produced by YP1. All the result indicates that orf14 encodes glycosyltransferase and glycosylation played an important role in the bioactivity of granaticin. Meanwhile, the blue pigment, deglycosylated intermediates, has favorable processing characteristics. Our finding supplies the function of orf14 and glycosylation, but also indicates a promising candidate of edible blue pigment applicated in food industry.

Organocatalytic activity of granaticin and its involvement in bactericidal function

We previously discovered that actinorhodin, a benzoisochromanequinone antibiotic produced by Streptomyces coelicolor A3(2), serves as a catalyst facilitating the oxidation of ascorbic acid and cysteine (PNAS 48:17,152, 2014). In the present study, we screened for similar ascorbic acid-oxidizing activity in the culture broth of various Streptomyces spp., and discovered marked activity in the culture broth of Streptomyces vietnamensis. The principle active compound was granaticin, a pigmented antibiotic that is structurally related to actinorhodin. The absence of any metals in the purified granaticin fraction indicated that granaticin was an organocatalyst. Granaticin catalyzed the oxidation of L-ascorbic acid, generating L-dehydroascorbic acid and hydrogen peroxide (H₂O₂) at a 1:1 stoichiometric ratio, with 15 times higher reactivity than that of actinorhodin at an optimum pH of 7.0. Granaticin also oxidizes sulfhydryl compounds, including L-cysteine and glutathione. Growth inhibitory assays demonstrated that knockout mutants of the catalase gene exhibit high sensitivity to granaticin. The results suggest that the bactericidal activity of granaticin is exerted by the oxidation of sulfhydryl groups of cellular components and the toxicity of H₂O₂ generated during the oxidation reaction.

Discovery of Mycothiogranaticins from Streptomyces vietnamensis GIMV4.0001 and the Regulatory Effect of Mycothiol on the Granaticin Biosynthesis

Granaticins are benzoisochromanequinone polyketides with remarkable antibacterial and anticancer activities. Three sulfur-containing granaticin congeners, mycothiogranaticins A (1), B (2) and granaticin MA (3) were discovered from a granaticin-producing strain of Streptomyces vietnamensis GIMV4.0001. Two of them were structurally determined with mycothiol or N-acetylcysteine moieties and found to be bio-actively reluctant. Disruption of the mshA gene (SVTN_RS20640) that encodes the D-inositol-3-phosphate glycosyltransferase crucial for mycothiol biosynthesis, fully abolished the production of mycothiogranaticins. The result substantiated that the newly discovered mycothiogranaticins are consequences of the combination of the granaticin and mycothiol biosynthetic pathways. The overall granaticin production of the ΔmshA mutant strain was unexpectedly decreased by at least more than 50%, while similar production level of granaticins to that of the wild type strain was observed in an mycothiol-S transferase gene (SVTN_RS22215) disruptant Δmst. These results indicated that the mycothiol deficiency was responsible for the decreased production of granaticins. Mycothiol may positively regulate the biosynthesis of granaticin possibly by maintaining the cellular redox balance. To the best of our knowledge, this is the first report that mycothiol can not only be a direct building block of polyketides but also play a regulatory role in the polyketide biosynthesis.

Progress and challenges in aminoacyl-tRNA synthetase-based therapeutics

Aminoacyl-tRNA synthetases (ARSs) are universal enzymes that catalyze the attachment of amino acids to the 3' ends of their cognate tRNAs. The resulting aminoacylated tRNAs are escorted to the ribosome where they enter protein synthesis. By specifically matching amino acids to defined anticodon sequences in tRNAs, ARSs are essential to the physical interpretation of the genetic code. In addition to their canonical role in protein synthesis, ARSs are also involved in RNA splicing, transcriptional regulation, translation, and other aspects of cellular homeostasis. Likewise, aminoacylated tRNAs serve as amino acid donors for biosynthetic processes distinct from protein synthesis, including lipid modification and antibiotic biosynthesis. Thanks to the wealth of details on ARS structures and functions and the growing appreciation of their additional roles regulating cellular homeostasis, opportunities for the development of clinically useful ARS inhibitors are emerging to manage microbial and parasite infections. Exploitation of these opportunities has been stimulated by the discovery of new inhibitor frameworks, the use of semi-synthetic approaches combining chemistry and genome engineering, and more powerful techniques for identifying leads from the screening of large chemical libraries. Here, we review the inhibition of ARSs by small molecules, including the various families of natural products, as well as inhibitors developed by either rational design or high-throughput screening as antibiotics and anti-parasitic therapeutics.

Update of the list of QPS-recommended biological agents intentionally added to food or feed as notified to EFSA 6: suitability of taxonomic units notified to EFSA until March 2017

The qualified presumption of safety (QPS) concept was developed to provide a harmonised generic pre‐evaluation to support safety risk assessments of biological agents performed by EFSA's scientific Panels. The identity, body of knowledge, safety concerns and antimicrobial resistance of valid taxonomic units were assessed. Safety concerns identified for a taxonomic unit are, where possible and reasonable in number, reflected as ‘qualifications’ which should be assessed at the strain level by the EFSA's scientific Panels. No new information was found that would change the previously recommended QPS taxonomic units and their qualifications. Between the end of September 2016 and March 2017, the QPS notification list was updated with 87 applications for market authorisation. From these, 32 biological agents already had a QPS status, and 37 were not included in the evaluation as they are filamentous fungi or enterococci. Streptomyces species (Streptomyces cinnamonensis, Streptomyces mobaraensis and Streptomyces violaceoruber), Bacillus circulans (three notifications) and Escherichia coli (seven notifications) were re‐confirmed not suitable for QPS. Streptomyces rubiginosus and Streptomyces netropsis, not evaluated within the previous mandate, were also not recommended for QPS. Streptomyces spp. and E. coli will be excluded from further QPS evaluations within the current QPS mandate. Hyphomicrobium denitrificans, which has never been evaluated before, was not recommended for the QPS list and for Pseudomonas amyloderamosa, the QPS assessment was not applicable because it is not a validated species. Lactobacillus animalis was a new taxonomic unit recommended to have the QPS status.

Bioactive compounds from marine actinomycetes

Actinomycetes are one of the most efficient groups of secondary metabolite producers and are very important from an industrial point of view. Among its various genera, Streptomyces, Saccharopolyspora, Amycolatopsis, Micromonospora and Actinoplanes are the major producers of commercially important biomolecules. Several species have been isolated and screened from the soil in the past decades. Consequently the chance of isolating a novel actinomycete strain from a terrestrial habitat, which would produce new biologically active metabolites, has reduced. The most relevant reason for discovering novel secondary metabolites is to circumvent the problem of resistant pathogens, which are no longer susceptible to the currently used drugs. Existence of actinomycetes has been reported in the hitherto untapped marine ecosystem. Marine actinomycetes are efficient producers of new secondary metabolites that show a range of biological activities including antibacterial, antifungal, anticancer, insecticidal and enzyme inhibition. Bioactive compounds from marine actinomycetes possess distinct chemical structures that may form the basis for synthesis of new drugs that could be used to combat resistant pathogens.

Structure, activity, synthesis and biosynthesis of aryl-C-glycosides

The focus of this review is to highlight the structure, bioactivity and biosynthesis of naturally occurring aryl-C-glycosides. General synthetic methods and their relevance to proposed biochemical mechanisms for the aryl-C-glycoside bond formation are also presented.