Cloning and sequence analysis of the highly expressed melanin-synthesizing gene operon from Streptomyces castaneoglobisporus

The Novel Role of Tyrosinase Enzymes in the Storage of Globally Significant Amounts of Carbon in Wetland Ecosystems

Over the last millennia, wetlands have been sequestering carbon from the atmosphere via photosynthesis at a higher rate than releasing it and, therefore, have globally accumulated 550 × 10¹⁵ g of carbon, which is equivalent to 73% of the atmospheric carbon pool. The accumulation of organic carbon in wetlands is effectuated by phenolic compounds, which suppress the degradation of soil organic matter by inhibiting the activity of organic-matter-degrading enzymes. The enzymatic removal of phenolic compounds by bacterial tyrosinases has historically been blocked by anoxic conditions in wetland soils, resulting from waterlogging. Bacterial tyrosinases are a subgroup of oxidoreductases that oxidatively remove phenolic compounds, coupled to the reduction of molecular oxygen to water. The biochemical properties of bacterial tyrosinases have been investigated thoroughly in vitro within recent decades, while investigations focused on carbon fluxes in wetlands on a macroscopic level have remained a thriving yet separated research area so far. In the wake of climate change, however, anoxic conditions in wetland soils are threatened by reduced rainfall and prolonged summer drought. This potentially allows tyrosinase enzymes to reduce the concentration of phenolic compounds, which in turn will increase the release of stored carbon back into the atmosphere. To offer compelling evidence for the novel concept that bacterial tyrosinases are among the key enzymes influencing carbon cycling in wetland ecosystems first, bacterial organisms indigenous to wetland ecosystems that harbor a TYR gene within their respective genome (tyr⁺) have been identified, which revealed a phylogenetically diverse community of tyr⁺ bacteria indigenous to wetlands based on genomic sequencing data. Bacterial TYR host organisms covering seven phyla (Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Nitrospirae, Planctomycetes, and Proteobacteria) have been identified within various wetland ecosystems (peatlands, marshes, mangrove forests, bogs, and alkaline soda lakes) which cover a climatic continuum ranging from high arctic to tropic ecosystems. Second, it is demonstrated that (in vitro) bacterial TYR activity is commonly observed at pH values characteristic for wetland ecosystems (ranging from pH 3.5 in peatlands and freshwater swamps to pH 9.0 in soda lakes and freshwater marshes) and toward phenolic compounds naturally present within wetland environments (p-coumaric acid, gallic acid, protocatechuic acid, p-hydroxybenzoic acid, caffeic acid, catechin, and epicatechin). Third, analyzing the available data confirmed that bacterial host organisms tend to exhibit in vitro growth optima at pH values similar to their respective wetland habitats. Based on these findings, it is concluded that, following increased aeration of previously anoxic wetland soils due to climate change, TYRs are among the enzymes capable of reducing the concentration of phenolic compounds present within wetland ecosystems, which will potentially destabilize vast amounts of carbon stored in these ecosystems. Finally, promising approaches to mitigate the detrimental effects of increased TYR activity in wetland ecosystems and the requirement of future investigations of the abundance and activity of TYRs in an environmental setting are presented.

Comparative analysis of mussel foot protein 3B co-expressed with tyrosinases provides a potential adhesive biomaterial

Mussel foot proteins (Mfps), which help mussels attach to various surfaces, are considered to be promising biomaterials due to their outstanding adhesive properties. However, limited production and lack of post-translational modifications of tyrosine residues into 3,4-dihydroxyphenylalanine (Dopa) in bacterial expression systems have hampered their applications. In the present study, for the first time we established the expression of recombinant Mytilus galloprovincialis foot protein type 3 variant B (fp-3B) in Escherichia coli; and achieved its viable production (~51 mg/L). Additionally, the Dopa content and adhesive properties of fp-3B co-expressed using various types of tyrosinases were compared. Consequently, the co-expression of fp-3B construct together with tyrosinase from Verrucomicrobium spinosum (TyrVs) yielded up to 87 mg/L of modified fp-3B; hydroxylation of tyrosine residues accounted for 57.18% by acid-borate difference spectroscopy. The modified fp-3B also showed significant coating and adhesive ability, and its bulk-scale adhesive strength was 2.9-fold higher than that of unmodified fp-3B. Compared with other type 3 mussel foot proteins, the high-yield expression and extensive hydroxylation level of the recombinant protein indicate that fp-3B co-expressed with TyrVs (3B-Vs) has the potential to be widely used as bioglues.

Application of bacterial tyrosinases in organic synthesis

Bacterial tyrosinases, as in the case of other bacterial oxidative enzymes, have been found to possess biochemical characteristics that typically make them more suited to applications requiring special operational conditions such as alkaline pH, high or low temperature, the presence of organic solvents, and the presence of inhibitors. Even though a great deal is known about fungal tyrosinases, bacterial tyrosinases still vastly remain underexplored for their potential application in organic synthesis. A literature survey in particular highlights the gaps in our knowledge pertaining to their biochemical properties. Bacterial tyrosinases have not only shown promise in the synthesis of medically important compounds such as L-3,4-dihydroxyphenylalanine (L-DOPA) and melanin but have also seen application in cross-linking reactions of proteins and the polymerization of environmental pollutants. Their ability to catalyse o-hydroxylation reactions have shown some degree of promise in the biocatalytic conversion of resveratrol to piceatannol, tyrosol to hydroxytyrosol, and many more. In this review, we will explore the world of bacterial tyrosinases, their current applications, and future perspectives for the application of these enzymes in organic synthesis.

