Crystal structures and mutational analyses of acyl-CoA carboxylase beta subunit of Streptomyces coelicolor

Enhancing armeniaspirols production through multi-level engineering of a native Streptomyces producer

BACKGROUND

Nature has provided unique molecular scaffolds for applications including therapeutics, agriculture, and food. Due to differences in ecological environments and laboratory conditions, engineering is often necessary to uncover and utilize the chemical diversity. Although we can efficiently activate and mine these often complex 3D molecules, sufficient production of target molecules for further engineering and application remain a considerable bottleneck. An example of these bioactive scaffolds is armeniaspirols, which are potent polyketide antibiotics against gram-positive pathogens and multi-resistance gram-negative Helicobacter pylori. Here, we examine the upregulation of armeniaspirols in an alternative Streptomyces producer, Streptomyces sp. A793.

RESULTS

Through an incidental observation of enhanced yields with the removal of a competing polyketide cluster, we observed seven-fold improvement in armeniaspirol production. To further investigate the improvement of armeniaspirol production, we examine upregulation of armeniaspirols through engineering of biosynthetic pathways and primary metabolism; including perturbation of genes in biosynthetic gene clusters and regulation of triacylglycerols pool.

CONCLUSION

With either overexpression of extender unit pathway or late-stage N-methylation, or the deletion of a competing polyketide cluster, we can achieve seven-fold to forty nine-fold upregulation of armeniaspirol production. The most significant upregulation was achieved by expression of heterologous fatty acyl-CoA synthase, where we observed not only a ninety seven-fold increase in production yields compared to wild type, but also an increase in the diversity of observed armeniaspirol intermediates and analogs.

Discovery, structure, and function of filamentous 3-methylcrotonyl-CoA carboxylase

3-methylcrotonyl-CoA carboxylase (MCC) is a biotin-dependent mitochondrial enzyme necessary for leucine catabolism in most organisms. While the crystal structure of recombinant bacterial MCC has been characterized, the structure and potential polymerization of native MCC remain elusive. Here, we discovered that native MCC from Leishmania tarentolae (LtMCC) forms filaments, and determined the structures of different filament regions at 3.4, 3.9, and 7.3 Å resolution using cryoEM. α6β6 LtMCCs assemble in a twisted-stacks architecture, manifesting as supramolecular rods up to 400 nm. Filamentous LtMCCs bind biotin non-covalently and lack coenzyme A. Filaments elongate by stacking α6β6 LtMCCs onto the exterior α-trimer of the terminal LtMCC. This stacking immobilizes the biotin carboxylase domains, sequestering the enzyme in an inactive state. Our results support a new model for LtMCC catalysis, termed the dual-swinging-domains model, and cast new light on the function of polymerization in the carboxylase superfamily and beyond.

Improvement of Rimocidin Biosynthesis by Increasing Supply of Precursor Malonyl-CoA via Over-expression of Acetyl-CoA Carboxylase in Streptomyces rimosus M527

Precursor engineering is an effective strategy for the overproduction of secondary metabolites. The polyene macrolide rimocidin, which is produced by Streptomyces rimosus M527, exhibits a potent activity against a broad range of phytopathogenic fungi. It has been predicted that malonyl-CoA is used as extender units for rimocidin biosynthesis. Based on a systematic analysis of three sets of time-series transcriptome microarray data of S. rimosus M527 fermented in different conditions, the differentially expressed acc_sr gene that encodes acetyl-CoA carboxylase (ACC) was found. To understand how the formation of rimocidin is being influenced by the expression of the acc_sr gene and by the concentration of malonyl-CoA, the acc_sr gene was cloned and over-expressed in the wild-type strain S. rimosus M527 in this study. The recombinant strain S. rimosus M527-ACC harboring the over-expressed acc_sr gene exhibited better performances based on the enzymatic activity of ACC, intracellular malonyl-CoA concentrations, and rimocidin production compared to S. rimosus M527 throughout the fermentation process. The enzymatic activity of ACC and intracellular concentration of malonyl-CoA of S. rimosus M527-ACC were 1.0- and 1.5-fold higher than those of S. rimosus M527, respectively. Finally, the yield of rimocidin produced by S. rimosus M527-ACC reached 320.7 mg/L, which was 34.0% higher than that of S. rimosus M527. These results confirmed that malonyl-CoA is an important precursor for rimocidin biosynthesis and suggested that an adequate supply of malonyl-CoA caused by acc_sr gene over-expression led to the improvement in rimocidin production.

