SanJ, an ATP-dependent picolinate-CoA ligase, catalyzes the conversion of picolinate to picolinate-CoA during nikkomycin biosynthesis in Streptomyces ansochromogenes

Complete Genome Sequence Analysis of Kribbella sp. CA-293567 and Identification of the Kribbellichelins A & B and Sandramycin Biosynthetic Gene Clusters

Minor genera actinomycetes are considered a promising source of new secondary metabolites. The strain Kribbella sp. CA-293567 produces sandramycin and kribbellichelins A & B In this work, we describe the complete genome sequencing of this strain and the in silico identification of biosynthetic gene clusters (BGCs), focusing on the pathways encoding sandramycin and kribbellichelins A-B. We also present a comparative analysis of the biosynthetic potential of 38 publicly available genomes from Kribbella strains.

Streptomyces: host for refactoring of diverse bioactive secondary metabolites

Microbial secondary metabolites are intensively explored due to their demands in pharmaceutical, agricultural and food industries. Streptomyces are one of the largest sources of secondary metabolites having diverse applications. In particular, the abundance of secondary metabolites encoding biosynthetic gene clusters and presence of wobble position in Streptomyces strains make it potential candidate as a native or heterologous host for secondary metabolite production including several cryptic gene clusters expression. Here, we have discussed the developments in Streptomyces strains genome mining, its exploration as a suitable host and application of synthetic biology for refactoring genetic systems for developing chassis for enhanced as well as novel secondary metabolites with reduced genome and cleaned background.

Identification and characterization of enzymes involved in the biosynthesis of pyrimidine nucleoside antibiotics

Covering: up to September 2020 Hundreds of nucleoside-based natural products have been isolated from various microorganisms, several of which have been utilized in agriculture as pesticides and herbicides, in medicine as therapeutics for cancer and infectious disease, and as molecular probes to study biological processes. Natural products consisting of structural modifications of each of the canonical nucleosides have been discovered, ranging from simple modifications such as single-step alkylations or acylations to highly elaborate modifications that dramatically alter the nucleoside scaffold and require multiple enzyme-catalyzed reactions. A vast amount of genomic information has been uncovered the past two decades, which has subsequently allowed the first opportunity to interrogate the chemically intriguing enzymatic transformations for the latter type of modifications. This review highlights (i) the discovery and potential applications of structurally complex pyrimidine nucleoside antibiotics for which genetic information is known, (ii) the established reactions that convert the canonical pyrimidine into a new nucleoside scaffold, and (iii) the important tailoring reactions that impart further structural complexity to these molecules.

Natural and engineered biosynthesis of nucleoside antibiotics in Actinomycetes

Nucleoside antibiotics constitute an important family of microbial natural products bearing diverse bioactivities and unusual structural features. Their biosynthetic logics are unique with involvement of complex multi-enzymatic reactions leading to the intricate molecules from simple building blocks. Understanding how nature builds this family of antibiotics in post-genomic era sets the stage for rational enhancement of their production, and also paves the way for targeted persuasion of the cell factories to make artificial designer nucleoside drugs and leads via synthetic biology approaches. In this review, we discuss the recent progress and perspectives on the natural and engineered biosynthesis of nucleoside antibiotics.

Novel nikkomycin analogues generated by mutasynthesis in Streptomyces ansochromogenes

Background

Nikkomycins are competitive inhibitors of chitin synthase and inhibit the growth of filamentous fungi, insects, acarids and yeasts. The gene cluster responsible for biosynthesis of nikkomycins has been cloned and the biosynthetic pathway was elucidated at the genetic, enzymatic and regulatory levels.

Results

Streptomyces ansochromogenes ΔsanL was constructed by homologous recombination and the mutant strain was fed with benzoic acid, 4-hydroxybenzoic acid, nicotinic acid and isonicotinic acid. Two novel nikkomycin analogues were produced when cultures were supplemented with nicotinic acid. These two compounds were identified as nikkomycin Px and Pz by electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR). Bioassays against Candida albicans and Alternaria longipes showed that nikkomycin Px and Pz exhibited comparatively strong inhibitory activity as nikkomycin X and Z produced by Streptomyces ansochromogenes 7100 (wild-type strain). Moreover, nikkomycin Px and Pz were found to be more stable than nikkomycin X and Z at different pH and temperature conditions.

