Biosynthesis of the Actinomycin Chromophore; Enzymatic Conversion of 4-Methyl-3-hydroxyanthranilic Acid to Actinocin

ActO, a positive cluster-situated regulator for actinomycins biosynthesis in Streptomyces antibioticus ZS

Actinomycins are a class of cyclic lipopeptide antibiotics produced by Streptomyces, which have rich biological activities and demonstrate great potential value. Among them, actinomycin D is currently the effective drug for some malignant tumor diseases. Although the chemical properties, biological activities and biosynthesis of actinomycins have been extensively studied, the regulation of their biosynthesis remains poorly understood. Streptomyces antibioticus ZS isolated from deep-sea corals is a producer of actinomycin D and actinomycin V. Here, we reported the characterization of a cluster-situated regulator ActO in actinomycins biosynthetic gene cluster (act cluster) of S. antibioticus ZS, which belongs to LmbU family. Deletion of actO completely blocked the synthesis of actinomycins. Overexpression of actO increased the yields of actinomycin D and actinomycin V by 4.4 fold and 2.6 fold, respectively. The result of RT-qPCR showed that ActO activates the transcription of all genes in act cluster. However, no specific binding of His6-ActO to the promoters of target genes was observed after electrophoretic mobility shift assay (EMSA). These results proved that ActO serves as a positive regulator involved in the biosynthesis of actinomycins, affecting the transcription of all genes related to the synthesis of intermediates, skeleton modification and extracellular transportation of final products. Moreover, we demonstrated that overexpression of actO is a novel strategy to increase the yields of actinomycins.

Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

High phenoxazinone synthase activity of two mononuclear cis-dichloro cobalt(ii) complexes with a rigid pyridyl scaffold

Two mononuclear cis-dichloro cobalt(II) complexes with bidentate pyridyl ligands have been successfully synthesized and employed as active o-aminophenol oxidation catalysts resulting in high turnover numbers under aerobic conditions.

Type II non-ribosomal peptide synthetase proteins: structure, mechanism, and protein-protein interactions

Covering: 1990 to 2019 Many medicinally-relevant compounds are derived from non-ribosomal peptide synthetase (NRPS) products. Type I NRPSs are organized into large modular complexes, while type II NRPS systems contain standalone or minimal domains that often encompass specialized tailoring enzymes that produce bioactive metabolites. Protein-protein interactions and communication between the type II biosynthetic machinery and various downstream pathways are critical for efficient metabolite production. Importantly, the architecture of type II NRPS proteins makes them ideal targets for combinatorial biosynthesis and metabolic engineering. Future investigations exploring the molecular basis or protein-protein recognition in type II NRPS pathways will guide these engineering efforts. In this review, we consolidate the broad range of NRPS systems containing type II proteins and focus on structural investigations, enzymatic mechanisms, and protein-protein interactions important to unraveling pathways that produce unique metabolites, including dehydrogenated prolines, substituted benzoic acids, substituted amino acids, and cyclopropanes.

Preparation and characterization of a highly stable phenoxazinone synthase nanogel

BACKGROUND

Phenoxazinone synthase (PHS) is a laccase-like multicopper oxidase originating from Streptomyces with great industrial application potential. In this paper, we prepared the PHS nanogel retaining 82 % of its initial activity by aqueous in situ polymerization at pH 9.3.

RESULTS

The average diameter of the PHS nanogel was 50.8 nm based on dynamic light scattering (DLS) analysis. Fluorescence analysis indicated the impressive preservation of the enzyme molecular structure upon modification. The PHS nanogel exhibited the most activity at pH 4.0-4.5 and 50 °C while the corresponding values were pH 4.5 and 40 °C for the native PHS. The K m and V max of the PHS nanogel were found to be 0.052 mM and 0.018 mM/min, whereas those of the native PHS were 0.077 mM and 0.021 mM/min, respectively. In addition, the PHS nanogel possessed higher thermal and storage stability and solvent tolerance compared with the native one. The half-life of the PHS nanogel was 1.71 h and multiplied around ninefold compared to 0.19 h for the native one.

CONCLUSION

In summary, the PHS nanogel could be a promising biocatalyst in industry.