Increasing the Ascomycin Yield by Relieving the Inhibition of Acetyl/Propionyl-CoA Carboxylase by the Signal Transduction Protein GlnB

Ascomycin (FK520) is a multifunctional antibiotic produced by Streptomyces hygroscopicus var. ascomyceticus. In this study, we demonstrated that the inactivation of GlnB, a signal transduction protein belonging to the PII family, can increase the production of ascomycin by strengthening the supply of the precursors malonyl-CoA and methylmalonyl-CoA, which are produced by acetyl-CoA carboxylase and propionyl-CoA carboxylase, respectively. Bioinformatics analysis showed that Streptomyces hygroscopicus var. ascomyceticus contains two PII family signal transduction proteins, GlnB and GlnK. Protein co-precipitation experiments demonstrated that GlnB protein could bind to the α subunit of acetyl-CoA carboxylase, and this binding could be disassociated by a sufficient concentration of 2-oxoglutarate. Coupled enzyme activity assays further revealed that the interaction between GlnB protein and the α subunit inhibited both the activity of acetyl-CoA carboxylase and propionyl-CoA carboxylase, and this inhibition could be relieved by 2-oxoglutarate in a concentration-dependent manner. Because GlnK protein can act redundantly to maintain metabolic homeostasis under the control of the global nitrogen regulator GlnR, the deletion of GlnB protein enhanced the supply of malonyl-CoA and methylmalonyl-CoA by restoring the activity of acetyl-CoA carboxylase and propionyl-CoA carboxylase, thereby improving the production of ascomycin to 390 ± 10 mg/L. On this basis, the co-overexpression of the β and ε subunits of propionyl-CoA carboxylase further increased the ascomycin yield to 550 ± 20 mg/L, which was 1.9-fold higher than that of the parent strain FS35 (287 ± 9 mg/L). Taken together, this study provides a novel strategy to increase the production of ascomycin, providing a reference for improving the yield of other antibiotics.

Reconstruction of a Genome-Scale Metabolic Model of Streptomyces albus J1074: Improved Engineering Strategies in Natural Product Synthesis

Streptomyces albus J1074 is recognized as an effective host for heterologous production of natural products. Its fast growth and efficient genetic toolbox due to a naturally minimized genome have contributed towards its advantage in expressing biosynthetic pathways for a diverse repertoire of products such as antibiotics and flavonoids. In order to develop precise model-driven engineering strategies for de novo production of natural products, a genome-scale metabolic model (GEM) was reconstructed for the microorganism based on protein homology to model species Streptomyces coelicolor while drawing annotated data from databases and literature for further curation. To demonstrate its capabilities, the Salb-GEM was used to predict overexpression targets for desirable compounds using flux scanning with enforced objective function (FSEOF). Salb-GEM was also utilized to investigate the effect of a minimized genome on metabolic gene essentialities in comparison to another Streptomyces species, S. coelicolor.

Functional Analysis of the PCCA and PCCB Gene Variants Predicted to Affect Splicing

It is estimated that up to one-third of all variants causing inherited diseases affect splicing; however, their deleterious effects and roles in disease pathogenesis are often not fully characterized. Given their prevalence and the development of various antisense-based splice-modulating approaches, pathogenic splicing variants have become an important object of genomic medicine. To improve the accuracy of variant interpretation in public mutation repositories, we applied the minigene splicing assay to study the effects of 24 variants that were predicted to affect normal splicing in the genes associated with propionic acidemia (PA)—PCCA and PCCB. As a result, 13 variants (including one missense and two synonymous variants) demonstrated a significant alteration of splicing with the predicted deleterious effect at the protein level and were characterized as spliceogenic loss-of-function variants. The analysis of the available data for the studied variants and application of the American College of Medical Genetics and the Association for Molecular Pathology (ACMG/AMP) guidelines allowed us to precisely classify five of the variants and change the pathogenic status of nine. Using the example of the PA genes, we demonstrated the utility of the minigene splicing assay in the fast and effective assessment of the spliceogenic effect for identified variants and highlight the necessity of their standardized classification.

CO2-converting enzymes for sustainable biotechnology: from mechanisms to application

To realize a circular, carbon-neutral economy, it will become important to utilize the greenhouse gas CO₂ as a sustainable carbon source. Carboxylases, the enzymes that capture and convert gaseous CO₂ are the prime candidates to pave the way towards realizing this vision of a CO₂-based bio-economy. In the last couple of years, the interest in using and engineering carboxylases has been steadily growing. Here, we discuss how basic research on the mechanism of CO₂ binding and activation by carboxylases opened the way to develop new-to-nature CO₂-fixing enzymes that found application in the development of synthetic CO₂-fixation pathways and their further realization in vitro and in vivo. These pioneering efforts in the field pave the way to realize a diverse CO₂-fixation biochemistry that can find application in biocatalysis, biotechnology, and artificial photosynthesis.