Expression, Purification, and Characterization of a Well-Adapted Tyrosinase from Peatlands Identified by Partial Community Analysis

In peatlands, bacterial tyrosinases (TYRs) are proposed to act as key regulators of carbon storage by removing phenolic compounds, which inhibit the degradation of organic carbon. Historically, TYR activity has been blocked by anoxia resulting from persistent waterlogging; however, recent events of prolonged summer drought have boosted TYR activity and, consequently, the release of carbon stored in the form of organic compounds from peatlands. Since 30% of the global soil carbon stock is stored in peatlands, a profound understanding of the production and activity of TYRs is essential to assess the impact of carbon dioxide emitted from peatlands on climate change. TYR partial sequences identified by degenerated primers suggest a versatile TYR enzyme community naturally present in peatlands, which is produced by a phylogenetically diverse spectrum of bacteria, including Proteobacteria and Actinobacteria. One full-length sequence of an extracellular TYR (SzTYR) identified from a soda-rich inland salt marsh has been heterologously expressed and purified. SzTYR exhibits a molecular mass of 30 891.8 Da and shows a pH optimum of 9.0. Spectroscopic studies and kinetic investigations characterized SzTYR as a tyrosinase and proved its activity toward monophenols (coumaric acid), diphenols (caffeic acid, protocatechuic acid), and triphenols (gallic acid) naturally present in peatlands.

The basicity of an active-site water molecule discriminates between tyrosinase and catechol oxidase activity

Tyrosinase (Ty) and catechol oxidase (CO) are members of type-3 copper enzymes. While Ty catalyzes both phenolase and catecholase reactions, CO catalyzes only the latter reaction. In the present study, Ty was found to catalyze the catecholase reaction, but hardly the phenolase reaction in the presence of the metallochaperon called "caddie protein (Cad)". The ability of the substrates to dissociate the motif shielding the active-site pocket seems to contribute critically to the substrate specificity of Ty. In addition, a mutation at the N191 residue, which forms a hydrogen bond with a water molecule near the active center, decreased the inherent ratio of phenolase versus catecholase activity. Unlike the wild-type complex, reaction intermediates were not observed when the catalytic reaction toward the Y98 residue of Cad was progressed in the crystalline state. The increased basicity of the water molecule may be necessary to inhibit the proton transfer from the conjugate acid to a hydroxide ion bridging the two copper ions. The deprotonation of the substrate hydroxyl by the bridging hydroxide seems to be significant for the efficient catalytic cycle of the phenolase reaction.

Catalytic mechanism of the tyrosinase reaction toward the Tyr98 residue in the caddie protein

Tyrosinase (EC 1.14.18.1), a copper-containing monooxygenase, catalyzes the conversion of phenol to the corresponding ortho-quinone. The Streptomyces tyrosinase is generated as a complex with a “caddie” protein that facilitates the transport of two copper ions into the active center. In our previous study, the Tyr⁹⁸ residue in the caddie protein, which is accommodated in the pocket of active center of tyrosinase, has been found to be converted to a reactive quinone through the formations of the μ-η²:η²-peroxo-dicopper(II) and Cu(II)-dopasemiquinone intermediates. Until now—despite extensive studies for the tyrosinase reaction based on the crystallographic analysis, low-molecular-weight models, and computer simulations—the catalytic mechanism has been unable to be made clear at an atomic level. To make the catalytic mechanism of tyrosinase clear, in the present study, the cryo-trapped crystal structures were determined at very high resolutions (1.16–1.70 Å). The structures suggest the existence of an important step for the tyrosinase reaction that has not yet been found: that is, the hydroxylation reaction is triggered by the movement of Cu^A, which induces the syn-to-anti rearrangement of the copper ligands after the formation of μ-η²:η²-peroxo-dicopper(II) core. By the rearrangement, the hydroxyl group of the substrate is placed in an equatorial position, allowing the electrophilic attack to the aromatic ring by the Cu₂O₂ oxidant.

The cryo-trapped crystal structures of tyrosinase in a complex with its “caddie” protein reveal structural insight into the catalytic mechanism of tyrosinase, the rate-limiting enzyme in the production of melanin.

Tyrosinase is an enzyme that controls a rate-limiting reaction of melanogenesis: it catalyzes the conversion of a phenol to the corresponding ortho-quinone. Streptomyces tyrosinase is formed as a complex, with a “caddie” protein that assists with the transport of the two copper ions into the enzyme’s active center. In our previous study, we showed that the Tyr⁹⁸ residue in the caddie protein, which is accommodated in the pocket of active center of tyrosinase, is converted to a reactive quinone through the formations of the μ-η²:η²-peroxo-dicopper(II) and Cu(II)-dopasemiquinone intermediates. Until now—despite extensive studies of the tyrosinase reaction based on the crystallographic analysis, low-molecular-weight model systems, and computer simulations—the catalytic mechanism was unclear at an atomic level. To understand the catalytic mechanism of tyrosinase in detail, we determined the cryo-trapped crystal structures at very high resolutions, which suggest an important new step for the tyrosinase reaction: the hydroxylation reaction triggered by the movement of Cu^A, which induces the syn-to-anti rearrangement of the copper ligands after the formation of μ-η²:η²-peroxo-dicopper(II) core.

