Biosynthesis of the dichloroacetyl component of chloramphenicol in Streptomyces venezuelae ISP5230: genes required for halogenation

Selective functionalization of C(sp3)-H bonds: catalytic chlorination and bromination by IronIII-acacen-halide under ambient condition

The oxidative catalytic halogenations of the C(sp³)-H bond of alkanes promoted by Fe^III(acacen)Cl (1III-Cl) and Fe^III(acacen)Br (1III-Br) in the presence of trifluoroacetic acid (TFA) were investigated. Four major steps were involved: (i) formation of [Fe^V(acacen)(oxo)X] species (X = Cl or Br), (ii) hydrogen-atom transfer, (iii) halogen atom rebound, and (iv) regeneration of 1III-Cl or 1III-Br. TFA played a significant role in (i) forming the high-valent iron-oxo intermediate and (ii) generating the reaction selectivity.

Colibrimycins, novel halogenated hybrid PKS-NRPS compounds produced by Streptomyces sp. CS147

The improvement on genome sequencing techniques has brought to light the biosynthetic potential of actinomycetes due to the high number of gene clusters they present compared to the number of known compounds. Genome mining is a recent strategy in the search for novel bioactive compounds, which involves the analysis of sequenced genomes to identify uncharacterized natural product biosynthetic gene clusters, many of which are cryptic or silent under laboratory conditions, and to develop experimental approaches to identify their products. Owing to the importance of halogenation in terms of structural diversity, bioavailability and bioactivity, searching for new halogenated bioactive compounds has become an interesting issue in the field of natural product discovery. Following this purpose, a screening for halogenase coding genes was performed on twelve Streptomyces strains isolated from fungus growing ants of the Attini tribe. Using the bioinformatics tools antiSMASH and BLAST, six halogenase coding genes were identified. Some of these genes were located within biosynthetic gene clusters (BGCs), which were studied by construction of several mutants for the identification of the putative halogenated compounds produced. The comparison of the metabolite production profile of wild type strains and their corresponding mutants by UPLC-UV and HPLC-MS allowed us the identification of a novel family of halogenated compounds in Streptomyces sp. CS147, designated as colibrimycins. Importance Genome mining has proven its usefulness in the search for novel bioactive compounds produced by microorganisms, and halogenases comprise an interesting starting point. In this work, we have identified a new halogenase coding gene, which led to the discovery of novel lipopetide NRPS/PKS-derived natural products, the colibrimycins, produced by Streptomyces sp. CS147, isolated from Attini ant niche. Some colibrimycins display an unusual α-ketoamide moiety in the peptide structure. Although its biosynthetic origin remains unknown, its presence might be related with a hypothetical inhibition of virus proteases and, together with the presence of the halogenase, it represents a feature to be incorporated in the arsenal of structural modifications available for combinatorial biosynthesis.

Halogenases: a palette of emerging opportunities for synthetic biology-synthetic chemistry and C-H functionalisation

The enzymatic generation of carbon-halogen bonds is a powerful strategy used by both nature and synthetic chemists to tune the bioactivity, bioavailability and reactivity of compounds, opening up the opportunity for selective C-H functionalisation. Genes encoding halogenase enzymes have recently been shown to transcend all kingdoms of life. These enzymes install halogen atoms into aromatic and less activated aliphatic substrates, achieving selectivities that are often challenging to accomplish using synthetic methodologies. Significant advances in both halogenase discovery and engineering have provided a toolbox of enzymes, enabling the ready use of these catalysts in biotransformations, synthetic biology, and in combination with chemical catalysis to enable late stage C-H functionalisation. With a focus on substrate scope, this review outlines the mechanisms employed by the major classes of halogenases, while in parallel, it highlights key advances in the utilisation of the combination of enzymatic halogenation and chemical catalysis for C-H activation and diversification.