Kinetic, Structural, and Mutational Analysis of Acyl-CoA Carboxylase From Thermobifida fusca YX

Acyl-CoA carboxylases (AcCCase) are biotin-dependent enzymes that are capable of carboxylating more than one short chain acyl-CoA substrate. We have conducted structural and kinetic analyses of such an AcCCase from Thermobifida fusca YX, which exhibits promiscuity in carboxylating acetyl-CoA, propionyl-CoA, and butyryl-CoA. The enzyme consists of two catalytic subunits (TfAcCCA and TfAcCCB) and a non-catalytic subunit, TfAcCCE, and is organized in quaternary structure with a A₆B₆E₆ stoichiometry. Moreover, this holoenzyme structure appears to be primarily assembled from two A₃ and a B₆E₆ subcomplexes. The role of the TfAcCCE subunit is to facilitate the assembly of the holoenzyme complex, and thereby activate catalysis. Based on prior studies of an AcCCase from Streptomyces coelicolor, we explored whether a conserved Asp residue in the TfAcCCB subunit may have a role in determining the substrate selectivity of these types of enzymes. Mutating this D427 residue resulted in alterations in the substrate specificity of the TfAcCCase, increasing proficiency for carboxylating acetyl-CoA, while decreasing carboxylation proficiency with propionyl-CoA and butyryl-CoA. Collectively these results suggest that residue D427 of AcCCB subunits is an important, but not sole determinant of the substrate specificity of AcCCase enzymes.

A new-to-nature carboxylation module to improve natural and synthetic CO2 fixation

The capture of CO2 by carboxylases is key to sustainable biocatalysis and a carbon-neutral bio-economy, yet currently limited to few naturally existing enzymes. Here, we developed glycolyl-CoA carboxylase (GCC), a new-to-nature enzyme, by combining rational design, high-throughput microfluidics and microplate screens. During this process, GCC’s catalytic efficiency improved by three orders of magnitude to match the properties of natural CO2-fixing enzymes. We verified our active-site redesign with an atomic-resolution, 1.96-Å cryo-electron microscopy structure and engineered two more enzymes that, together with GCC, form a carboxylation module for the conversion of glycolate (C2) to glycerate (C3). We demonstrate how this module can be interfaced with natural photorespiration, ethylene glycol conversion and synthetic CO2 fixation. Based on stoichiometrical calculations, GCC is predicted to increase the carbon efficiency of all of these processes by up to 150% while reducing their theoretical energy demand, showcasing how expanding the solution space of natural metabolism provides new opportunities for biotechnology and agriculture.

AccR, a TetR Family Transcriptional Repressor, Coordinates Short-Chain Acyl Coenzyme A Homeostasis in Streptomyces avermitilis

Malonyl coenzyme A (malonyl-CoA) and methylmalonyl-CoA are the most common extender units for the biosynthesis of fatty acids and polyketides in Streptomyces, an industrially important producer of polyketides. Carboxylation of acetyl- and propionyl-CoAs is an essential source of malonyl- and methylmalonyl-CoAs; therefore, acyl-CoA carboxylases (ACCases) play key roles in primary and secondary metabolism. The regulation of the expression of ACCases in Streptomyces spp. has not been investigated previously. We characterized a TetR family transcriptional repressor, AccR, that mediates intracellular acetyl-, propionyl-, methylcrotonyl-, malonyl-, and methylmalonyl-CoA levels by controlling the transcription of genes that encode the main ACCase and enzymes associated with branched-chain amino acid metabolism in S. avermitilis AccR bound to a 16-nucleotide palindromic binding motif (GTTAA-N6-TTAAC) in promoter regions and repressed the transcription of the accD1A1-hmgL-fadE4 operon, echA8, echA9, and fadE2, which are involved in the production and assimilation of acetyl- and propionyl-CoAs. Methylcrotonyl-, propionyl-, and acetyl-CoAs acted as effectors to release AccR from its target DNA, resulting in enhanced transcription of target genes by derepression. The affinity of methylcrotonyl- and propionyl-CoAs to AccR was stronger than that of acetyl-CoA. Deletion of accR resulted in increased concentrations of short-chain acyl-CoAs (acetyl-, propionyl-, malonyl-, and methylmalonyl-CoAs), leading to enhanced avermectin production. Avermectin production was increased by 14.5% in an accR deletion mutant of the industrial high-yield strain S. avermitilis A8. Our findings clarify the regulatory mechanisms that maintain the homeostasis of short-chain acyl-CoAs in Streptomyces IMPORTANCE Acyl-CoA carboxylases play key roles in primary and secondary metabolism. However, the regulation of ACCase genes transcription in Streptomyces spp. remains unclear. Here, we demonstrated that AccR responded to intracellular acetyl-, propionyl-, and methylcrotonyl-CoA availability and mediated transcription of the genes related to production and assimilation of these compounds in S. avermitilis When intracellular concentrations of these compounds are low, AccR binds to target genes and represses their transcription, resulting in low production of malonyl- and methylmalonyl-CoAs. When intracellular acetyl-, propionyl-, and methylcrotonyl-CoA concentrations are high, these compounds bind to AccR to dissociate AccR from target DNA, promoting the conversion of these compounds to malonyl- and methylmalonyl-CoAs. This investigation revealed how AccR coordinates short-chain acyl-CoA homeostasis in Streptomyces.