Molecular genetics of naringenin biosynthesis, a typical plant secondary metabolite produced by Streptomyces clavuligerus

Background

Some types of flavonoid intermediates seemed to be restricted to plants. Naringenin is a typical plant metabolite, that has never been reported to be produced in prokariotes. Naringenin is formed by the action of a chalcone synthase using as starter 4-coumaroyl-CoA, which in dicotyledonous plants derives from phenylalanine by the action of a phenylalanine ammonia lyase.

Results

A compound produced by Streptomyces clavuligerus has been identified by LC–MS and NMR as naringenin and coelutes in HPLC with a naringenin standard. Genome mining of S. clavuligerus revealed the presence of a gene for a chalcone synthase (ncs), side by side to a gene encoding a P450 cytochrome (ncyP) and separated from a gene encoding a Pal/Tal ammonia lyase (tal). Deletion of any of these genes results in naringenin non producer mutants. Complementation with the deleted gene restores naringenin production in the transformants. Furthermore, naringenin production increases in cultures supplemented with phenylalanine or tyrosine.

Conclusion

This is the first time that naringenin is reported to be produced naturally in a prokariote. Interestingly three non-clustered genes are involved in naringenin production, which is unusual for secondary metabolites. A tentative pathway for naringenin biosynthesis has been proposed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0373-7) contains supplementary material, which is available to authorized users.

High-Level Heterologous Production of D-Cycloserine by Escherichia coli

Previously, we successfully cloned a d-cycloserine (d-CS) biosynthetic gene cluster consisting of 10 open reading frames (designated dcsA to dcsJ) from d-CS-producing Streptomyces lavendulae ATCC 11924. In this study, we put four d-CS biosynthetic genes (dcsC, dcsD, dcsE, and dcsG) in tandem under the control of the T7 promoter in an Escherichia coli host. SDS-PAGE analysis demonstrated that the 4 gene products were simultaneously expressed in host cells. When l-serine and hydroxyurea (HU), the precursors of d-CS, were incubated together with the E. coli resting cell suspension, the cells produced significant amounts of d-CS (350 ± 20 μM). To increase the productivity of d-CS, the dcsJ gene, which might be responsible for the d-CS excretion, was connected downstream of the four genes. The E. coli resting cells harboring the five genes produced d-CS at 660 ± 31 μM. The dcsD gene product, DcsD, forms O-ureido-l-serine from O-acetyl-l-serine (OAS) and HU, which are intermediates in d-CS biosynthesis. DcsD also catalyzes the formation of l-cysteine from OAS and H2S. To repress the side catalytic activity of DcsD, the E. coli chromosomal cysJ and cysK genes, encoding the sulfite reductase α subunit and OAS sulfhydrylase, respectively, were disrupted. When resting cells of the double-knockout mutant harboring the four d-CS biosynthetic genes, together with dcsJ, were incubated with l-serine and HU, the d-CS production was 980 ± 57 μM, which is comparable to that of d-CS-producing S. lavendulae ATCC 11924 (930 ± 36 μM).

Mechanisms of copper homeostasis in bacteria

Copper is an important micronutrient required as a redox co-factor in the catalytic centers of enzymes. However, free copper is a potential hazard because of its high chemical reactivity. Consequently, organisms exert a tight control on Cu(+) transport (entry-exit) and traffic through different compartments, ensuring the homeostasis required for cuproprotein synthesis and prevention of toxic effects. Recent studies based on biochemical, bioinformatics, and metalloproteomics approaches, reveal a highly regulated system of transcriptional regulators, soluble chaperones, membrane transporters, and target cuproproteins distributed in the various bacterial compartments. As a result, new questions have emerged regarding the diversity and apparent redundancies of these components, their irregular presence in different organisms, functional interactions, and resulting system architectures.

Pseudoalteromonas xiamenensis sp. nov., a marine bacterium isolated from coastal surface seawater

A Gram-negative, oxidase- and catalase-positive, rod-shaped, non-spore-forming, motile, aerobic bacterium, designated Y2(T), was isolated from surface seawater of Yundang Lake, Xiamen, China. The strain was able to grow in the presence of 0.5-6.0% NaCl (optimum 1.0-1.5%), at pH 5-10 (optimum pH 8) and at 10-40 °C (optimum 25 °C). Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain Y2(T) belongs to the genus Pseudoalteromonas, with the highest sequence similarity of 94.9% to Pseudoalteromonas tunicata D2(T); within the genus Pseudoalteromonas, it showed the lowest similarity of 92.8% to Pseudoalteromonas denitrificans ATCC 43337(T). The G+C content of the chromosomal DNA of strain Y2(T) was 45.1 mol%. The predominant fatty acids were summed feature 3 (C(16 : 1)ω6c and/or C(16 : 1)ω7c), C(16 : 0), C(12 : 0) 3-OH and summed feature 8 (C(18 : 1)ω6c and/or C(18 : 1)ω7c). The only respiratory quinone detected was Q-8. Based on the phylogenetic and phenotypic characteristics, strain Y2(T) represents a novel species of the genus Pseudoalteromonas, for which the name Pseudoalteromonas xiamenensis sp. nov. is proposed; the type strain is Y2(T) ( = CGMCC 1.12157(T) = JCM 18779(T)).