Direct C-H bond halogenation and pseudohalogenation of hydrocarbons mediated by high-valent 3d metal-oxo species

Late-stage direct functionalization of the C-H bond is synthetically desirable. Metalloenzymes having metal-oxo active sites are well known to selectively catalyze hydroxylation and halogenation reactions with high efficiency. This review highlights the recent developments in the field of direct C-H halogenation and pseudohalogenation reactions catalyzed by the functional models of metalloenzymes.

Microbial Synthesis and Transformation of Inorganic and Organic Chlorine Compounds

Organic and inorganic chlorine compounds are formed by a broad range of natural geochemical, photochemical and biological processes. In addition, chlorine compounds are produced in large quantities for industrial, agricultural and pharmaceutical purposes, which has led to widespread environmental pollution. Abiotic transformations and microbial metabolism of inorganic and organic chlorine compounds combined with human activities constitute the chlorine cycle on Earth. Naturally occurring organochlorines compounds are synthesized and transformed by diverse groups of (micro)organisms in the presence or absence of oxygen. In turn, anthropogenic chlorine contaminants may be degraded under natural or stimulated conditions. Here, we review phylogeny, biochemistry and ecology of microorganisms mediating chlorination and dechlorination processes. In addition, the co-occurrence and potential interdependency of catabolic and anabolic transformations of natural and synthetic chlorine compounds are discussed for selected microorganisms and particular ecosystems.

Regulation of antibiotic biosynthesis in actinomycetes: Perspectives and challenges

Actinomycetes are the main sources of antibiotics. The onset and level of production of each antibiotic is subject to complex control by multi-level regulators. These regulators exert their functions at hierarchical levels. At the lower level, cluster-situated regulators (CSRs) directly control the transcription of neighboring genes within the gene cluster. Higher-level pleiotropic and global regulators exert their functions mainly through modulating the transcription of CSRs. Advances in understanding of the regulation of antibiotic biosynthesis in actinomycetes have inspired us to engineer these regulators for strain improvement and antibiotic discovery.

Halogenase engineering and its utility in medicinal chemistry

Halogenation is commonly used in medicinal chemistry to improve the potency of pharmaceutical leads. While synthetic methods for halogenation present selectivity and reactivity challenges, halogenases have evolved over time to perform selective reactions under benign conditions. The optimization of halogenation biocatalysts has utilized enzyme evolution and structure-based engineering alongside biotransformation in a variety of systems to generate stable site-selective variants. The recent improvements in halogenase-catalyzed reactions has demonstrated the utility of these biocatalysts for industrial purposes, and their ability to achieve a broad substrate scope implies a synthetic tractability with increasing relevance in medicinal chemistry.

Unprecedented (μ-1,1-Peroxo)diferric Structure for the Ambiphilic Orange Peroxo Intermediate of the Nonheme N-Oxygenase CmlI

The final step in the biosynthesis of the antibiotic chloramphenicol is the oxidation of an aryl-amine substrate to an aryl-nitro product catalyzed by the N-oxygenase CmlI in three two-electron steps. The CmlI active site contains a diiron cluster ligated by three histidine and four glutamate residues and activates dioxygen to perform its role in the biosynthetic pathway. It was previously shown that the active oxidant used by CmlI to facilitate this chemistry is a peroxo-diferric intermediate (CmlIP). Spectroscopic characterization demonstrated that the peroxo binding geometry of CmlIP is not consistent with the μ-1,2 mode commonly observed in nonheme diiron systems. Its geometry was tentatively assigned as μ-η2:η1 based on comparison with resonance Raman (rR) features of mixed-metal model complexes in the absence of appropriate diiron models. Here, X-ray absorption spectroscopy (XAS) and rR studies have been used to establish a refined structure for the diferric cluster of CmlIP. The rR experiments carried out with isotopically labeled water identified the symmetric and asymmetric vibrations of an Fe-O-Fe unit in the active site at 485 and 780 cm-1, respectively, which was confirmed by the 1.83 Å Fe-O bond observed by XAS. In addition, a unique Fe···O scatterer at 2.82 Å observed from XAS analysis is assigned as arising from the distal O atom of a μ-1,1-peroxo ligand that is bound symmetrically between the irons. The (μ-oxo)(μ-1,1-peroxo)diferric core structure associated with CmlIP is unprecedented among diiron cluster-containing enzymes and corresponding biomimetic complexes. Importantly, it allows the peroxo-diferric intermediate to be ambiphilic, acting as an electrophilic oxidant in the initial N-hydroxylation of an arylamine and then becoming a nucleophilic oxidant in the final oxidation of an aryl-nitroso intermediate to the aryl-nitro product.