De novo biosynthesis of myricetin, kaempferol and quercetin in Streptomyces albus and Streptomyces coelicolor

Flavonols are a flavonoid subfamily widely distributed in plants, including several ones of great importance in human and animal diet (apple, tomato, broccoli, onion, beans, tea). These polyphenolic nutraceuticals exert potent antimicrobial (membrane potential disruptors), antioxidant (free-radical scavengers), pharmacokinetic (CYP450 modulators), anti-inflammatory (lipoxygenase inhibitors), antiangiogenic (VEGF inhibitors) and antitumor (cyclin inhibitors) activities. Biotechnological production of these nutraceuticals, for example via heterologous biosynthesis in industrial actinomycetes, is favored since in plants these polyphenols appear as inactive glycosylated derivatives, in low concentrations or as part of complex mixtures with other polyphenolic compounds. In this work, we describe the de novo biosynthesis of three important flavonols, myricetin, kaempferol and quercetin, in the industrially relevant actinomycetes Streptomyces coelicolor and S. albus. De novo biosynthesis of kaempferol, myricetin and quercetin in actinomycetes has not been described before.

Type II fatty acid and polyketide synthases: deciphering protein-protein and protein-substrate interactions

Covering: up to April 5, 2018 Metabolites from type II fatty acid synthase (FAS) and polyketide synthase (PKS) pathways differ broadly in their identities and functional roles. The former are considered primary metabolites that are linear hydrocarbon acids, while the latter are complex aromatic or polyunsaturated secondary metabolites. Though the study of bacterial FAS has benefitted from decades of biochemical and structural investigations, type II PKSs have remained less understood. Here we review the recent approaches to understanding the protein-protein and protein-substrate interactions in these pathways, with an emphasis on recent chemical biology and structural applications. New approaches to the study of FAS have highlighted the critical role of the acyl carrier protein (ACP) with regard to how it stabilizes intermediates through sequestration and selectively delivers cargo to successive enzymes within these iterative pathways, utilizing protein-protein interactions to guide and organize enzymatic timing and specificity. Recent tools that have shown promise in FAS elucidation should find new approaches to studying type II PKS systems in the coming years.

Daring metabolic designs for enhanced plant carbon fixation

Increasing agricultural productivity is one of the major challenges our society faces. While multiple strategies to enhance plant carbon fixation have been suggested, and partially implemented, most of them are restricted to relatively simple modifications of endogenous metabolism, i.e., "low hanging fruit". Here, I portray the next generation of metabolic solutions to increase carbon fixation rate and yield. These strategies involve major rewiring of central metabolism, including dividing Rubisco's catalysis between several enzymes, replacing Rubisco with a different carboxylation reaction, substituting the Calvin Cycle with alternative carbon fixation pathways, and engineering photorespiration bypass routes that do not release carbon. While the barriers for implementing these elaborated metabolic architectures are quite significant, if we truly want to revolutionize carbon fixation, only daring engineering efforts will lead the way.

3-methylcrotonyl Coenzyme A (CoA) carboxylase complex is involved in the Xanthomonas citri subsp. citri lifestyle during citrus infection

Citrus canker is a disease caused by the phytopathogen Xanthomonas citri subsp. citri (Xcc), bacterium which is unable to survive out of the host for extended periods of time. Once established inside the plant, the pathogen must compete for resources and evade the defenses of the host cell. However, a number of aspects of Xcc metabolic and nutritional state, during the epiphytic stage and at different phases of infection, are poorly characterized. The 3-methylcrotonyl-CoA carboxylase complex (MCC) is an essential enzyme for the catabolism of the branched-chain amino acid leucine, which prevents the accumulation of toxic intermediaries, facilitates the generation of branched chain fatty acids and/or provides energy to the cell. The MCC complexes belong to a group of acyl-CoA carboxylases (ACCase) enzymes dependent of biotin. In this work, we have identified two ORFs (XAC0263 and XAC0264) encoding for the α and β subunits of an acyl-CoA carboxylase complex from Xanthomonas and demonstrated that this enzyme has MCC activity both in vitro and in vivo. We also found that this MCC complex is conserved in a group of pathogenic gram negative bacteria. The generation and analysis of an Xcc mutant strain deficient in MCC showed less canker lesions in the interaction with the host plant, suggesting that the expression of these proteins is necessary for Xcc fitness during infection.

