Brucella BioR regulator defines a complex regulatory mechanism for bacterial biotin metabolism

Advances and prospects in microbial production of biotin

Biotin, serving as a coenzyme in carboxylation reactions, is a vital nutrient crucial for the natural growth, development, and overall well-being of both humans and animals. Consequently, biotin is widely utilized in various industries, including feed, food, and pharmaceuticals. Despite its potential advantages, the chemical synthesis of biotin for commercial production encounters environmental and safety challenges. The burgeoning field of synthetic biology now allows for the creation of microbial cell factories producing bio-based products, offering a cost-effective alternative to chemical synthesis for biotin production. This review outlines the pathway and regulatory mechanism involved in biotin biosynthesis. Then, the strategies to enhance biotin production through both traditional chemical mutagenesis and advanced metabolic engineering are discussed. Finally, the article explores the limitations and future prospects of microbial biotin production. This comprehensive review not only discusses strategies for biotin enhancement but also provides in-depth insights into systematic metabolic engineering approaches aimed at boosting biotin production.

Advances in biotin biosynthesis and biotechnological production in microorganisms

Biotin, also known as vitamin H or B7, acts as a crucial cofactor in the central metabolism processes of fatty acids, amino acids, and carbohydrates. Biotin has important applications in food additives, biomedicine, and other fields. While the ability to synthesize biotin de novo is confined to microorganisms and plants, humans and animals require substantial daily intake, primarily through dietary sources and intestinal microflora. Currently, chemical synthesis stands as the primary method for commercial biotin production, although microbial biotin production offers an environmentally sustainable alternative with promising prospects. This review presents a comprehensive overview of the pathways involved in de novo biotin synthesis in various species of microbes and insights into its regulatory and transport systems. Furthermore, diverse strategies are discussed to improve the biotin production here, including mutation breeding, rational metabolic engineering design, artificial genetic modification, and process optimization. The review also presents the potential strategies for addressing current challenges for industrial-scale bioproduction of biotin in the future. This review is very helpful for exploring efficient and sustainable strategies for large-scale biotin production.

Three enigmatic BioH isoenzymes are programmed in the early stage of mycobacterial biotin synthesis, an attractive anti-TB drug target

Tuberculosis (TB) is one of the leading infectious diseases of global concern, and one quarter of the world’s population are TB carriers. Biotin metabolism appears to be an attractive anti-TB drug target. However, the first-stage of mycobacterial biotin synthesis is fragmentarily understood. Here we report that three evolutionarily-distinct BioH isoenzymes (BioH1 to BioH3) are programmed in biotin synthesis of Mycobacterium smegmatis. Expression of an individual bioH isoform is sufficient to allow the growth of an Escherichia coli ΔbioH mutant on the non-permissive condition lacking biotin. The enzymatic activity in vitro combined with biotin bioassay in vivo reveals that BioH2 and BioH3 are capable of removing methyl moiety from pimeloyl-ACP methyl ester to give pimeloyl-ACP, a cognate precursor for biotin synthesis. In particular, we determine the crystal structure of dimeric BioH3 at 2.27Å, featuring a unique lid domain. Apart from its catalytic triad, we also dissect the substrate recognition of BioH3 by pimeloyl-ACP methyl ester. The removal of triple bioH isoforms (ΔbioH1/2/3) renders M. smegmatis biotin auxotrophic. Along with the newly-identified Tam/BioC, the discovery of three unusual BioH isoforms defines an atypical ‘BioC-BioH(3)’ paradigm for the first-stage of mycobacterial biotin synthesis. This study solves a long-standing puzzle in mycobacterial nutritional immunity, providing an alternative anti-TB drug target.