Crystal structure of CmlI, the arylamine oxygenase from the chloramphenicol biosynthetic pathway

The diiron cluster-containing oxygenase CmlI catalyzes the conversion of the aromatic amine precursor of chloramphenicol to the nitroaromatic moiety of the active antibiotic. The X-ray crystal structures of the fully active, N-terminally truncated CmlIΔ33 in the chemically reduced Fe(2+)/Fe(2+) state and a cis μ-1,2(η (1):η (1))-peroxo complex are presented. These structures allow comparison with the homologous arylamine oxygenase AurF as well as other types of diiron cluster-containing oxygenases. The structural model of CmlIΔ33 crystallized at pH 6.8 lacks the oxo-bridge apparent from the enzyme optical spectrum in solution at higher pH. In its place, residue E236 forms a μ-1,3(η (1):η (2)) bridge between the irons in both models. This orientation of E236 stabilizes a helical region near the cluster which closes the active site to substrate binding in contrast to the open site found for AurF. A very similar closed structure was observed for the inactive dimanganese form of AurF. The observation of this same structure in different arylamine oxygenases may indicate that there are two structural states that are involved in regulation of the catalytic cycle. Both the structural studies and single crystal optical spectra indicate that the observed cis μ-1,2(η (1):η (1))-peroxo complex differs from the μ-η (1):η (2)-peroxo proposed from spectroscopic studies of a reactive intermediate formed in solution by addition of O2 to diferrous CmlI. It is proposed that the structural changes required to open the active site also drive conversion of the µ-1,2-peroxo species to the reactive form.

JadX is a Disparate Natural Product Binding Protein

We report that JadX, a protein of previously undetermined function coded for in the jadomycin biosynthetic gene cluster of Streptomyces venezuelae ISP5230, affects both chloramphenicol and jadomycin production levels in blocked mutants. Characterization of recombinant JadX through protein-ligand interactions by chemical shift perturbation and WaterLOGSY NMR spectroscopy resulted in the observation of binding between JadX and a series of jadomycins and between JadX and chloramphenicol, another natural product produced by S. venezuelae ISP5230. These results suggest JadX to be an unusual class of natural product binding protein involved in binding structurally disparate natural products. The ability for JadX to bind two different natural products in vitro and the ability to affect production of these secondary metabolites in vivo suggest a potential role in regulation or signaling. This is the first example of functional characterization of these JadX-like proteins, and provides insight into a previously unobserved regulatory process.

New insights into chloramphenicol biosynthesis in Streptomyces venezuelae ATCC 10712

Comparative genome analysis revealed seven uncharacterized genes, sven0909 to sven0915, adjacent to the previously identified chloramphenicol biosynthetic gene cluster (sven0916–sven0928) of Streptomyces venezuelae strain ATCC 10712 that was absent in a closely related Streptomyces strain that does not produce chloramphenicol. Transcriptional analysis suggested that three of these genes might be involved in chloramphenicol production, a prediction confirmed by the construction of deletion mutants. These three genes encode a cluster-associated transcriptional activator (Sven0913), a phosphopantetheinyl transferase (Sven0914), and a Na⁺/H⁺ antiporter (Sven0915). Bioinformatic analysis also revealed the presence of a previously undetected gene, sven0925, embedded within the chloramphenicol biosynthetic gene cluster that appears to encode an acyl carrier protein, bringing the number of new genes likely to be involved in chloramphenicol production to four. Microarray experiments and synteny comparisons also suggest that sven0929 is part of the biosynthetic gene cluster. This has allowed us to propose an updated and revised version of the chloramphenicol biosynthetic pathway.