Identification of Mycobacterium tuberculosis BioA inhibitors by using structure-based virtual screening

Background

7,8-Diaminopelargonic acid synthase (BioA), an enzyme of biotin biosynthesis pathway, is a well-known promising target for anti-tubercular drug development.

Methods

In this study, structure-based virtual screening was employed against the active site of BioA to identify new chemical entities for BioA inhibition and top ranking compounds were evaluated for their ability to inhibit BioA enzymatic activity.

Results

Seven compounds inhibited BioA enzymatic activity by greater than 60% at 100 μg/mL with most potent compounds being A36, A35 and A65, displaying IC₅₀ values of 10.48 μg/mL (28.94 μM), 33.36 μg/mL (88.16 μM) and 39.17 μg/mL (114.42 μM), respectively. Compounds A65 and A35 inhibited Mycobacterium tuberculosis (M. tuberculosis) growth with MIC₉₀ of 20 μg/mL and 80 μg/mL, respectively, whereas compound A36 exhibited relatively weak inhibition of M. tuberculosis growth (83% inhibition at 200 μg/mL). Compound A65 emerged as the most potent compound identified in our study that inhibited BioA enzymatic activity and growth of the pathogen and possessed drug-like properties.

Conclusion

Our study has identified a few hit molecules against M. tuberculosis BioA that can act as potential candidates for further development of potent anti-tubercular therapeutic agents.

Engineered polyketides: Synergy between protein and host level engineering

Metabolic engineering efforts toward rewiring metabolism of cells to produce new compounds often require the utilization of non-native enzymatic machinery that is capable of producing a broad range of chemical functionalities. Polyketides encompass one of the largest classes of chemically diverse natural products. With thousands of known polyketides, modular polyketide synthases (PKSs) share a particularly attractive biosynthetic logic for generating chemical diversity. The engineering of modular PKSs could open access to the deliberate production of both existing and novel compounds. In this review, we discuss PKS engineering efforts applied at both the protein and cellular level for the generation of a diverse range of chemical structures, and we examine future applications of PKSs in the production of medicines, fuels and other industrially relevant chemicals.

Striking Diversity in Holoenzyme Architecture and Extensive Conformational Variability in Biotin-Dependent Carboxylases

Biotin-dependent carboxylases are widely distributed in nature and have central roles in the metabolism of fatty acids, amino acids, carbohydrates, and other compounds. The last decade has seen the accumulation of structural information on most of these large holoenzymes, including the 500-kDa dimeric yeast acetyl-CoA carboxylase, the 750-kDa α₆β₆ dodecameric bacterial propionyl-CoA carboxylase, 3-methylcrotonyl-CoA carboxylase, and geranyl-CoA carboxylase, the 720-kDa hexameric bacterial long-chain acyl-CoA carboxylase, the 500-kDa tetrameric bacterial single-chain pyruvate carboxylase, the 370-kDa α₂β₄ bacterial two-subunit pyruvate carboxylase, and the 130-kDa monomeric eukaryotic urea carboxylase. A common theme that has emerged from these studies is the dramatic structural flexibility of these holoenzymes despite their strong overall sequence conservation, evidenced both by the extensive diversity in the architectures of the holoenzymes and by the extensive conformational variability of their domains and subunits. This structural flexibility is crucial for the function and regulation of these enzymes and identifying compounds that can interfere with it represents an attractive approach for developing novel modulators and drugs. The extensive diversity observed in the structures so far and its biochemical and functional implications will be the focus of this review.

A crotonyl-CoA reductase-carboxylase independent pathway for assembly of unusual alkylmalonyl-CoA polyketide synthase extender units

Type I modular polyketide synthases assemble diverse bioactive natural products. Such multienzymes typically use malonyl and methylmalonyl-CoA building blocks for polyketide chain assembly. However, in several cases more exotic alkylmalonyl-CoA extender units are also known to be incorporated. In all examples studied to date, such unusual extender units are biosynthesized via reductive carboxylation of α, β-unsaturated thioesters catalysed by crotonyl-CoA reductase/carboxylase (CCRC) homologues. Here we show using a chemically-synthesized deuterium-labelled mechanistic probe, and heterologous gene expression experiments that the unusual alkylmalonyl-CoA extender units incorporated into the stambomycin family of polyketide antibiotics are assembled by direct carboxylation of medium chain acyl-CoA thioesters. X-ray crystal structures of the unusual β-subunit of the acyl-CoA carboxylase (YCC) responsible for this reaction, alone and in complex with hexanoyl-CoA, reveal the molecular basis for substrate recognition, inspiring the development of methodology for polyketide bio-orthogonal tagging via incorporation of 6-azidohexanoic acid and 8-nonynoic acid into novel stambomycin analogues.

Polyketides are typically assembled from a starter unit and malonyl- and/or methylmalonyl-CoA-derived extender units, but the macrolide antibiotics stambomycins incorporate non-standard alkylmalonyl-CoA extender units. Here, the authors describe the biosynthetic pathway responsible for this unusual synthesis.