RNase III is required for actinomycin production in Streptomyces antibioticus

Using insertional mutagenesis, we have disrupted the RNase III gene, rnc, of the actinomycin-producing streptomycete, Streptomyces antibioticus. Disruption was verified by Southern blotting. The resulting strain grows more vigorously than its parent on actinomycin production medium but produces significantly lower levels of actinomycin. Complementation of the rnc disruption with the wild-type rnc gene from S. antibioticus restored actinomycin production to nearly wild-type levels. Western blotting experiments demonstrated that the disruptant did not produce full-length or truncated forms of RNase III. Thus, as is the case in Streptomyces coelicolor, RNase III is required for antibiotic production in S. antibioticus. No differences in the chemical half-lives of bulk mRNA were observed in a comparison of the S. antibioticus rnc mutant and its parental strain.

Basic and applied features of multicopper oxidases, CueO, bilirubin oxidase, and laccase

Multicopper oxidases (MCOs) such as CueO, bilirubin oxidase, and laccase contain four Cu centers, type 1 Cu, type II Cu, and a pair of type III Cu's in a protein molecule consisting of three domains with homologous structure to cupredoxin containing only type I Cu. Type I Cu mediates electron transfer between the substrate and the trinuclear Cu center formed by a type II Cu and a pair of type III Cu's, where the final electron acceptor O(2) is converted to H(2)O without releasing activated oxygen species. During the process, O(2) is reduced by MCOs such as lacquer laccase and bilirubin oxidase; the reaction intermediate II with a possible doubly OH(-)-bridged structure in the trinuclear Cu center has been detected. The preceding reaction intermediate I has been detected by the reaction of the lacquer laccase in a mixed valence state, at which type I Cu was cuprous and the trinuclear Cu center was fully reduced, and by the reaction of the Cys --> Ser mutant for the type I Cu site in bilirubin oxidase and CueO. An acidic amino acid residue located adjacent to the trinuclear Cu center was proved to function as a proton donor to these reaction intermediates. The substrate specificity of MCO for organic substrates is produced by the integrated effects of the shape of the substrate-binding site and the specific interaction of the substrate with the amino acid located adjacent to the His residue coordinating to the type I Cu. In contrast, the substrate specificity of the cuprous oxidase, CueO, is produced by the segment covering the Cu(I)-binding site so as to obstruct the access of organic substrates. Truncating the segment spanning helix 5 to helix 7 greatly reduced the specificity of CueO for Cu(I) and prominently enhanced the low oxidizing activity for the organic substrates, indicating the success of protein engineering to modify the substrate specificity of MCO.

Kinetics and mechanism of the ferroxime(ii)-catalysed biomimetic oxidation of 2-aminophenol by dioxygen. A functional phenoxazinone synthase model

[Fe(Hdmg)(2)(MeIm)(2)](1), referred to as ferroxime(II), is the precursor of a selective catalyst for the oxidative dehydrogenation of 2-aminophenol (Hap) to 2-amino-3H-phenoxazine-3-one (apx) by dioxygen under ambient conditions. The superoxoferroxime(III) species has been detected by ES-MS, and a 4-substituted 2-aminophenoxyl free radical by the ESR technique. The kinetics of the reaction was followed spectrophotometrically and by monitoring dioxygen uptake at constant pressure. According to the proposed mechanism, solvolysis of 1 is followed by O(2) binding to afford a superoxoferroxime, which abstracts an H-atom from Hap in the rate-determining step via an H-bonded intermediate, generating the free radical. This is supported by the observed primary deuterium kinetic isotope effect of 2.63. The system studied is a functional phenoxazinone synthase model.

Nonribosomal biosynthesis of microbial chromopeptides

Nonribosomal chromopeptides and mixed chromopeptide-polyketides contain aromatic or heteroaromatic side groups which are important recognition elements for interaction with cellular targets such as DNA and proteins, resulting in the biological activities of these natural products. In the chromopeptide lactones and arylpeptide-siderophores from bacteria, the chromophore moiety--an aryl carboxylate amidated to the peptide chain--constitutes the formal amino terminus and is the starter residue of peptide assembly. Common to many arylpeptide systems is the activation by stand-alone adenylation domains and loading of the starter to discrete aryl carrier proteins (ArCPs) or ArCP domains which interact with the modules of the respective nonribosomal peptide synthetase (NRPS), assembling the next residues of the chain. Chain modification is another mechanism of nonribosomal chromopeptide synthesis where heteroaromatic rings such as thiazoles and oxazoles in peptides and polyketides are generated by heterocylizations of acyl- or peptidyl-cysteinyl or -serinyl/threonyl intermediates in each elongation step. In this review the basic mechanisms of chromophore acquisition in nonribosomal chromopeptide synthesis and mixed peptide/polyketide synthesis are illustrated by comparing the biosynthesis systems of various chromopeptides and chromopeptidic polyketide compounds.