Actinobacterial melanins: current status and perspective for the future

Melanins are enigmatic pigments that are produced by a wide variety of microorganisms including several species of bacteria and fungi. Melanins are biological macromolecules with multiple important functions, yet their structures are not well understood. Melanins are frequently used in medicine, pharmacology, and cosmetics preparations. Melanins also have great application potential in agriculture industry. They have several biological functions including photoprotection, thermoregulation, action as free radical sinks, cation chelators, and antibiotics. Plants and insects incorporate melanins as cell wall and cuticle strengtheners, respectively. Actinobacteria are the most economically as well as biotechnologically valuable prokaryotes. However, the melanin properties are, in general, poorly understood. In this review an evaluation is made on the present state of research on actinobacterial melanins and its perspectives. The highlights include the production and biotechnological applications of melanins in agriculture, food, cosmetic and medicinal fields. With increasing advancement in science and technology, there would be greater demands in the future for melanins produced by actinobacteria from various sources.

Experimental and bioinformatic investigation of the proteolytic degradation of the C-terminal domain of a fungal tyrosinase

Proteolytic processing is a key step in the production of polyphenol oxidases such as tyrosinases, converting the inactive proenzyme to an active form. In general, the fungal tyrosinase gene codes for a ~60 kDa protein that is, however, isolated as an active enzyme of ~40 kDa, lacking the C-terminal domain. Using the secreted tyrosinase 2 from Trichoderma reesei as a model protein, we performed a mutagenesis study of the residues in proximity of the experimentally determined cleavage site which are possibly involved in the proteolytic process. However, the mutant forms of tyrosinase 2 were not secreted in a full-length form retaining the C-terminal domain, but they were processed to give a ~45 kDa active form. Aiming at explaining this phenomenon, we analysed in silico the properties of the C-terminal domain of tyrosinase 2, of 23 previously retrieved homologous tyrosinase sequences from fungi (C. Gasparetti, G. Faccio, M. Arvas, J. Buchert, M. Saloheimo, K. Kruus, Appl. Microbiol. Biotechnol. 86 (2010) 213-226) and of nine well-characterised polyphenol oxidases. Based on the results of our study, we exclude the key role of specific amino acids at the cleavage site in the proteolytic process and report an overall higher sensitivity to proteolysis of the linker region and of the whole C-terminal domain of fungal tyrosinases.

Heme protein and hydroxyarginase necessary for biosynthesis of D-cycloserine

We have recently cloned a D-cycloserine (DCS) biosynthetic gene cluster that consists of 10 genes, designated dcsA~dcsJ, from Streptomyces lavendulae ATCC 11924 (16). In the predicted pathway of hydroxyurea (HU) formation in DCS biosynthesis, L-arginine (L-Arg) must first be hydroxylated, prior to the hydrolysis of N(ω)-hydroxy-L-arginine (NHA) by DcsB, an arginase homolog. The hydroxylation of L-Arg is known to be catalyzed by nitric oxide synthase (NOS). In this study, to verify the supply route of HU, we created a dcsB-disrupted mutant, ΔdcsB. While the mutant lost DCS productivity, its productivity was restored by complementation of dcsB, and also by the addition of HU but not NHA, suggesting that HU is supplied by DcsB. A NOS-encoding gene, nos, from S. lavendulae chromosome was cloned, to create a nos-disrupted mutant. However, the mutant maintained the DCS productivity, suggesting that NOS is not necessary for DCS biosynthesis. To clarify the identity of an enzyme necessary for NHA formation, a dcsA-disrupted mutant, designated ΔdcsA, was also created. The mutant lost DCS productivity, whereas the DCS productivity was restored by complementation of dcsA. The addition of NHA to the culture medium of ΔdcsA mutant was also effective to restore DCS production. These results indicate that the dcsA gene product, DcsA, is an enzyme essential to generate NHA as a precursor in the DCS biosynthetic pathway. Spectroscopic analyses of the recombinant DcsA revealed that it is a heme protein, supporting an idea that DcsA is an enzyme catalyzing hydroxylation.

A molecular mechanism for copper transportation to tyrosinase that is assisted by a metallochaperone, caddie protein

The Cu(II)-soaked crystal structure of tyrosinase that is present in a complex with a protein, designated "caddie," which we previously determined, possesses two copper ions at its catalytic center. We had identified two copper-binding sites in the caddie protein and speculated that copper bound to caddie may be transported to the tyrosinase catalytic center. In our present study, at a 1.16-1.58 Å resolution, we determined the crystal structures of tyrosinase complexed with caddie prepared by altering the soaking time of the copper ion and the structures of tyrosinase complexed with different caddie mutants that display little or no capacity to activate tyrosinase. Based on these structures, we propose a molecular mechanism by which two copper ions are transported to the tyrosinase catalytic center with the assistance of caddie acting as a metallochaperone.