XRE-Type Regulator BioX Acts as a Negative Transcriptional Factor of Biotin Metabolism in Riemerella anatipestifer

Biotin is essential for the growth and pathogenicity of microorganisms. Damage to biotin biosynthesis results in impaired bacterial growth and decreased virulence in vivo. However, the mechanisms of biotin biosynthesis in Riemerella anatipestifer remain unclear. In this study, two R. anatipestifer genes associated with biotin biosynthesis were identified. AS87_RS05840 encoded a BirA protein lacking the N-terminal winged helix-turn-helix DNA binding domain, identifying it as a group I biotin protein ligase, and AS87_RS09325 encoded a BioX protein, which was in the helix-turn-helix xenobiotic response element family of transcription factors. Electrophoretic mobility shift assays demonstrated that BioX bound to the promoter region of bioF. In addition, the R. anatipestifer genes bioF (encoding 7-keto-8-aminopelargonic acid synthase), bioD (encoding dethiobiotin synthase), and bioA (encoding 7,8-diaminopelargonic acid synthase) were in an operon and were regulated by BioX. Quantitative reverse transcription-PCR showed that transcription of the bioFDA operon increased in the mutant Yb2ΔbioX in the presence of excessive biotin, compared with that in the wild-type strain Yb2, suggesting that BioX acted as a repressor of biotin biosynthesis. Streptavidin blot analysis showed that BirA caused biotinylation of BioX, indicating that biotinylated BioX was involved in metabolic pathways. Moreover, as determined by the median lethal dose, the virulence of Yb2ΔbioX was attenuated 500-fold compared with that of Yb2. To summarize, the genes birA and bioX were identified in R. anatipestifer, and BioX was found to act as a repressor of the bioFDA operon involved in the biotin biosynthesis pathway and identified as a bacterial virulence factor. IMPORTANCE Riemerella anatipestifer is a causative agent of diseases in ducks, geese, turkeys, and various other domestic and wild birds. Our study reveals that biotin synthesis of R. anatipestifer is regulated by the BioX through binding to the promoter region of the bioF gene to inhibit transcription of the bioFDA operon. Moreover, bioX is required for R. anatipestifer pathogenicity, suggesting that BioX is a potential target for treatment of the pathogen. R. anatipestifer BioX has thus been identified as a novel negative regulator involved in biotin metabolism and associated with bacterial virulence in this study.

Peptide nucleic acid restores colistin susceptibility through modulation of MCR-1 expression in Escherichia coli

BACKGROUND

Plasmid-mediated mechanisms of drug resistance accelerate the spread of polymyxin resistance, leaving clinicians with few or no antibacterial options for the treatment of infections caused by MDR bacteria, especially carbapenemase-producing strains.

OBJECTIVES

To evaluate the associations among promoter sequence variation, mcr-1 expression, host factors and levels of colistin resistance and to propose antisense agents such as peptide nucleic acids (PNAs) targeting mcr-1 as a tool to restore colistin susceptibility through modulation of MCR-1 expression in Escherichia coli.

METHODS

A β-galactosidase assay was performed to study mcr-1 promoter activity. Quantitative real-time PCR and western blot assays were used to identify the expression level of MCR-1 in WT strains and transformants. Three PNAs targeting different regions of mcr-1 were designed and synthesized to determine whether they can effectively inhibit MCR-1 expression. MIC was measured to test colistin susceptibility in the presence or absence of PNA-1 in mcr-1-carrying E. coli.

RESULTS

Variation in the mcr-1 promoter sequence and host species affect promoter activity, MCR-1 expression levels and colistin MICs. One PNA targeting the ribosome-binding site fully inhibited the expression of mcr-1 at a concentration of 4 μM, resulting in significantly increased susceptibility to colistin. The MIC90 of colistin decreased from 8 to 2 mg/L (P < 0.05) in the presence of 4 μM PNA.

CONCLUSIONS

These findings suggest that the antisense approach is a possible strategy to combat mcr-1-mediated resistance as well as other causes of emerging global resistance.