Actinomycin production persists in a strain of Streptomyces antibioticus lacking phenoxazinone synthase

Truncated fragments of the phenoxazinone synthase gene, phsA, were prepared by the PCR. The resulting fragments were cloned into conjugative plasmid pKC1132 and transferred to Streptomyces antibioticus by conjugation from Escherichia coli. Two of the resulting constructs were integrated into the S. antibioticus chromosome by homologous recombination, and each of the resulting strains, designated 3720/pJSE173 and 3720/pJSE174, contained a disrupted phsA gene. Strain 3720/pJSE173 grew poorly, and Southern blotting suggested that genetic changes other than the disruption of the phsA gene might have occurred during the construction of that strain. Strain 3720/pJSE174 sporulated well and grew normally on the medium used to prepare inocula for antibiotic production. Strain 3720/pJSE174 also grew as well as the wild-type strain on antibiotic production medium containing either 1 or 5.7 mM phosphate. Strain 3720/pJSE174 was shown to be devoid of phenoxazinone synthase (PHS) activity, and PHS protein was undetectable in this strain by Western blotting. Despite the absence of detectable PHS activity, strain 3720/pJSE174 produced slightly more actinomycin than did the wild-type parent strain in medium containing 1 or 5.7 mM phosphate. The observation that strain 3720/pJSE174, lacking detectable PHS protein or enzyme activity, retained the ability to produce actinomycin supports the conclusion that PHS is not required for actinomycin biosynthesis in S. antibioticus.

Molecular characterization of the genes of actinomycin synthetase I and of a 4-methyl-3-hydroxyanthranilic acid carrier protein involved in the assembly of the acylpeptide chain of actinomycin in Streptomyces

Actinomycin synthetase I (ACMS I) activates 4-methyl-3-hydroxyanthranilic acid, the precursor of the chromophoric moiety of the actinomycin, as adenylate. The gene acmA of ACMS I was identified upstream of the genes acmB and acmC encoding the two peptide synthetases ACMS II and ACMS III, respectively, which assemble the pentapeptide lactone rings of the antibiotic. Sequence analysis and expression of acmA in Streptomyces lividans as enzymatically active hexa-His-fusion confirmed the acmA gene product to be ACMS I. An open reading frame of 234 base pairs (acmD), which encodes a 78-amino acid protein with similarity to various acyl carrier proteins, is located downstream of acmA. The acmD gene was expressed in Escherichia coli as hexa-His-fusion protein (Acm acyl carrier protein (AcmACP)). ACMS I in the presence of ATP acylated the purified AcmACP with radioactive p-toluic acid, used as substrate in place of 4-MHA. Only 10% of the AcmACP from E. coli was acylated, suggesting insufficient modification with 4'-phosphopantetheine cofactor. Incubation of this AcmACP with a holo-ACP synthase and coenzyme A quantitatively established the holo-form of AcmACP. Enzyme assays in the presence of ACMS II showed that toluyl-AcmACP directly acylated the thioester-bound threonine on ACMS II. Thus, AcmACP is a 4-MHA carrier protein in the peptide chain initiation of actinomycin synthesis.