Bacterial tyrosinases: old enzymes with new relevance to biotechnology

Tyrosinases are copper-containing dioxygen activating enzymes found in many species of bacteria and are usually associated with melanin production. These proteins have a strong preference for phenolic and diphenolic substrates and are somewhat limited in their reaction scope, always producing an activated quinone as product. Despite this fact they have potential in several biotechnological applications, including the production of novel mixed melanins, protein cross-linking, phenolic biosensors, production of l-DOPA, phenol and dye removal and biocatalysis. Although most studies have used Streptomyces sp. enzymes, there are several other examples of these proteins that are also of potential interest. For instance a solvent tolerant enzyme has been described, as well as an enzyme with both tyrosinase and laccase activities, enzymes with altered substrate preferences, an enzyme produced as an inactive zymogen as well as examples which do not require auxiliary proteins for copper insertion (unlike the Streptomyces sp. enzymes which do require such a protein). This article will summarise the reports on the biotechnological applications of bacterial tyrosinases as well as the current information available on the different types of this enzyme.

Role of the C-terminal extension in a bacterial tyrosinase

The well studied bacterial tyrosinases from the Streptomyces sp. bacteria are distinguishable from their eukaryotic counterparts by the absence of a C-terminal extension. In the present study, we report that the tyrosinase from the bacterium Verrucomicrobium spinosum also has such a C-terminal extension, thus making it distinct from the Streptomyces enzymes. The entire tyrosinase gene from V. spinosum codes for a 57 kDa protein (full-length unprocessed form), which has a twin arginine translocase type signal peptide, the two copper-binding motifs typical of the tyrosinase protein family and the aforementioned C-terminal extension. We expressed various mutants of the recombinant enzyme in Escherichia coli and found that removal of the C-terminal extension by genetic engineering or limited trypsin digest of the pro-form results in a more active enzyme (i.e. 30-100-fold increase in monophenolase and diphenolase activities). Further studies also revealed the importance of a phenylalanine residue in this C-terminal domain. These results demonstrate that the V. spinosum tyrosinase is a new example of this interesting family of enzymes. In addition, we show that this enzyme can be readily overproduced and purified and that it will prove useful in furthering the understanding of these enzymes, as well as their biotechnological application.

Molecular cloning and heterologous expression of a biosynthetic gene cluster for the antitubercular agent D-cycloserine produced by Streptomyces lavendulae

In the present study, we successfully cloned a 21-kb DNA fragment containing a d-cycloserine (DCS) biosynthetic gene cluster from a DCS-producing Streptomyces lavendulae strain, ATCC 11924. The putative gene cluster consists of 10 open reading frames (ORFs), designated dcsA to dcsJ. This cluster includes two ORFs encoding D-alanyl-D-alanine ligase (dcsI) and a putative membrane protein (dcsJ) as the self-resistance determinants of the producer organism, indicated by our previous work. When the 10 ORFs were introduced into DCS-nonproducing Streptomyces lividans 66 as a heterologous host cell, the transformant acquired DCS productivity. This reveals that the introduced genes are responsible for the biosynthesis of DCS. As anticipated, the disruption of dcsG, seen in the DCS biosynthetic gene cluster, made it possible for the strain ATCC 11924 to lose its DCS production. We here propose the DCS biosynthetic pathway. First, L-serine is O acetylated by a dcsE-encoded enzyme homologous to homoserine O-acetyltransferase. Second, O-acetyl-L-serine accepts hydroxyurea via an O-acetylserine sulfhydrylase homolog (dcsD product) and forms O-ureido-L-serine. The hydroxyurea must be supplied by the catalysis of a dcsB-encoded arginase homolog using the L-arginine derivative, N(G)-hydroxy-L-arginine. The resulting O-ureido-L-serine is then racemized to O-ureido-D-serine by a homolog of diaminopimelate epimerase. Finally, O-ureido-D-serine is cyclized to form DCS with the release of ammonia and carbon dioxide. The cyclization must be done by the dcsG or dcsH product, which belongs to the ATP-grasp fold family of protein.

A novel tyrosine-phosphorylated protein inhibiting the growth of Streptomyces cells

Very few of the tyrosine-phosphorylated proteins in Streptomyces have been identified. Here, we identify a tyrosine-phosphorylated protein from Streptomyces coelicolor A3(2), designated as SCO5717. The protein possesses Walker motifs and a tyrosine cluster at the C-terminus. When sco5717 harboring its own promoter was introduced into the S. coelicolor cell, the growth was inhibited. An sco5717-disrupted mutant formed aerial mycelium earlier than the wild-type strain, suggesting that SCO5717 controls the cell growth of S. coelicolor. Although the recombinant SCO5717 showed an ATPase activity, it lacked self-phosphorylation ability, suggesting that SCO5717 is a novel tyrosine-phosphorylated protein, which is distinguishable from bacterial protein tyrosine kinases known so far.