Conclusions

Two novel nikkomycin analogues (nikkomycin Px and Pz) were generated by mutasynthesis with the sanL inactivated mutant of Streptomyces ansochromogenes 7100. Although antifungal activities of these two compounds are similar to those of nikkomycin X and Z, their stabilities are much better than nikkomycin X and Z under different pHs and temperatures.

GouR, a TetR family transcriptional regulator, coordinates the biosynthesis and export of gougerotin in Streptomyces graminearus

Gougerotin is a peptidyl nucleoside antibiotic. It functions as a specific inhibitor of protein synthesis by binding ribosomal peptidyl transferase and exhibits a broad spectrum of biological activities. gouR, situated in the gougerotin biosynthetic gene cluster, encodes a TetR family transcriptional regulatory protein. Gene disruption and genetic complementation revealed that gouR plays an important role in the biosynthesis of gougerotin. Transcriptional analysis suggested that GouR represses the transcription of the gouL-to-gouB operon consisting of 11 structural genes and activates the transcription of the major facilitator superfamily (MFS) transporter gene (gouM). Electrophoresis mobility shift assays (EMSAs) and DNase I footprinting experiments showed that GouR has specific DNA-binding activity for the promoter regions of gouL, gouM, and gouR. Our data suggested that GouR modulates gougerotin production by coordinating its biosynthesis and export in Streptomyces graminearus.

Characterization of the PLP-dependent aminotransferase NikK from Streptomyces tendae and its putative role in nikkomycin biosynthesis

As inhibitors of chitin synthase, nikkomycins have attracted interest as potential antibiotics. The biosynthetic pathway to these peptide nucleosides in Streptomyces tendae is only partially known. In order to elucidate the last step of the biosynthesis of the aminohexuronic building block, we have heterologously expressed a predicted aminotransferase encoded by the gene nikK from S. tendae in Escherichia coli. The purified protein, which is essential for nikkomycin biosynthesis, has a pyridoxal-5'-phosphate cofactor bound as a Schiff base to lysine 221. The enzyme possesses aminotransferase activity and uses several standard amino acids as amino group donors with a preference for glutamate (Glu > Phe > Trp > Ala > His > Met > Leu). Therefore, we propose that NikK catalyses the introduction of the amino group into the ketohexuronic acid precursor of nikkomycins. At neutral pH, the UV-visible absorbance spectrum of NikK has two absorbance maxima at 357 and 425 nm indicative of the presence of the deprotonated and protonated aldimine with an estimated pK(a) of 8.3. The rate of donor substrate deamination is faster at higher pH, indicating that an alkaline environment favours the deamination reaction.

SabR enhances nikkomycin production via regulating the transcriptional level of sanG, a pathway-specific regulatory gene in Streptomyces ansochromogenes

BACKGROUND

sabR is a pleiotropic regulatory gene which has been shown to positively regulate the nikkomycin biosynthesis and negatively affect the sporulation of Streptomyces ansochromogenes. In this study, we investigate the mechanism of SabR on modulating nikkomycin production in Streptomyces ansochromogenes.

RESULTS

The transcription start point of sabR was determined by high-resolution S1 nuclease mapping and localized at the nucleotide T at position 37 bp upstream of the potential sabR translation start codon (GTG). Disruption of sabR enhanced its own transcription, but retarded the nikkomycin production. Over-expression of sabR enhanced nikkomycin biosynthesis in Streptomyces ansochromogenes. EMSA analysis showed that SabR bound to the upstream region of sanG, but it did not bind to the upstream region of its encoding gene (sabR), sanF and the intergenic region between sanN and sanO. DNase 1 footprinting assays showed that the SabR-binding site upstream of sanG was 5'-CTTTAAGTCACCTGGCTCATTCGCGTTCGCCCAGCT-3' which was designated as SARE. Deletion of SARE resulted in the delay of nikkomycin production that was similar to that of sabR disruption mutant.

CONCLUSIONS

These results indicated that SabR modulated nikkomycin biosynthesis as an enhancer via interaction with the promoter region of sanG, and expanded our understanding about regulatory cascade in nikkomycin biosynthesis.