Biochemical and structural characterization of the BioZ enzyme engaged in bacterial biotin synthesis pathway

Biotin is an essential micro-nutrient across the three domains of life. The paradigm earlier step of biotin synthesis denotes "BioC-BioH" pathway in Escherichia coli. Here we report that BioZ bypasses the canonical route to begin biotin synthesis. In addition to its origin of Rhizobiales, protein phylogeny infers that BioZ is domesticated to gain an atypical role of β-ketoacyl-ACP synthase III. Genetic and biochemical characterization demonstrates that BioZ catalyzes the condensation of glutaryl-CoA (or ACP) with malonyl-ACP to give 5'-keto-pimeloyl ACP. This intermediate proceeds via type II fatty acid synthesis (FAS II) pathway, to initiate the formation of pimeloyl-ACP, a precursor of biotin synthesis. To further explore molecular basis of BioZ activity, we determine the crystal structure of Agrobacterium tumefaciens BioZ at 1.99 Å, of which the catalytic triad and the substrate-loading tunnel are functionally defined. In particular, we localize that three residues (S84, R147, and S287) at the distant bottom of the tunnel might neutralize the charge of free C-carboxyl group of the primer glutaryl-CoA. Taken together, this study provides molecular insights into the BioZ biotin synthesis pathway.

Crystal structure and acetylation of BioQ suggests a novel regulatory switch for biotin biosynthesis in Mycobacterium smegmatis

Biotin (vitamin B7), a sulfur-containing fatty acid derivative, is a nutritional virulence factor in certain mycobacterial species. Tight regulation of biotin biosynthesis is important because production of biotin is an energetically expensive process requiring 15-20 equivalents of ATP. The Escherichia coli bifunctional BirA is a prototypical biotin regulatory system. In contrast, mycobacterial BirA is an unusual biotin protein ligase without DNA-binding domain. Recently, we established a novel two-protein paradigm of BioQ-BirA. However, structural and molecular mechanism for BioQ is poorly understood. Here, we report crystal structure of the M. smegmatis BioQ at 1.9 Å resolution. Structure-guided functional mapping defined a seven residues-requiring motif for DNA-binding activity. Western blot and MALDI-TOF MS allowed us to unexpectedly discover that the K47 acetylation activates crosstalking of BioQ to its cognate DNA. More intriguingly, excess of biotin augments the acetylation status of BioQ in M. smegmatis. It seems likely that BioQ acetylation proceeds via a non-enzymatic mechanism. Mutation of this acetylation site K47 in BioQ significantly impairs its regulatory role in vivo. This explains in part (if not all) why BioQ has no detectable requirement of the presumable bio-5'-AMP effecter, which is a well-known ligand for the paradigm E. coli BirA regulator system. Unlike the scenario seen with E. coli carrying a single biotinylated protein, AccB, genome-wide search and Streptavidin blot revealed that no less than seven proteins require the rare post-translational modification, biotinylation in M. smegmatis, validating its physiological demand for biotin at relatively high level. Taken together, our finding defines a novel biotin regulatory machinery by BioQ, posing a possibility that development of new antibiotics targets biotin, the limited nutritional virulence factor in certain pathogenic mycobacterial species.

Mechanistic insights into transferable polymyxin resistance among gut bacteria

Polymyxins such as colistin are antibiotics used as a final line of defense in the management of infections with multidrug-resistant Gram-negative bacteria. Although natural resistance to polymyxins is rare, the discovery of a mobilized colistin resistance gene (mcr-1) in gut bacteria has raised significant concern. As an intramembrane enzyme, MCR-1 catalyzes the transfer of phosphoethanolamine (PEA) to the 1 (or 4')-phosphate group of the lipid A moiety of lipopolysaccharide, thereby conferring colistin resistance. However, the structural and biochemical mechanisms used by this integral membrane enzyme remain poorly understood. Here, we report the modeled structure of the full-length MCR-1 membrane protein. Together with molecular docking, our structural and functional dissection of the complex of MCR-1 with its phosphatidylethanolamine (PE) substrate suggested the presence of a 12 residue-containing cavity for substrate entry, which is critical for both enzymatic activity and its resultant phenotypic resistance to colistin. More importantly, two periplasm-facing helices (PH2 and PH2') of the trans-membrane domain were essential for MCR-1 activity. MALDI-TOF MS and thin-layer chromatography assays provide both in vivo and in vitro evidence that MCR-1 catalyzes the transfer of PEA from the PE donor substrate to its recipient substrate lipid A. Also, the chemical modification of lipid A species was detected in clinical species of bacteria carrying mcr-1 Our results provide mechanistic insights into transferable MCR-1 polymyxin resistance, raising the prospect of rational design of small molecules that reverse bacterial polymyxin resistance, as a last-resort clinical option to combat pathogens with carbapenem resistance.