Burkholderia cenocepacia C5424 produces a pigment with antioxidant properties using a homogentisate intermediate

Burkholderia cenocepacia is a gram-negative opportunistic pathogen that belongs to the Burkholderia cepacia complex. B. cenocepacia can survive intracellularly within phagocytic cells, and some epidemic strains produce a brown melanin-like pigment that can scavenge free radicals, resulting in the attenuation of the host cell oxidative burst. In this work, we demonstrate that the brown pigment produced by B. cenocepacia C5424 is synthesized from a homogentisate (HGA) precursor. The disruption of BCAL0207 (hppD) by insertional inactivation resulted in loss of pigmentation. Steady-state kinetic analysis of the BCAL0207 gene product demonstrated that it has 4-hydroxyphenylpyruvic acid dioxygenase (HppD) activity. Pigmentation could be restored by complementation providing hppD in trans. The hppD mutant was resistant to paraquat challenge but sensitive to H2O2 and to extracellularly generated superoxide anions. Infection experiments in RAW 264.7 murine macrophages showed that the nonpigmented bacteria colocalized in a dextran-positive vacuole, suggesting that they are being trafficked to the lysosome. In contrast, the wild-type strain did not localize with dextran. Colocalization of the nonpigmented strain with dextran was reduced in the presence of the NADPH oxidase inhibitor diphenyleneiodonium, and also the inducible nitric oxide inhibitor aminoguanidine. Together, these observations suggest that the brown pigment produced by B. cenocepacia C5424 is a pyomelanin synthesized from an HGA intermediate that is capable of protecting the organism from in vitro and in vivo sources of oxidative stress.

Identification and kinetic study of tyrosinase inhibitors found in sake lees

The present study found that the n-hexane extract of freeze-dried sake lees inhibits tyrosinase activity and showed that the constituents isolated from the n-hexane extract are the mixture of triacylglycerols. The inhibitory effects of triolein and trilinolein found as the triacylglycerols were examined using tyrosinases from mushroom and Streptomyces castaneoglobisporus. The IC50 values of the triacylglycerol mixture for the oxidase activity on mushroom and Streptomyces tyrosinases were 20 and 0.14 microg/mL, respectively. The IC50 values of trilinolein for the oxidase activity on mushroom and Streptomyces tyrosinases were 8.4 and 0.1 microM, respectively. However, the inhibitory effect of triolein (IC50=30 microM) was lower than that of trilinolein, even when the Streptomyces tyrosinase was used for the assay. Kinetic analyses indicate that both trilinolein and triolein inhibit the tyrosinase activity noncompetitively. When transformed with a plasmid carrying the Streptomyces tyrosinase gene, the melanin-synthesizing ability of the transformed Escherichia coli host was dose-dependently interfered with by trilinolein.

Crystallographic Evidence That the Dinuclear Copper Center of Tyrosinase Is Flexible during Catalysis

At high resolution, we determined the crystal structures of copper-bound and metal-free tyrosinase in a complex with ORF378 designated as a "caddie" protein because it assists with transportation of two CuII ions into the tyrosinase catalytic center. These structures suggest that the caddie protein covers the hydrophobic molecular surface of tyrosinase and interferes with the binding of a substrate tyrosine to the catalytic site of tyrosinase. The caddie protein, which consists of one six-strandedbeta-sheet and one alpha-helix, has no similarity with all proteins deposited into the Protein Data Bank. Although tyrosinase and catechol oxidase are classified into the type 3 copper protein family, the latter enzyme lacks monooxygenase activity. The difference in catalytic activity is based on the structural observations that a large vacant space is present just above the active center of tyrosinase and that one of the six His ligands for the two copper ions is highly flexible. These structural characteristics of tyrosinase suggest that, in the reaction that catalyzes the ortho-hydroxylation of monophenol, one of the two Cu(II) ions is coordinated by the peroxide-originated oxygen bound to the substrate. Our crystallographic study shows evidence that the tyrosinase active center formed by dinuclear coppers is flexible during catalysis.

Bacterial tyrosinases

Tyrosinases are nearly ubiquitously distributed in all domains of life. They are essential for pigmentation and are important factors in wound healing and primary immune response. Their active site is characterized by a pair of antiferromagnetically coupled copper ions, CuA and CuB, which are coordinated by six histidine residues. Such a "type 3 copper centre" is the common feature of tyrosinases, catecholoxidases and haemocycanins. It is also one of several other copper types found in the multi-copper oxidases (ascorbate oxidase, laccase). The copper pair of tyrosinases binds one molecule of atmospheric oxygen to catalyse two different kinds of enzymatic reactions: (1) the ortho-hydroxylation of monophenols (cresolase activity) and (2) the oxidation of o-diphenols to o-diquinones (catecholase activity). The best-known function is the formation of melanins from L-tyrosine via L-dihydroxyphenylalanine (L-dopa). The complicated hydroxylation mechanism at the active centre is still not completely understood, because nothing is known about their tertiary structure. One main reason for this deficit is that hitherto tyrosinases from eukaryotic sources could not be isolated in sufficient quantities and purities for detailed structural studies. This is not the case for prokaryotic tyrosinases from different Streptomyces species, having been intensively characterized genetically and spectroscopically for decades. The Streptomyces tyrosinases are non-modified monomeric proteins with a low molecular mass of ca. 30kDa. They are secreted to the surrounding medium, where they are involved in extracellular melanin production. In the species Streptomyces, the tyrosinase gene is part of the melC operon. Next to the tyrosinase gene (melC2), this operon contains an additional ORF called melC1, which is essential for the correct expression of the enzyme. This review summarizes the present knowledge of bacterial tyrosinases, which are promising models in order to get more insights in structure, enzymatic reactions and functions of "type 3 copper" proteins in general.