The pleiotropic regulator AdpA-L directly controls the pathway-specific activator of nikkomycin biosynthesis in Streptomyces ansochromogenes

The nikkomycin-producing strain Streptomyces ansochromogenes has a homologue (adpA-L) of the key pleiotropic Streptomyces regulatory gene adpA. Gene disruption and genetic complementation revealed that adpA-L was required for both nikkomycin biosynthesis and morphological differentiation. Transcriptional analysis suggested that the transcription of sanG, the specific activator gene for nikkomycin biosynthesis, was dependent on AdpA-L. In gel-shift and DNase 1 footprinting assays, the purified His(6)-tagged recombinant AdpA-L protein bound the upstream region of sanG at five sites, which are spread over more than one kilobase of DNA and most of which is inside the transcribed region. A consensus AdpA-L-binding sequence, 5'-TGGCNNVWHN-3' (V: C, A or G; W: A or T; H: A, T or C; N: any nucleotide) was found in these binding sites. Transcriptional analysis of sanG carrying mutated AdpA-L binding sites showed that transcription of sanG was eliminated when site I was mutated and its trascription was decreased when site V was mutated, whereas it was increased when the binding sites II, III or IV were mutated. Meanwhile, nikkomycin production of the mutated site III strain was enhanced comparing with the wild-type strain as control. This work highlights a new level of complexity in the regulation of nikkomycin biosynthesis.

Antimicrobial nucleoside antibiotics targeting cell wall assembly: recent advances in structure-function studies and nucleoside biosynthesis

The quest for new antibiotics, especially those with activity against Gram-negative bacteria, is urgent; however, very few new antibiotics have been marketed in the last 40 years, with this limited number falling into only four new structural classes. Several nucleoside natural product antibiotics target bacterial translocase MraY, involved in the lipid-linked cycle of peptidoglycan biosynthesis, and fungal chitin synthase. Biosynthetic studies on the nikkomycin, caprazamycin and pacidamycin/mureidomycin families are also reviewed.

Selectively improving nikkomycin Z production by blocking the imidazolone biosynthetic pathway of nikkomycin X and uracil feeding in Streptomyces ansochromogenes

BACKGROUND

Nikkomycins are a group of peptidyl nucleoside antibiotics and act as potent inhibitors of chitin synthases in fungi and insects. Nikkomycin X and Z are the main components produced by Streptomyces ansochromogenes. Of them, nikkomycin Z is a promising antifungal agent with clinical significance. Since highly structural similarities between nikkomycin Z and X, separation of nikkomycin Z from the culture medium of S. ansochromogenes is difficult. Thus, generating a nikkomycin Z selectively producing strain is vital to scale up the nikkomycin Z yields for clinical trials.

RESULTS

A nikkomycin Z producing strain (sanPDM) was constructed by blocking the imidazolone biosynthetic pathway of nikkomycin X via genetic manipulation and yielded 300 mg/L nikkomycin Z and abolished the nikkomycin X production. To further increase the yield of nikkomycin Z, the effects of different precursors on its production were investigated. Precursors of nucleoside moiety (uracil or uridine) had a stimulatory effect on nikkomycin Z production while precursors of peptidyl moiety (L-lysine and L-glutamate) had no effect. sanPDM produced the maximum yields of nikkomycin Z (800 mg/L) in the presence of uracil at the concentration of 2 g/L and it was approximately 2.6-fold higher than that of the parent strain.

CONCLUSION

A high nikkomycin Z selectively producing was obtained by genetic manipulation combined with precursors feeding. The strategy presented here might be applicable in other bacteria to selectively produce targeted antibiotics.

Mycobacterial phenolic glycolipid virulence factor biosynthesis: mechanism and small-molecule inhibition of polyketide chain initiation

Phenolic glycolipids (PGLs) are polyketide-derived virulence factors produced by Mycobacterium tuberculosis, M. leprae, and other mycobacterial pathogens. We have combined bioinformatic, genetic, biochemical, and chemical biology approaches to illuminate the mechanism of chain initiation required for assembly of the p-hydroxyphenyl-polyketide moiety of PGLs. Our studies have led to the identification of a stand-alone, didomain initiation module, FadD22, comprised of a p-hydroxybenzoic acid adenylation domain and an aroyl carrier protein domain. FadD22 forms an acyl-S-enzyme covalent intermediate in the p-hydroxyphenyl-polyketide chain assembly line. We also used this information to develop a small-molecule inhibitor of PGL biosynthesis. Overall, these studies provide insights into the biosynthesis of an important group of small-molecule mycobacterial virulence factors and support the feasibility of targeting PGL biosynthesis to develop new drugs to treat mycobacterial infections.