Structural and Functional Characterization of the FadR Regulatory Protein from Vibrio alginolyticus

The structure of Vibrio cholerae FadR (VcFadR) complexed with the ligand oleoyl-CoA suggests an additional ligand-binding site. However, the fatty acid metabolism and its regulation is poorly addressed in Vibrio alginolyticus, a species closely-related to V. cholerae. Here, we show crystal structures of V. alginolyticus FadR (ValFadR) alone and its complex with the palmitoyl-CoA, a long-chain fatty acyl ligand different from the oleoyl-CoA occupied by VcFadR. Structural comparison indicates that both VcFadR and ValFadR consistently have an additional ligand-binding site (called site 2), which leads to more dramatic conformational-change of DNA-binding domain than that of the E. coli FadR (EcFadR). Isothermal titration calorimetry (ITC) analyses defines that the ligand-binding pattern of ValFadR (2:1) is distinct from that of EcFadR (1:1). Together with surface plasmon resonance (SPR), electrophoresis mobility shift assay (EMSA) demonstrates that ValFadR binds fabA, an important gene of unsaturated fatty acid (UFA) synthesis. The removal of fadR from V. cholerae attenuates fabA transcription and results in the unbalance of UFA/SFA incorporated into membrane phospholipids. Genetic complementation of the mutant version of fadR (Δ42, 136-177) lacking site 2 cannot restore the defective phenotypes of ΔfadR while the wild-type fadR gene and addition of exogenous oleate can restore them. Mice experiments reveals that VcFadR and its site 2 have roles in bacterial colonizing. Together, the results might represent an additional example that illustrates the Vibrio FadR-mediated lipid regulation and its role in pathogenesis.

Control of biotin biosynthesis in mycobacteria by a pyruvate carboxylase dependent metabolic signal

Biotin is an essential cofactor utilized by all domains of life, but only synthesized by bacteria, fungi and plants, making biotin biosynthesis a target for antimicrobial development. To understand biotin biosynthesis in mycobacteria, we executed a genetic screen in Mycobacterium smegmatis for biotin auxotrophs and identified pyruvate carboxylase (Pyc) as required for biotin biosynthesis. The biotin auxotrophy of the pyc::tn strain is due to failure to transcriptionally induce late stage biotin biosynthetic genes in low biotin conditions. Loss of bioQ, the repressor of biotin biosynthesis, in the pyc::tn strain reverted biotin auxotrophy, as did reconstituting the last step of the pathway through heterologous expression of BioB and provision of its substrate DTB. The role of Pyc in biotin regulation required its catalytic activities and could be supported by M. tuberculosis Pyc. Quantitation of the kinetics of depletion of biotinylated proteins after biotin withdrawal revealed that Pyc is the most rapidly depleted biotinylated protein and metabolomics revealed a broad metabolic shift in wild type cells upon biotin withdrawal which was blunted in cell lacking Pyc. Our data indicate that mycobacterial cells monitor biotin sufficiency through a metabolic signal generated by dysfunction of a biotinylated protein of central metabolism.