Genomic mining for novel FADH₂-dependent halogenases in marine sponge-associated microbial consortia

Many marine sponges (Porifera) are known to contain large amounts of phylogenetically diverse microorganisms. Sponges are also known for their large arsenal of natural products, many of which are halogenated. In this study, 36 different FADH₂-dependent halogenase gene fragments were amplified from various Caribbean and Mediterranean sponges using newly designed degenerate PCR primers. Four unique halogenase-positive fosmid clones, all containing the highly conserved amino acid motif "GxGxxG", were identified in the microbial metagenome of Aplysina aerophoba. Sequence analysis of one halogenase-bearing fosmid revealed notably two open reading frames with high homologies to efflux and multidrug resistance proteins. Single cell genomic analysis allowed for a taxonomic assignment of the halogenase genes to specific symbiotic lineages. Specifically, the halogenase cluster S1 is predicted to be produced by a deltaproteobacterial symbiont and halogenase cluster S2 by a poribacterial sponge symbiont. An additional halogenase gene is possibly produced by an actinobacterial symbiont of marine sponges. The identification of three novel, phylogenetically, and possibly also functionally distinct halogenase gene clusters indicates that the microbial consortia of sponges are a valuable resource for novel enzymes involved in halogenation reactions.

CmlI is an N-oxygenase in the biosynthesis of chloramphenicol

The N-oxygenation of an amine group is one of the steps in the biosynthesis of the antibiotic chloramphenicol. The non-heme di-iron enzyme CmlI was identified as the enzyme catalyzing this reaction through bioinformatics studies and reconstitution of enzymatic activity. In vitro reconstitution was achieved using phenazine methosulfate and NADH as electron mediators, while in vivo activity was demonstrated in Escherichia coli using two substrates. Kinetic analysis showed a biphasic behavior of the enzyme. Oxidized hydroxylamine and nitroso compounds in the reaction were detected both in vitro and in vivo based on LC-MS. The active site metal was confirmed to be iron based on a ferrozine assay. These findings provide new insights into the biosynthesis of chloramphenicol and could lead to further development of CmlI as a useful biocatalyst.

Enzymatic chlorination and bromination

Our knowledge about the enzymes catalyzing the incorporation of halide ions during the biosynthesis of halometabolites has increased tremendously during the last 15 years. Between 1960 and 1995, haloperoxidases were the only halogenating enzymes known. However, absolute proof for the connection of haloperoxidases to the biosynthesis of halometabolites is still missing. In 1997, FADH(2)-dependent halogenases were identified as the type of halogenating enzymes responsible for the incorporation of chloride and bromide atoms into aromatic and aliphatic compounds activated for electrophilic attack. FADH(2)-dependent halogenases are two-component systems consisting of a flavin reductase providing the FADH(2) required by the halogenase. Elucidation of the three-dimensional structure of FADH(2)-dependent halogenases led to the understanding of the reaction mechanism, which involves the formation of hypohalous acids. Unactivated carbon atoms were found to be halogenated by nonheme iron, α-ketoglutarate- and O(2)-dependent halogenases. The reaction mechanism of this type of halogenase was shown to involve the formation of a substrate radical. These two types of halogenating enzymes, together with the much less common fluorinases, are the major types of halogenating enzymes. However, the existence of other types of halogenating enzymes, yet not detected, cannot be completely ruled out. Here, we describe the detection, purification, characterization, and reaction mechanisms of flavin-dependent halogenases and of nonheme iron, α-ketoglutarate- and O(2)-dependent halogenases.