Nucleotide sequence, transcriptional analysis, and glucose regulation of the phenoxazinone synthase gene (phsA) from Streptomyces antibioticus

The nucleotide sequence of a 2.3-kb SphI fragment containing the structural gene (phsA) for phenoxazinone synthase (PHS) of Streptomyces antibioticus was determined. The sequence was found to contain an open reading frame (ORF) with a G+C content of 71.5% oriented in the direction of transcription that was confirmed by primer extension. The ORF encodes a protein with an M(r) of 70,223 consisting of 642 amino acids and is preceded by a potential ribosome-binding site. The codon usage pattern is in agreement with the general pattern for streptomycete genes, with a 92.5 mol% G+C content in the third position. The N-terminal sequence of the mature PHS subunit corresponds exactly to that predicted from the nucleotide sequence. Neither ATG nor GTG initiator codons were identified for the protein. However, a TTG codon was located near the amino terminus of the mature protein and is a good candidate for the initiator codon. The transcriptional start point of phsA was located 36 bp upstream of the start codon by primer extension. The -10 region of the putative promoter showed some similarity to the consensus sequence for the major class of prokaryotic promoters, but the -35 region was less similar. Comparison of the primary amino acid sequence of PHS of S. antibioticus with other amino acid sequences indicated that PHS is a blue copper protein with copper binding domains in the N-terminal and C-terminal regions of the polypeptide chain. A BsrBI fragment containing the promoter region of phsA and a portion of the ORF was shown to promote xylE expression when cloned in the streptomycete promoter probe vector pIJ2843. This phsA promoter-dependent xylE expression could be repressed by glucose in S. antibioticus when the organism was grown on glucose or galactose plus glucose. Thus, the cloned promoter region appears to contain the sequences responsible for catabolite repression of PHS production.

Stoichiometry and spectroscopic identity of copper centers in phenoxazinone synthase: a new addition to the blue copper oxidase family

Phenoxazinone synthase catalyzes the oxidative condensation of two molecules of substituted o-aminophenols to the phenoxazinone chromophore of actinomycin. Cyclization occurs with the concomitant reduction of molecular oxygen to water. We have shown that the enzyme requires 4-5 copper atoms/monomer for full activity and the additional copper inhibits the enzyme. The optical absorption spectrum of phenoxazinone synthase is also dependent on the Cu per monomer ratio, and the absorption peak at 598 nm has a maximum extinction coefficient of 4000 +/- 150 M-1 cm-1 at a ratio of 4-5 Cu atoms per monomer. The electron paramagnetic resonance (EPR) spectrum of enzyme as isolated with low copper content (0.8 Cu/monomer) only shows the presence of type 1 (blue) copper centers (g parallel = 2.24, A = 0.0067 cm-1, and g perpendicular = 2.07). Enzyme incubated with 4-5 Cu per monomer demonstrates the presence of both type 1 and type 2 copper centers with a stoichiometry of one type 1 center per monomer and the remainder bound as type 2 Cu2+. Anaerobic incubation of substrate with enzyme containing five Cu atoms per subunit results in bleaching of the blue center. The EPR spectrum of the enzyme reduced under these conditions suggests that one of the type 2 Cu2+ centers with a g parallel = 2.34, A = 0.016 cm-1, and g perpendicular = 2.07 remains oxidized and is not involved in catalysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning and in vitro expression of a silent phenoxazinone synthase gene from Streptomyces lividans

Phenoxazinone synthase (PHS) catalyzes a step in actinomycin D biosynthesis in Streptomyces antibioticus. Two sequences from Streptomyces lividans that hybridize to the phs gene of S. antibioticus have been cloned in Escherichia coli K-12 using the plasmid pBR322. Although there was some similarity in the restriction maps of the two cloned fragments, neither insert appeared to be a direct subset of the other nor of the S. antibioticus phs gene. In vitro expression studies, in a streptomycete coupled transcription-translation system, showed that a 3.98-kb SphI fragment encoded a PHS-related protein. These observations provide additional support for the existence of silent genes for antibiotic production in streptomycetes.

Genetics of actinomycin C production in Streptomyces chrysomallus

Three distinct classes of mutations affecting the biosynthesis of actinomycin have been established in Streptomyces chyrsomallus by crossing various actinomycin-nonproducing mutants with each other by protoplast fusion. In crosses between members of different classes of mutations, actinomycin-producing recombinant progeny arose, whereas in crosses between members of the same class, no actinomycin-producing recombinants were seen. Biochemical examination of a number of mutants revealed that the expression of all actinomycin synthetases was reduced by about 1 order of magnitude in mutants belonging to class II. In mutants of class I, the specific activities of the actinomycin synthetases were comparable with those measured in their actinomycin-producing parents. Feeding experiments with 4-methyl-3-hydroxyanthranilic acid (4-MHA), the biosynthetic precursor of the chromophore moiety of actinomycin, with representative mutants of the three genetic classes revealed formation of actinomycin in minute amounts by mutants of class I. It is suggested that mutants belonging to class I are mutated at a genetic locus involved in the biosynthesis of 4-MHA. Mutants belonging to class II appear to carry mutations at a locus involved in the regulation of the expression of the actinomycin synthetases. The role of the locus in class III mutations could not be assigned. Mapping studies in S. chrysomallus based on conjugal matings revealed the chromosomal linkage of all three loci. Mutations belonging to classes I and III were closely linked. Their genetic loci could be localized in a map interval of the chromosomal linkage group which is significantly distant from the gene locus represented by mutations belonging to class II.