Deciphering MCR-2 Colistin Resistance

Antibiotic resistance is a prevalent problem in public health worldwide. In general, the carbapenem β-lactam antibiotics are considered a final resort against lethal infections by multidrug-resistant bacteria. Colistin is a cationic polypeptide antibiotic and acts as the last line of defense for treatment of carbapenem-resistant bacteria. Very recently, a new plasmid-borne colistin resistance gene, mcr-2, was revealed soon after the discovery of the paradigm gene mcr-1, which has disseminated globally. However, the molecular mechanisms for MCR-2 colistin resistance are poorly understood. Here we show a unique transposon unit that facilitates the acquisition and transfer of mcr-2 Evolutionary analyses suggested that both MCR-2 and MCR-1 might be traced to their cousin phosphoethanolamine (PEA) lipid A transferase from a known polymyxin producer, Paenibacillus Transcriptional analyses showed that the level of mcr-2 transcripts is relatively higher than that of mcr-1 Genetic deletions revealed that the transmembrane regions (TM1 and TM2) of both MCR-1 and MCR-2 are critical for their location and function in bacterial periplasm, and domain swapping indicated that the TM2 is more efficient than TM1. Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) confirmed that all four MCR proteins (MCR-1, MCR-2, and two chimeric versions [TM1-MCR-2 and TM2-MCR-1]) can catalyze chemical modification of lipid A moiety anchored on lipopolysaccharide (LPS) with the addition of phosphoethanolamine to the phosphate group at the 4' position of the sugar. Structure-guided site-directed mutagenesis defined an essential 6-residue-requiring zinc-binding/catalytic motif for MCR-2 colistin resistance. The results further our mechanistic understanding of transferable colistin resistance, providing clues to improve clinical therapeutics targeting severe infections by MCR-2-containing pathogens.IMPORTANCE Carbapenem and colistin are the last line of refuge in fighting multidrug-resistant Gram-negative pathogens. MCR-2 is a newly emerging variant of the mobilized colistin resistance protein MCR-1, posing a potential challenge to public health. Here we report transfer of the mcr-2 gene by a unique transposal event and its possible origin. Distribution of MCR-2 in bacterial periplasm is proposed to be a prerequisite for its role in the context of biochemistry and the colistin resistance. We also define the genetic requirement of a zinc-binding/catalytic motif for MCR-2 colistin resistance. This represents a glimpse of transferable colistin resistance by MCR-2.

Comparative genomics and evolution of transcriptional regulons in Proteobacteria

Comparative genomics approaches are broadly used for analysis of transcriptional regulation in bacterial genomes. In this work, we identified binding sites and reconstructed regulons for 33 orthologous groups of transcription factors (TFs) in 196 reference genomes from 21 taxonomic groups of Proteobacteria. Overall, we predict over 10 600 TF binding sites and identified more than 15 600 target genes for 1896 TFs constituting the studied orthologous groups of regulators. These include a set of orthologues for 21 metabolism-associated TFs from Escherichia coli and/or Shewanella that are conserved in five or more taxonomic groups and several additional TFs that represent non-orthologous substitutions of the metabolic regulators in some lineages of Proteobacteria. By comparing gene contents of the reconstructed regulons, we identified the core, taxonomy-specific and genome-specific TF regulon members and classified them by their metabolic functions. Detailed analysis of ArgR, TyrR, TrpR, HutC, HypR and other amino-acid-specific regulons demonstrated remarkable differences in regulatory strategies used by various lineages of Proteobacteria. The obtained genomic collection of in silico reconstructed TF regulons contains a large number of new regulatory interactions that await future experimental validation. The collection provides a framework for future evolutionary studies of transcriptional regulatory networks in Bacteria. It can be also used for functional annotation of putative metabolic transporters and enzymes that are abundant in the reconstructed regulons.

Functional definition of BirA suggests a biotin utilization pathway in the zoonotic pathogen Streptococcus suis

Biotin protein ligase is universal in three domains of life. The paradigm version of BPL is the Escherichia coli BirA that is also a repressor for the biotin biosynthesis pathway. Streptococcus suis, a leading bacterial agent for swine diseases, seems to be an increasingly-important opportunistic human pathogen. Unlike the scenario in E. coli, S. suis lacks the de novo biotin biosynthesis pathway. In contrast, it retains a bioY, a biotin transporter-encoding gene, indicating an alternative survival strategy for S. suis to scavenge biotin from its inhabiting niche. Here we report functional definition of S. suis birA homologue. The in vivo functions of the birA paralogue with only 23.6% identity to the counterpart of E. coli, was judged by its ability to complement the conditional lethal mutants of E. coli birA. The recombinant BirA protein of S. suis was overexpressed in E. coli, purified to homogeneity and verified with MS. Both cellulose TLC and MALDI-TOFF-MS assays demonstrated that the S. suis BirA protein catalyzed the biotinylation reaction of its acceptor biotin carboxyl carrier protein. EMSA assays confirmed binding of the bioY gene to the S. suis BirA. The data defined the first example of the bifunctional BirA ligase/repressor in Streptococcus.