SanT, a bidomain protein, is essential for nikkomycin biosynthesis of Streptomyces ansochromogenes

Nikkomycins act as a competitive inhibitor of chitin synthetase and display potent activities against phytopathogenic and human pathogenic fungi. sanT is located in the gene cluster of nikkomycin biosynthesis in Streptomyces ansochromogenes. Sequence analysis revealed that the deduced product of sanT has an unusual domain structure, which consists of an N-terminal acyl carrier protein (ACP) domain and a C-terminal aminotransferase (AMT) domain. Gene disruption and complementation indicated that sanT is essential for nikkomycin biosynthesis. Each domain of SanT was overexpressed in Escherichia coli and then purified. ACP domain is posttranslationally modified with phosphopantetheine (Ppant) prosthetic group at Ser-33. AMT domain catalyzes the transamination of 4-pyridyl-2-oxo-4-hydroxyisovalerate (POHIV), a precursor of peptidyl moiety of nikkomycins, to pyridylhomothreonine (PHT) in vitro. The two domains function independently but both are essential for nikkomycin biosynthesis. The biochemical and genetic evidences suggested that SanT is possibly a bifunctional protein, participating in the biosynthesis of peptidyl moiety and the assembly of nikkomycins.

Identification and Characterization of sanH and sanI Involved in the Hydroxylation of Pyridyl Residue During Nikkomycin Biosynthesis in Streptomyces ansochromogenes

Nikkomycins are highly potent inhibitors of chitin synthase. The nikkomycin biosynthetic gene cluster has been cloned from Streptomyces asochromogenes. Two cytochrome P450 monooxygenase genes (sanQ, sanH) and one ferredoxin gene (sanI) were found in the cluster. It was reported that SanQ is involved in the hydroxylation of L-His, a key step in 4-formyl-4-imidazolin-2-one base biosynthesis. Here, we have studied the function of sanH and sanI. Disruption of sanH abolished the production of nikkomycin X and Z, but it accumulated one dominant component nikkomycin Lx, which is the nikkomycin X analog lacking the hydroxy group at the pyridyl residue. The sanI disruption mutant accumulated predominantly nikkomycin Lx in addition to nikkomycin X and Z. The nikkomycin production profile of the sanH and sanI double disruption mutant was the same as that of the sanH disruption mutant. These results confirmed that SanH is essential for the hydroxylation of pyridyl residue in nikkomycin biosynthesis of S. ansochromogenes and first demonstrated that SanI is an effective electron donor for SanH, but not for SanQ in vivo.

SanM catalyzes the formation of 4-pyridyl-2-oxo-4-hydroxyisovalerate in nikkomycin biosynthesis by interacting with SanN

Nikkomycins are peptidyl nucleoside antibiotics with potent activities against phytopathogenic and human pathogenic fungi. The sanM and sanN genes are required for the nikkomycin biosynthesis of Streptomyces ansochromogenes. In the present study, interaction between SanM and SanN was identified by yeast two-hybrid and co-immunoprecipitation assays. Moreover, SanM and SanN were heterologously expressed and purified. Further biochemical assay demonstrated that the SanM-SanN interaction is essential for SanM aldolase activity but not for SanN dehydrogenase activity. SanM converts piconaldehyde and 2-oxobutyrate to 4-pyridyl-2-oxo-4-hydroxyisovalerate in nikkomycin biosynthesis by interacting with SanN. Steady state kinetics analysis revealed that K(m) and k(cat)/K(m) of SanM are 123.2 microM and 11.4 mM(-1)s(-1) for picolinaldehyde, while 335.6 microM and 4.0 mM(-1)s(-1) for 2-oxobutyrate, respectively. However, SanN as a dehydrogenase is independent of SanM.

Antibiotic biosynthetic pathways and pathway engineering--a growing research field in China

This review describes the recent research activities in China in relation to studies on antibiotic biosynthetic pathways and pathway engineering in actinomycetes. 75 references are cited.