Purification and characterization of tryptophan dioxygenase from Streptomyces parvulus

Tryptophan dioxygenase, derived from Streptomyces parvulus, was purified to near homogeneity and shown to have a native Mr of 88,000. Kinetic parameters of the enzyme were determined and evidence suggesting that it is a hemoprotein was obtained. Tryptophan dioxygenase has a high specificity toward L-tryptophan with an apparent Km of 0.3 mM. L-3-Hydroxykynurenine was a competitive inhibitor with respect to L-tryptophan with a Ki of 0.16 mM. In vitro, the enzyme displayed little activity in the absence of a reducing agent; ascorbate, at 50 mM, was the preferred reductant providing almost a 50-fold increase in enzyme activity. The regulation of tryptophan dioxygenase synthesis and activity is described. The expression of the enzyme is correlated with the biosynthesis of actinomycin D in S. parvulus. These results support the hypothesis that tryptophan dioxygenase functions as the first enzyme in the sequence converting L-tryptophan to the chromophore of this antibiotic.

Actinomycin synthesis in Streptomyces antibioticus: enzymatic conversion of 3-hydroxyanthranilic acid to 4-methyl-3-hydroxyanthranilic acid

A methyltransferase which utilizes 3-hydroxyanthranilic acid (HAA) as a substrate was identified in detergent-treated extracts of the bacterium Streptomyces antibioticus. The enzyme catalyzes the transfer of methyl groups from [14C]S-adenosylmethionine to HAA, but does not catalyze the methylation of 3-hydroxy-DL-kynurenine. Enzyme, substrate, time, and pH dependencies for the methyl transfer reaction were examined. Reaction products obtained from scaled-up reaction mixtures were fractionated by chromatography on Dowex 1, and the Dowex 1 fractions were examined by paper and thin-layer chromatography. One Dowex fraction was shown to contain a radioactive product with the chromatographic properties of 4-methyl-3-hydroxyanthranilic acid (MHA), a known intermediate in the biosynthesis of actinomycin. Available evidence indicates that the conversion of HAA to MHA is an early step in the biosynthesis of actinomycin by S. antibioticus and other actinomycin-producing streptomycetes.

Regulation of phenoxazinone synthase expression in Streptomyces antibioticus

The cloned gene for the subunit of phenoxazinone synthase (PHS), an enzyme implicated in the biosynthesis of actinomycin in Streptomyces antibioticus, was used as a probe to study the regulation of the enzyme. The direction of transcription of the PHS gene was determined with end-labeled restriction fragments derived from the gene. Low-resolution S1 mapping revealed that transcription was initiated at a position which may lie within the SphI restriction site, which represents the limit of the cloned sequence. Northern blotting allowed the identification of the putative PHS message. This RNA appeared to be significantly larger than the size required to encode the PHS subunit. RNA dot blotting showed that the increase in PHS specific activity observed in cultures grown on antibiotic production medium, with galactose as a carbon source, was due in part to an increased production of PHS mRNA. PHS was also more stable than most cellular proteins and appeared to be protected against degradation under conditions in which most other proteins are broken down. This protective effect also contributed to the increase in PHS specific activity observed in S. antibioticus cultures grown on production medium. The repression of PHS synthesis by glucose was also reflective of a transcriptional control mechanism. At early time points postinoculation, PHS mRNA levels were lower in cultures grown on glucose as a carbon source than in cultures of the same age grown on galactose. mRNA levels presumably begin to increase only after all the glucose in the medium is utilized. The ability of 5-fluorouracil to stimulate PHS production in young cultures was also due to the synthesis of new mRNA for the enzyme.