Deciphering a unique biotin scavenging pathway with redundant genes in the probiotic bacterium Lactococcus lactis

Biotin protein ligase (BPL) is widespread in the three domains of the life. The paradigm BPL is the Escherichia coli BirA protein, which also functions as a repressor for the biotin biosynthesis pathway. Here we report that Lactococcus lactis possesses two different orthologues of birA (birA1_{_LL} and birA2_{_LL}). Unlike the scenario in E. coli, L. lactis appears to be auxotrophic for biotin in that it lacks a full biotin biosynthesis pathway. In contrast, it retains two biotin transporter-encoding genes (bioY1_{_LL} and bioY2_{_LL}), suggesting the use of a scavenging strategy to obtain biotin from the environment. The in vivo function of the two L. lactis birA genes was judged by their abilities to complement the conditional lethal E. coli birA mutant. Thin-layer chromatography and mass spectroscopy assays demonstrated that these two recombinant BirA proteins catalyze the biotinylation reaction of the acceptor biotin carboxyl carrier protein (BCCP), through the expected biotinoyl-AMP intermediate. Gel shift assays were used to characterize bioY1_{_LL} and BirA1_{_LL}. We also determined the ability to uptake ³H-biotin by L. lactis. Taken together, our results deciphered a unique biotin scavenging pathway with redundant genes present in the probiotic bacterium L. lactis.

Two novel regulators of N-acetyl-galactosamine utilization pathway and distinct roles in bacterial infections

Bacterial pathogens can exploit metabolic pathways to facilitate their successful infection cycles, but little is known about roles of d‐galactosamine (GalN)/N‐acetyl‐d‐galactosamine (GalNAc) catabolism pathway in bacterial pathogenesis. Here, we report the genomic reconstruction of GalN/GalNAc utilization pathway in Streptococci and the diversified aga regulons. We delineated two new paralogous AgaR regulators for the GalN/GalNAc catabolism pathway. The electrophoretic mobility shift assays experiment demonstrated that AgaR2 (AgaR1) binds the predicted palindromes, and the combined in vivo data from reverse transcription quantitative polymerase chain reaction and RNA‐seq suggested that AgaR2 (not AgaR1) can effectively repress the transcription of the target genes. Removal of agaR2 (not agaR1) from Streptococcus suis 05ZYH33 augments significantly the abilities of both adherence to Hep‐2 cells and anti‐phagocytosis against RAW264.7 macrophage. As anticipated, the dysfunction in AgaR2‐mediated regulation of S. suis impairs its pathogenicity in experimental models of both mice and piglets. Our finding discovered two novel regulators specific for GalN/GalNAc catabolism and assigned them distinct roles into bacterial infections. To the best of our knowledge, it might represent a first paradigm that links the GalN/GalNAc catabolism pathway to bacterial pathogenesis.

Binding of Shewanella FadR to the fabA fatty acid biosynthetic gene: implications for contraction of the fad regulon

The Escherichia coli fadR protein product, a paradigm/prototypical FadR regulator, positively regulates fabA and fabB, the two critical genes for unsaturated fatty acid (UFA) biosynthesis. However the scenario in the other Ɣ-proteobacteria, such as Shewanella with the marine origin, is unusual in that Rodionov and coworkers predicted that only fabA (not fabB) has a binding site for FadR protein. It raised the possibility of fad regulon contraction. Here we report that this is the case. Sequence alignment of the FadR homologs revealed that the N-terminal DNA-binding domain exhibited remarkable similarity, whereas the ligand-accepting motif at C-terminus is relatively-less conserved. The FadR homologue of S. oneidensis (referred to FadR_she) was over-expressed and purified to homogeneity. Integrative evidence obtained by FPLC (fast protein liquid chromatography) and chemical cross-linking analyses elucidated that FadR_she protein can dimerize in solution, whose identity was determined by MALDI-TOF-MS. In vitro data from electrophoretic mobility shift assays suggested that FadR_she is almost functionally-exchangeable/equivalent to E. coli FadR (FadR_ec) in the ability of binding the E. coli fabA (and fabB) promoters. In an agreement with that of E. coli fabA, S. oneidensis fabA promoter bound both FadR_she and FadR_ec, and was disassociated specifically with the FadR regulatory protein upon the addition of long-chain acyl-CoA thioesters. To monitor in vivo effect exerted by FadR on Shewanella fabA expression, the native promoter of S. oneidensis fabA was fused to a LacZ reporter gene to engineer a chromosome fabA-lacZ transcriptional fusion in E. coli. As anticipated, the removal of fadR gene gave about 2-fold decrement of Shewanella fabA expression by β-gal activity, which is almost identical to the inhibitory level by the addition of oleate. Therefore, we concluded that fabA is contracted to be the only one member of fad regulon in the context of fatty acid synthesis in the marine bacteria Shewanella genus.