Role of Wax Ester Synthase/Acyl Coenzyme A:Diacylglycerol Acyltransferase in Oleaginous Streptomyces sp. Strain G25

Recently, we isolated a novel Streptomyces strain which can accumulate extraordinarily large amounts of triacylglycerol (TAG) and consists of 64% fatty acids (dry weight) when cultivated with glucose and 50% fatty acids (dry weight) when cultivated with cellobiose. To identify putative gene products responsible for lipid storage and cellobiose utilization, we analyzed its draft genome sequence. A single gene encoding a wax ester synthase/acyl coenzyme A (CoA):diacylglycerol acyltransferase (WS/DGAT) was identified and heterologously expressed in Escherichia coli The purified enzyme AtfG25 showed acyltransferase activity with C12- or C16-acyl-CoA, C12 to C18 alcohols, or dipalmitoyl glycerol. This acyltransferase exhibits 24% amino acid identity to the model enzyme AtfA from Acinetobacter baylyi but has high sequence similarities to WS/DGATs from other Streptomyces species. To investigate the impact of AtfG25 on lipid accumulation, the respective gene, atfG25, was inactivated in Streptomyces sp. strain G25. However, cells of the insertion mutant still exhibited DGAT activity and were able to store TAG, albeit in lower quantities and at lower rates than the wild-type strain. These findings clearly indicate that AtfG25 has an important, but not exclusive, role in TAG biosynthesis in the novel Streptomyces isolate and suggest the presence of alternative metabolic pathways for lipid accumulation which are discussed in the present study.

IMPORTANCE

A novel Streptomyces strain was isolated from desert soil, which represents an extreme environment with high temperatures, frequent drought, and nutrient scarcity. We believe that these harsh conditions promoted the development of the capacity for this strain to accumulate extraordinarily large amounts of lipids. In this study, we present the analysis of its draft genome sequence with a special focus on enzymes potentially involved in its lipid storage. Furthermore, the activity and importance of the detected acyltransferase were studied. As discussed in this paper, and in contrast to many other bacteria, streptomycetes seem to possess a complex metabolic network to synthesize lipids, whereof crucial steps are still largely unknown. This paper therefore provides insights into a range of topics, including extremophile bacteria, the physiology of lipid accumulation, and the biotechnological production of bacterial lipids.

The Role of Biotin in Bacterial Physiology and Virulence: a Novel Antibiotic Target for Mycobacterium tuberculosis

Biotin is an essential cofactor for enzymes present in key metabolic pathways such as fatty acid biosynthesis, replenishment of the tricarboxylic acid cycle, and amino acid metabolism. Biotin is synthesized de novo in microorganisms, plants, and fungi, but this metabolic activity is absent in mammals, making biotin biosynthesis an attractive target for antibiotic discovery. In particular, biotin biosynthesis plays important metabolic roles as the sole source of biotin in all stages of the Mycobacterium tuberculosis life cycle due to the lack of a transporter for scavenging exogenous biotin. Biotin is intimately associated with lipid synthesis where the products form key components of the mycobacterial cell membrane that are critical for bacterial survival and pathogenesis. In this review we discuss the central role of biotin in bacterial physiology and highlight studies that demonstrate the importance of its biosynthesis for virulence. The structural biology of the known biotin synthetic enzymes is described alongside studies using structure-guided design, phenotypic screening, and fragment-based approaches to drug discovery as routes to new antituberculosis agents.

RAP-PCR fingerprinting reveals time-dependent expression of development-related genes following differentiation process of Bacillus thuringiensis

Gene expression profiles are important data to reveal the functions of genes putatively involved in crucial biological processes. RNA arbitrarily primed polymerase chain reaction (RAP-PCR) and specifically primed reverse transcription polymerase chain reaction (RT-PCR) were combined to screen differentially expressed genes following development of a commercial Bacillus thuringiensis subsp. kurstaki strain 8010 (serotype 3a3b). Six differentially expressed transcripts (RAP1 to RAP6) were obtained. RAP1 encoded a putative triple helix repeat-containing collagen or an exosporium protein H related to spore pathogenicity. RAP2 was homologous to a ClpX protease and an ATP-dependent protease La (LonB), which likely acted as virulence factors. RAP3 was homologous to a beta subunit of propionyl-CoA carboxylase required for the development of Myxococcus xanthus. RAP4 had homology to a quinone oxidoreductase involved in electron transport and ATP formation. RAP5 showed significant homology to a uridine kinase that mediates phosphorylation of uridine and azauridine. RAP6 shared high sequence identity with 3-methyl-2-oxobutanoate-hydroxymethyltransferase (also known as ketopantoate hydroxymethyltransferase or PanB) involved in the operation of the tricarboxylic acid cycle. The findings described here would help to elucidate the molecular mechanisms underlying the differentiation process of B. thuringiensis and unravel novel pathogenic genes.