"Pseudo" gamma-butyrolactone receptors respond to antibiotic signals to coordinate antibiotic biosynthesis

In actinomycetes, the onset of secondary metabolite biosynthesis is often triggered by the quorum-sensing signal gamma-butyrolactones (GBLs) via specific binding to their cognate receptors. However, the presence of multiple putative GBL receptor homologues in the genome suggests the existence of an alternative regulatory mechanism. Here, in the model streptomycete Streptomyces coelicolor, ScbR2 (SCO6286, a homologue of GBL receptor) is shown not to bind the endogenous GBL molecule SCB1, hence designated "pseudo" GBL receptor. Intriguingly, it could bind the endogenous antibiotics actinorhodin and undecylprodigiosin as ligands, leading to the derepression of KasO, an activator of a cryptic type I polyketide synthase gene cluster. Likewise, JadR2 is also a putative GBL receptor homologue in Streptomyces venezuelae, the producer of chloramphenicol and cryptic antibiotic jadomycin. It is shown to coordinate their biosynthesis via direct repression of JadR1, which activates jadomycin biosynthesis while repressing chloramphenicol biosynthesis directly. Like ScbR2, JadR2 could also bind these two disparate antibiotics, and the interactions lead to the derepression of jadR1. The antibiotic responding activities of these pseudo GBL receptors were further demonstrated in vivo using the lux reporter system. Overall, these results suggest that pseudo GBL receptors play a novel role to coordinate antibiotic biosynthesis by binding and responding to antibiotics signals. Such an antibiotic-mediated regulatory mechanism could be a general strategy to coordinate antibiotic biosynthesis in the producing bacteria.

Isolation and identification of chlorinated genistein from Actinoplanes sp. HBDN08 with antioxidant and antitumor activities

A strain Actinoplanes sp. HBDN08 was screened by PCR-guided method using primers derived from conserved regions of halogenase genes. A new chlorinated isoflavone, 3',8-dichlorogenistein (1), along with 8-chlorogenistein (2) were isolated from the fermentation broth of Actinoplanes sp. HBDN08. Their structures were elucidated on the basis of extensive 1D and 2D NMR as well as HRESI-MS, ESI-MS, UV, and IR spectroscopic analyses. The origin of the two compounds was also investigated by high-performance liquid chromatography (HPLC) analysis. The results demonstrated that they were not biosynthesized but derived from the biotransformation of genistein by Actinoplanes sp. HBDN08. The antioxidant activities of the isolated compounds 1 and 2 were evaluated by using the lipid peroxidation assay. Their antitumor activities were calculated according to the inhibitory rate of cell proliferation against the human breast cancer cell line MDA-MB-231. The results indicated that compounds 1 (IC(50) = 5.2 microM) and 2 (IC(50) = 7.5 microM) showed stronger antioxidant activities than genistein (IC(50) = 13.6 microM). In comparison with the antitumor activities of genistein, those of compounds 1 and 2 increased 7.7- and 2.6-fold, respectively. These results suggest that the PCR-guided screening strategy is a rapid method for obtaining halometabolite-producing strains. Moreover, these results reveal that chlorination has significant effects on the bioactivities of genistein. This could be important information for studying the structure-activity relationships of genistein.

Chloramphenicol biosynthesis: the structure of CmlS, a flavin-dependent halogenase showing a covalent flavin-aspartate bond