Paracoccus denitrificans possesses two BioR homologs having a role in regulation of biotin metabolism

Recently, we determined that BioR, the GntR family of transcription factor, acts as a repressor for biotin metabolism exclusively distributed in certain species of α-proteobacteria, including the zoonotic agent Brucella melitensis and the plant pathogen Agrobacterium tumefaciens. However, the scenario is unusual in Paracoccus denitrificans, another closely related member of the same phylum α-proteobacteria featuring with denitrification. Not only does it encode two BioR homologs Pden_1431 and Pden_2922 (designated as BioR1 and BioR2, respectively), but also has six predictive BioR-recognizable sites (the two bioR homolog each has one site, whereas the two bio operons (bioBFDAGC and bioYB) each contains two tandem BioR boxes). It raised the possibility that unexpected complexity is present in BioR-mediated biotin regulation. Here we report that this is the case. The identity of the purified BioR proteins (BioR1 and BioR2) was confirmed with LC-QToF-MS. Phylogenetic analyses combined with GC percentage raised a possibility that the bioR2 gene might be acquired by horizontal gene transfer. Gel shift assays revealed that the predicted BioR-binding sites are functional for the two BioR homologs, in much similarity to the scenario seen with the BioR site of A. tumefaciens bioBFDAZ. Using the A. tumefaciens reporter system carrying a plasmid-borne LacZ fusion, we revealed that the two homologs of P. denitrificans BioR are functional repressors for biotin metabolism. As anticipated, not only does the addition of exogenous biotin stimulate efficiently the expression of bioYB operon encoding biotin transport/uptake system BioY, but also inhibits the transcription of the bioBFDAGC operon resembling the de novo biotin synthetic pathway. EMSA-based screening failed to demonstrate that the biotin-related metabolite is involved in BioR-DNA interplay, which is consistent with our former observation with Brucella BioR. Our finding defined a complex regulatory network for biotin metabolism in P. denitrificans by two BioR proteins.

A new regulatory mechanism for bacterial lipoic acid synthesis

Lipoic acid, an essential enzyme cofactor, is required in three domains of life. In the past 60 years since its discovery, most of the pathway for lipoic acid synthesis and metabolism has been elucidated. However, genetic control of lipoic acid synthesis remains unclear. Here, we report integrative evidence that bacterial cAMP-dependent signaling is linked to lipoic acid synthesis in Shewanella species, the certain of unique marine-borne bacteria with special ability of metal reduction. Physiological requirement of protein lipoylation in γ-proteobacteria including Shewanella oneidensis was detected using Western blotting with rabbit anti-lipoyl protein primary antibody. The two genes (lipB and lipA) encoding lipoic acid synthesis pathway were proved to be organized into an operon lipBA in Shewanella, and the promoter was mapped. Electrophoretic mobility shift assays confirmed that the putative CRP-recognizable site (AAGTGTGATCTATCTTACATTT) binds to cAMP-CRP protein with origins of both Escherichia coli and Shewanella. The native lipBA promoter of Shewanella was fused to a LacZ reporter gene to create a chromosome lipBA-lacZ transcriptional fusion in E. coli and S. oneidensis, allowing us to directly assay its expression level by β-galactosidase activity. As anticipated, the removal of E. coli crp gene gave above fourfold increment of lipBA promoter-driven β-gal expression. The similar scenario was confirmed by both the real-time quantitative PCR and the LacZ transcriptional fusion in the crp mutant of Shewanella. Furthermore, the glucose effect on the lipBA expression of Shewanella was evaluated in the alternative microorganism E. coli. As anticipated, an addition of glucose into media effectively induces the transcriptional level of Shewanella lipBA in that the lowered cAMP level relieves the repression of lipBA by cAMP-CRP complex. Therefore, our finding might represent a first paradigm mechanism for genetic control of bacterial lipoic acid synthesis.