A Novel o-Aminophenol Oxidase Responsible for Formation of the Phenoxazinone Chromophore of Grixazone

Grixazone contains a phenoxazinone chromophore and is a secondary metabolite produced by Streptomyces griseus. In the grixazone biosynthesis gene cluster, griF (encoding a tyrosinase homolog) and griE (encoding a protein similar to copper chaperons for tyrosinases) are encoded. An expression study of GriE and GriF in Escherichia coli showed that GriE activated GriF by transferring copper ions to GriF, as has been observed for a Streptomyces melanogenesis system in which the MelC1 copper chaperon transfers copper ions to MelC2 tyrosinase. In contrast with tyrosinases, GriF showed no monophenolase activity, although it oxidized various o-aminophenols as preferable substrates rather than catechol-type substrates. Deletion of the griEF locus on the chromosome resulted in accumulation of 3-amino-4-hydroxybenzaldehyde (3,4-AHBAL) and its acetylated compound, 3-acetylamino-4-hydroxybenzaldehyde. GriF oxidized 3,4-AHBAL to yield an o-quinone imine derivative, which was then non-enzymatically coupled with another molecule of the o-quinone imine to form a phenoxazinone. The coexistence of N-acetylcysteine in the in vitro oxidation of 3,4-AH-BAL by GriF resulted in the formation of grixazone A, suggesting that the -SH group of N-acetylcysteine is conjugated to the o-quinone imine formed from 3,4-AHBAL and that the conjugate is presumably coupled with another molecule of the o-quinone imine. GriF is thus a novel o-aminophenol oxidase that is responsible for the formation of the phenoxazinone chromophore in the grixazone biosynthetic pathway.

Catalytic Properties of an Organic Solvent–Resistant Tyrosinase from Streptomyces sp. REN-21 and Its High-Level Production in E. coli

An organic solvent-resistant tyrosinase (OSRT) from Streptomyces sp. REN-21 is a unique enzyme showing high activity in the presence of organic solvents. The OSRT-catalyzed oxidation of monophenols such as tyrosine-containing peptides and proteins was examined. The catalytic properties of OSRT were compared with those of mushroom tyrosinase. OSRT was shown to oxidize Gly-l-Tyr most effectively among four peptide substrates tested. On the other hand, mushroom tyrosinase showed the highest activity toward l-Tyr-Gly under the condition of 1 mM substrate. OSRT oxidized several proteins, including casein and hemoglobin, with relatively higher activity compared with mushroom tyrosinase under the condition of 1% (w/v) substrate. Thus, it was clarified that the catalytic properties of OSRT toward tyrosine-containing peptides and proteins are different from those of mushroom tyrosinase under these conditions. The OSRT-encoding gene operon was cloned, and found to consist of two genes, designated ORF-OSRT and ORF-393. The former encodes apo-OSRT, and the latter encodes the putative activator protein of apo-OSRT. A binuclear copper-binding site (type-3 copper site) characteristic of tyrosinases is contained in the deduced amino acid sequence for apo-OSRT. A high-level production system for the OSRT was constructed using pET20b(+) and Escherichia coli BL21(DE3)pLysS. Approximately 54 mg of active OSRT was synthesized in a 1-liter broth culture by this system. The properties of the recombinant OSRT were similar to those of the wild-type enzyme. In conclusion, we succeeded in constructing a high-level production system for OSRT.

Characterization of Streptomyces scabies mutants deficient in melanin biosynthesis

Streptomyces scabies, a causal agent of common scab, produces both melanin and a secondary metabolite called thaxtomin A. To establish a possible relation between melanin and thaxtomin A production in S. scabies, we carried out N-methyl-N′-nitro-N-nitrosoguanidine (NTG) mutagenesis and isolated 11 melanin-negative mutants of S. scabies EF-35. These mutants were characterized for thaxtomin A production, pathogenicity, sporulation, and stress resistance. Nine of these mutants showed a significant reduction in thaxtomin A production when compared with the wild strain. However, only a few mutants exhibited a reduced level of virulence or a loss in their ability to induce common scab symptoms on potato tubers. Other pleiotrophic effects, such as higher sensitivity to heavy metals and incapacity to sporulate under certain stress conditions, were also associated with a deficiency in melanin production.Key words: common scab, potato, secondary metabolism, stress, thaxtomin.