Chloramphenicol is a halogenated natural product bearing an unusual dichloroacetyl moiety that is critical for its antibiotic activity. The operon for chloramphenicol biosynthesis in Streptomyces venezuelae encodes the chloramphenicol halogenase CmlS, which belongs to the large and diverse family of flavin-dependent halogenases (FDH's). CmlS was previously shown to be essential for the formation of the dichloroacetyl group. Here we report the X-ray crystal structure of CmlS determined at 2.2 A resolution, revealing a flavin monooxygenase domain shared by all FDHs, but also a unique 'winged-helix' C-terminal domain that creates a T-shaped tunnel leading to the halogenation active site. Intriguingly, the C-terminal tail of this domain blocks access to the halogenation active site, suggesting a structurally dynamic role during catalysis. The halogenation active site is notably nonpolar and shares nearly identical residues with Chondromyces crocatus tyrosyl halogenase (CndH), including the conserved Lys (K71) that forms the reactive chloramine intermediate. The exception is Y350, which could be used to stabilize enolate formation during substrate halogenation. The strictly conserved residue E44, located near the isoalloxazine ring of the bound flavin adenine dinucleotide (FAD) cofactor, is optimally positioned to function as a remote general acid, through a water-mediated proton relay, which could accelerate the reaction of the chloramine intermediate during substrate halogenation, or the oxidation of chloride by the FAD(C4alpha)-OOH intermediate. Strikingly, the 8alpha carbon of the FAD cofactor is observed to be covalently attached to D277 of CmlS, a residue that is highly conserved in the FDH family. In addition to representing a new type of flavin modification, this has intriguing implications for the mechanism of FDHs. Based on the crystal structure and in analogy to known halogenases, we propose a reaction mechanism for CmlS.

Biohalogenation: nature's way to synthesize halogenated metabolites

Halogenated natural products are widely distributed in nature, some of them showing potent biological activities. Incorporation of halogen atoms in drug leads is a common strategy to modify molecules in order to vary their bioactivities and specificities. Chemical halogenation, however, often requires harsh reaction conditions and results in unwanted byproduct formation. It is thus of great interest to investigate the biosynthesis of halogenated natural products and the biotechnological potential of halogenating enzymes. This review aims to give a comprehensive overview on the current knowledge concerning biological halogenations.

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.

What's new in enzymatic halogenations

The halogenation of thousands of natural products occurs during biosynthesis and often confers important functional properties. While haloperoxidases had been the default paradigm for enzymatic incorporation of halogens, via X+ equivalents into organic scaffolds, a combination of microbial genome sequencing, enzymatic studies and structural biology have provided deep new insights into enzymatic transfer of halide equivalents in three oxidation states. These are (1) the halide ions (X-) abundant in nature, (2) halogen atoms (X*), and (3) the X+ equivalents. The mechanism of halogen incorporation is tailored to the electronic demands of specific substrates and involves enzymes with distinct redox coenzyme requirements.

Inferring the chemical mechanism from structures of enzymes

Chemistry has once again embraced the study of enzyme mechanism as a core discipline. Chemists are uniquely able to contribute to the analysis of enzymes through their understanding of the reactivity of atoms. In this tutorial review for the Corday-Morgan medal, I will concentrate on the work from my lab over the past six years. I discuss enzymes which transform carbohydrates and incorporate halogens. The tutorial review will emphasise the strengths and limitations of structural biology as a means to deducing the chemical mechanism.

Flavin-dependent halogenases involved in secondary metabolism in bacteria

The understanding of biological halogenation has increased during the last few years. While haloperoxidases were the only halogenating enzymes known until 1997, it is now clear that haloperoxidases are hardly, if at all, involved in biosynthesis of more complex halogenated compounds in microorganisms. A novel type of halogenating enzymes, flavin-dependent halogenases, has been identified as a major player in the introduction of chloride and bromide into activated organic molecules. Flavin-dependent halogenases require the activity of a flavin reductase for the production of reduced flavin, required by the actual halogenase. A number of flavin-dependent tryptophan halogenases have been investigated in some detail, and the first three-dimensional structure of a member of this enzyme subfamily, tryptophan 7-halogenase, has been elucidated. This structure suggests a mechanism involving the formation of hypohalous acid, which is used inside the enzyme for regioselective halogenation of the respective substrate. The introduction of halogen atoms into non-activated alkyl groups is catalysed by non-heme FeII alpha-ketoglutarate- and O2-dependent halogenases. Examples for the use of flavin-dependent halogenases for the formation of novel halogenated compounds in in vitro and in vivo reactions promise a bright future for the application of biological halogenation reactions.