Structure, activity, and inhibition of the Carboxyltransferase β-subunit of acetyl coenzyme A carboxylase (AccD6) from Mycobacterium tuberculosis

In Mycobacterium tuberculosis, the carboxylation of acetyl coenzyme A (acetyl-CoA) to produce malonyl-CoA, a building block in long-chain fatty acid biosynthesis, is catalyzed by two enzymes working sequentially: a biotin carboxylase (AccA) and a carboxyltransferase (AccD). While the exact roles of the three different biotin carboxylases (AccA1 to -3) and the six carboxyltransferases (AccD1 to -6) in M. tuberculosis are still not clear, AccD6 in complex with AccA3 can synthesize malonyl-CoA from acetyl-CoA. A series of 10 herbicides that target plant acetyl-CoA carboxylases (ACC) were tested for inhibition of AccD6 and for whole-cell activity against M. tuberculosis. From the tested herbicides, haloxyfop, an arylophenoxypropionate, showed in vitro inhibition of M. tuberculosis AccD6, with a 50% inhibitory concentration (IC50) of 21.4 ± 1 μM. Here, we report the crystal structures of M. tuberculosis AccD6 in the apo form (3.0 Å) and in complex with haloxyfop-R (2.3 Å). The structure of M. tuberculosis AccD6 in complex with haloxyfop-R shows two molecules of the inhibitor bound on each AccD6 subunit. These results indicate the potential for developing novel therapeutics for tuberculosis based on herbicides with low human toxicity.

Structure and function of biotin-dependent carboxylases

Biotin-dependent carboxylases include acetyl-CoA carboxylase (ACC), propionyl-CoA carboxylase (PCC), 3-methylcrotonyl-CoA carboxylase (MCC), geranyl-CoA carboxylase, pyruvate carboxylase (PC), and urea carboxylase (UC). They contain biotin carboxylase (BC), carboxyltransferase (CT), and biotin-carboxyl carrier protein components. These enzymes are widely distributed in nature and have important functions in fatty acid metabolism, amino acid metabolism, carbohydrate metabolism, polyketide biosynthesis, urea utilization, and other cellular processes. ACCs are also attractive targets for drug discovery against type 2 diabetes, obesity, cancer, microbial infections, and other diseases, and the plastid ACC of grasses is the target of action of three classes of commercial herbicides. Deficiencies in the activities of PCC, MCC, or PC are linked to serious diseases in humans. Our understanding of these enzymes has been greatly enhanced over the past few years by the crystal structures of the holoenzymes of PCC, MCC, PC, and UC. The structures reveal unanticipated features in the architectures of the holoenzymes, including the presence of previously unrecognized domains, and provide a molecular basis for understanding their catalytic mechanism as well as the large collection of disease-causing mutations in PCC, MCC, and PC. This review will summarize the recent advances in our knowledge on the structure and function of these important metabolic enzymes.

Resistance to spiromesifen in Trialeurodes vaporariorum is associated with a single amino acid replacement in its target enzyme acetyl-coenzyme A carboxylase

Spiromesifen is a novel insecticide and is classed as a tetronic acid derivative. It targets the insects' acetyl-coenzyme A carboxylase (ACCase) enzyme, causing a reduction in lipid biosynthesis. At the time of this publication, there are no reports of resistance to this class of insecticides in insects although resistance has been observed in several mite species. The greenhouse whitefly Trialeurodes vaporariorum (Westwood) is a serious pest of protected vegetable and ornamental crops in temperate regions of the world and spiromesifen is widely used in its control. Mortality rates of UK and European populations of T. vaporariorum to spiromesifen were calculated and up to 26-fold resistance was found. We therefore sought to examine the molecular mechanism underlying spiromesifen resistance in this important pest. Pre-treatment with piperonyl butoxide did not synergize spiromesifen, suggesting a target-site resistance mechanism. The full length ACCase gene was sequenced for a range of T. vaporariorum strains and a strong association was found between spiromesifen resistance and a glutamic acid substitution with lysine in position 645 (E645K) of this gene. A TaqMan allelic discrimination assay confirmed these findings. Although this resistance is not considered sufficient to compromise the field performance of spiromesifen, this association of E645K with resistance is the first report of a potential target site mechanism affecting an ACCase inhibitor in an arthropod species.