Novel genes that influence development in Streptomyces coelicolor

Filamentous soil bacteria of the genus Streptomyces carry out complex developmental cycles that result in sporulation and production of numerous secondary metabolites with pharmaceutically important activities. To further characterize the molecular basis of these developmental events, we screened for mutants of Streptomyces coelicolor that exhibit aberrant morphological differentiation and/or secondary metabolite production. On the basis of this screening analysis and the subsequent complementation analysis of the mutants obtained we assigned developmental roles to a gene involved in methionine biosynthesis (metH) and two previously uncharacterized genes (SCO6938 and SCO2525) and we reidentified two previously described developmental genes (bldA and bldM). In contrast to most previously studied genes involved in development, the genes newly identified in the present study all appear to encode biosynthetic enzymes instead of regulatory proteins. The MetH methionine synthase appears to be required for conversion of aerial hyphae into chains of spores, SCO6938 is a probable acyl coenzyme A dehydrogenase that contributes to the proper timing of aerial mycelium formation and antibiotic production, and SCO2525 is a putative methyltransferase that influences various aspects of colony growth and development.

An efficient method for the overexpression and purification of active tyrosinase from Streptomyces castaneoglobisporus

The melanin-synthesizing gene operon cloned from Streptomyces castaneoglobisporus HUT6202 consists of two genes, designated tyrC and orf378, which encode apotyrosinase (TYRC) and its activator protein (ORF378), respectively. We have suggested that ORF378 may facilitate the incorporation of Cu(II) into apotyrosinase to express tyrosinase activity. To overproduce ORF378 and TYRC in Escherichia coli BL21(DE3)-pLysS, tyrC, and orf378 were independently but not polycistronically placed under the control of a T7 promoter in a vector, pET-21a(+). His(6)-tagged TYRC and His(6)-tagged ORF378 were simultaneously overproduced in an E. coli strain harboring a plasmid, designated pET-mel2, and the two proteins were co-purified with a Ni(II)-bound affinity column. Gel filtration analysis revealed that the two proteins form a heterodimer complex. The complexed protein was retrieved at a high efficiency (11 mg/L). To obtain an active TYRC, which is a Cu(II)-bound form of tyrosinase, we constructed pET-mel3 that carries orf378 without His(6)-tag and His(6)-tagged tyrC. After the cell-free extract from E. coli harboring pET-mel3 was subjected to Cu(II)-bound affinity column chromatography, His(6)-tagged TYRC, eluted from the column, exhibited the tyrosinase activity. The k(cat) and K(m) values for l-3,4-dihydroxyphenylalanine (l-DOPA) of His(6)-tagged TYRC, which catalyzes the oxidation of l-DOPA to dopaquinone, were 880+/-80s(-1) and 8.1+/-0.9 mM, respectively.

Identification and cloning of a type III polyketide synthase required for diffusible pigment biosynthesis in Saccharopolyspora erythraea

The soluble, diffusible red-brown pigment produced by a Saccharopolyspora erythraea "red variant" has been shown to contain glycosylated and polymerized derivatives of 2,5,7-trihydroxy-1,4-naphthoquinone (flaviolin). Flaviolin is a spontaneous oxidation product of 1,3,6,8-tetrahydroxynaphthalene (THN), which is biosynthesized in bacteria by a chalcone synthase-like (CS-like) type III polyketide synthase (PKS). A fragment of the gene responsible for THN biosynthesis in S. erythraea E_8-7 was amplified by polymerase chain reaction (PCR) using degenerate primers based on conserved regions of known plant CS and bacterial CS-like genes. From the isolated fragment, a suicide vector was prepared, which was subsequently used to disrupt the red-brown pigment-producing (rpp) locus in S. erythraea, generating a mutant that displayed an albino phenotype. Chromosomal DNA from the albino mutant was subsequently used in a vector-recapture protocol to isolate a plasmid that contained an insert spanning the entire rpp locus. Sequencing of the insert revealed that the disrupted open reading frame (ORF) encodes a CS-like protein displaying 69% sequence identity to the rppA gene of Streptomyces griseus. The S. griseus rppA gene encodes RppA, the first characterized bacterial CS-like protein, which is sufficient in vitro for the synthesis of THN from malonyl-CoA. The rppA disruption mutant and rppA sequence provided a means by which to address the mechanism of diffusible pigment biosynthesis, as well as to investigate any link between this and the modulation of erythromycin A titre, which has been observed for S. erythraea variants.

Cloning and overexpression of a tyrosinase gene mel from Pseudomonas maltophila

The tyrosinase gene (mel), which is responsible for melanin formation, was isolated by shotgun cloning of SalI fragments of Pseudomonas maltophila DNA. A 0.7-kb SalI fragment in the recombinant plasmid pWSY8 imparted the ability to synthesize melanin to an Escherichia coli host HB101. The nucleotide sequence of this DNA fragment revealed an open reading frame of 504 bp, encoding a protein of 169 amino acids. The fragment containing the mel gene was then cloned into an expression plasmid pPAS1 under the control of a promoter isolated from the host, P. maltophilia AT18. This strain increased the melanin production by 70.6% compared with the strain HB101/pWSY8, in which the cloned mel gene was under the control of the lac promoter from the vector pUC18.