Recent developments in enzymatic chlorination

While the existence of chlorinated natural products has been known for over 100 years, our understanding of the enzymology of biological chlorination reactions has been limited to chloroperoxidases, which are now known not to play a significant role in chlorometabolite biosynthesis. The discoveries of new classes of halogenases, described in this Highlight, have shed new light on the mechanisms of enzymatic chlorination of aromatic and aliphatic compounds.

Tryptophan 7-halogenase (PrnA) structure suggests a mechanism for regioselective chlorination

Chlorinated natural products include vancomycin and cryptophycin A. Their biosynthesis involves regioselective chlorination by flavin-dependent halogenases. We report the structural characterization of tryptophan 7-halogenase (PrnA), which regioselectively chlorinates tryptophan. Tryptophan and flavin adenine dinucleotide (FAD) are separated by a 10 angstrom-long tunnel and bound by distinct enzyme modules. The FAD module is conserved in halogenases and is related to flavin-dependent monooxygenases. On the basis of biochemical studies, crystal structures, and by analogy with monooxygenases, we predict that FADH2 reacts with O2 to make peroxyflavin, which is decomposed by Cl-. The resulting HOCl is guided through the tunnel to tryptophan, where it is activated to participate in electrophilic aromatic substitution.

An unexpected role for the putative 4'-phosphopantetheinyl transferase-encoding gene nysF in the regulation of nystatin biosynthesis in Streptomyces noursei ATCC 11455

The nysF gene encoding a putative 4'-phosphopantetheinyl transferase (PPTase) is located at the 5' border of the nystatin biosynthesis gene cluster in Streptomyces noursei. PPTases carry out post-translational modification of the acyl carrier protein domains on the polyketide synthases (PKS) required for their full functionality, and hence NysF was assumed to be involved in similar modification of the nystatin PKS. At the same time, DNA sequence analysis of the genomic region adjacent to the nysF gene revealed a gene cluster for a putative lantibiotic biosynthesis. This finding created some uncertainty regarding which gene cluster nysF functionally belongs to. To resolve this ambiguity, nysF was inactivated by both insertion of a kanamycin (Km) resistance marker into its coding region, and by in-frame deletion. Surprisingly, the nystatin production in both the nysF::Km(R) and DeltanysF mutants increased by ca. 60% compared to the wild-type, suggesting a negative role of nysF in the nystatin biosynthesis. The expression of xylE reporter gene under control of different promoters from the nystatin gene cluster in the DeltanysF mutant was studied. The data obtained clearly show enhanced expression of xylE from the promoters of several structural and regulatory genes in the DeltanysF mutant, implying that NysF negatively regulates the nystatin biosynthesis.

Robust in vitro activity of RebF and RebH, a two-component reductase/halogenase, generating 7-chlorotryptophan during rebeccamycin biosynthesis

The indolocarbazole antitumor agent rebeccamycin is modified by chlorine atoms on each of two indole moieties of the aglycone scaffold. These halogens are incorporated during the initial step of its biosynthesis from conversion of L-Trp to 7-chlorotryptophan. Two genes in the biosynthetic cluster, rebF and rebH, are predicted to encode the flavin reductase and halogenase components of an FADH2-dependent halogenase, a class of enzymes involved in the biosynthesis of numerous halogenated natural products. Here, we report that, in the presence of O2, chloride ion, and L-Trp as cosubstrates, purified RebH displays robust regiospecific halogenating activity to generate 7-chlorotryptophan over at least 50 catalytic cycles. Halogenation by RebH required the addition of RebF, which catalyzes the NADH-dependent reduction of FAD to provide FADH2 for the halogenase. Maximal rates were achieved at a RebF/RebH ratio of 3:1. In air-saturated solutions, a k(cat) of 1.4 min(-1) was observed for the RebF/RebH system but increased at least 10-fold in low-pO2 conditions. RebH was also able to use bromide ions to generate monobrominated Trp. The demonstration of robust chlorinating activity by RebF/RebH sets up this system for the probing of mechanistic questions regarding this intriguing class of enzymes.