Biotin biosynthesis in Mycobacterium tuberculosis: physiology, biochemistry and molecular intervention

Biotin is an important micronutrient that serves as an essential enzyme cofactor. Bacteria obtain biotin either through de novo synthesis or by active uptake from exogenous sources. Mycobacteria are unusual amongst bacteria in that their primary source of biotin is through de novo synthesis. Here we review the importance of biotin biosynthesis in the lifecycle of Mycobacteria. Genetic screens designed to identify key metabolic processes have highlighted a role for the biotin biosynthesis in bacilli growth, infection and survival during the latency phase. These studies help to establish the biotin biosynthetic pathway as a potential drug target for new anti-tuberculosis agents.

Structure, function and selective inhibition of bacterial acetyl-coa carboxylase

Acetyl-CoA carboxylase (ACC) catalyses the first committed step in fatty acid biosynthesis: a metabolic pathway required for several important biological processes including the synthesis and maintenance of cellular membranes. ACC employs a covalently attached biotin moiety to bind a carboxyl anion and then transfer it to acetyl-CoA, yielding malonyl-CoA. These activities occur at two different subsites: the biotin carboxylase (BC) and carboxyltransferase (CT). Structural biology, together with small molecule inhibitor studies, has provided new insights into the molecular mechanisms that govern ACC catalysis, specifically the BC and CT subunits. Here, we review these recent findings and highlight key differences between the bacterial and eukaryotic isozymes with a view to establish those features that provide an opportunity for selective inhibition. Especially important are examples of highly selective small molecule inhibitors capable of differentiating between ACCs from different phyla. The implications for early stage antibiotic discovery projects, stemming from these studies, are discussed.

Early evolution of the biotin-dependent carboxylase family

Background

Biotin-dependent carboxylases are a diverse family of carboxylating enzymes widespread in the three domains of life, and thus thought to be very ancient. This family includes enzymes that carboxylate acetyl-CoA, propionyl-CoA, methylcrotonyl-CoA, geranyl-CoA, acyl-CoA, pyruvate and urea. They share a common catalytic mechanism involving a biotin carboxylase domain, which fixes a CO₂ molecule on a biotin carboxyl carrier peptide, and a carboxyl transferase domain, which transfers the CO₂ moiety to the specific substrate of each enzyme. Despite this overall similarity, biotin-dependent carboxylases from the three domains of life carrying their reaction on different substrates adopt very diverse protein domain arrangements. This has made difficult the resolution of their evolutionary history up to now.

Results

Taking advantage of the availability of a large amount of genomic data, we have carried out phylogenomic analyses to get new insights on the ancient evolution of the biotin-dependent carboxylases. This allowed us to infer the set of enzymes present in the last common ancestor of each domain of life and in the last common ancestor of all living organisms (the cenancestor). Our results suggest that the last common archaeal ancestor had two biotin-dependent carboxylases, whereas the last common bacterial ancestor had three. One of these biotin-dependent carboxylases ancestral to Bacteria most likely belonged to a large family, the CoA-bearing-substrate carboxylases, that we define here according to protein domain composition and phylogenetic analysis. Eukaryotes most likely acquired their biotin-dependent carboxylases through the mitochondrial and plastid endosymbioses as well as from other unknown bacterial donors. Finally, phylogenetic analyses support previous suggestions about the existence of an ancient bifunctional biotin-protein ligase bound to a regulatory transcription factor.

Conclusions

The most parsimonious scenario for the early evolution of the biotin-dependent carboxylases, supported by the study of protein domain composition and phylogenomic analyses, entails that the cenancestor possessed two different carboxylases able to carry out the specific carboxylation of pyruvate and the non-specific carboxylation of several CoA-bearing substrates, respectively. These enzymes may have been able to participate in very diverse metabolic pathways in the cenancestor, such as in ancestral versions of fatty acid biosynthesis, anaplerosis, gluconeogenesis and the autotrophic fixation of CO₂.

Fatty acid biosynthesis in actinomycetes

All organisms that produce fatty acids do so via a repeated cycle of reactions. In mammals and other animals, these reactions are catalyzed by a type I fatty acid synthase (FAS), a large multifunctional protein to which the growing chain is covalently attached. In contrast, most bacteria (and plants) contain a type II system in which each reaction is catalyzed by a discrete protein. The pathway of fatty acid biosynthesis in Escherichia coli is well established and has provided a foundation for elucidating the type II FAS pathways in other bacteria (White et al., 2005). However, fatty acid biosynthesis is more diverse in the phylum Actinobacteria: Mycobacterium, possess both FAS systems while Streptomyces species have only the multienzyme FAS II system and Corynebacterium species exclusively FAS I. In this review, we present an overview of the genome organization, biochemical properties and physiological relevance of the two FAS systems in the three genera of actinomycetes mentioned above. We also address in detail the biochemical and structural properties of the acyl-CoA carboxylases (ACCases) that catalyzes the first committed step of fatty acid synthesis in actinomycetes, and discuss the molecular bases of their substrate specificity and the structure-based identification of new ACCase inhibitors with antimycobacterial properties.