Mycobacterium smegmatis BioQ defines a new regulatory network for biotin metabolism

Biotin (vitamin H), the sulfur-containing enzyme cofactor, is an essential micronutrient for three domains of life. Given the fact that biotin is an energetically expensive molecule whose de novo biosynthesis demands 20 ATP equivalents each, it is reasonable that bacteria have evolved diversified mechanisms in various microorganisms to tightly control biotin metabolism. Unlike the Escherichia coli BirA, the prototypical bi-functional version of biotin protein ligase (BPL) in that it acts as a repressor for biotin biosynthesis pathway, the BirA protein of Mycobacterium smegmatis (M. smegmatis), a closely relative of the tuberculosis-causing pathogen, Mycobacterium tuberculosis, lacked the DNA-binding activity. It raised a possibility that an alternative new regulator might be present to compensate the loss of regulatory function. Here we report that this is the case. Genomic context analyses of M. smegmatis detected a newly identified BioQ homolog classified into the TetR family of transcription factor and its recognizable palindromes. The M. smegmatis BioQ protein was overexpressed and purified to homogeneity. Size-exclusion chromatography combined with chemical cross-linking studies demonstrated that the BioQ protein had a propensity to dimerize. The promoters of bioFD and bioQ/B were mapped using 5'-RACE. Electrophoretic mobility shift assays revealed that BioQ binds specifically to the promoter regions of bioFD and bioQ/B. Further DNase I foot-printing elucidated the BioQ-binding palindromes. Site-directed mutagenesis suggested the important residues critical for BioQ/DNA binding. The isogenic mutant of bioQ (ΔbioQ) was generated using the approach of homologous recombination. The in vivo data from the real-time qPCR combined with the lacZ transcriptional fusion experiments proved that removal of bioQ gave significant increment with expression of bio operons. Also, expression of bio operons were repressed by exogenous addition of biotin, and this repression seemed to depend on the presence of BioQ protein. Thereby, we believed that M. smegmatis BioQ is not only a negative auto-regulator but also a repressor for bioFD and bioB operons involved in the biotin biosynthesis pathway. Collectively, this finding defined the two-protein paradigm of BirA and BioQ, representing a new mechanism for bacterial biotin metabolism.

PdhR, the pyruvate dehydrogenase repressor, does not regulate lipoic acid synthesis

Lipoic acid is a covalently-bound enzyme cofactor required for central metabolism all three domains of life. In the last 20 years the pathway of lipoic acid synthesis and metabolism has been established in Escherichia coli. Expression of the genes of the lipoic acid biosynthesis pathway was believed to be constitutive. However, in 2010 Kaleta and coworkers (BMC Syst. Biol. 4:116) predicted a binding site for the pyruvate dehydrogenase operon repressor, PdhR (referred to lipA site 1) upstream of lipA, the gene encoding lipoic acid synthase and concluded that PdhR regulates lipA transcription. We report in vivo and in vitro evidence that lipA is not controlled by PdhR and that the putative regulatory site deduced by the prior workers is nonfunctional and physiologically irrelevant. E. coli PdhR was purified to homogeneity and used for electrophoretic mobility shift assays. The lipA site 1 of Kaleta and coworkers failed to bind PdhR. The binding detected by these workers is due to another site (lipA site 3) located far upstream of the lipA promoter. Relative to the canonical PdhR binding site lipA site 3 is a half-palindrome and as expected had only weak PdhR binding ability. Manipulation of lipA site 3 to construct a palindrome gave significantly enhanced PdhR binding affinity. The native lipA promoter and the version carrying the artificial lipA3 palindrome were transcriptionally fused to a LacZ reporter gene to directly assay lipA expression. Deletion of pdhR gave no significant change in lipA promoter-driven β-galactosidase activity with either the native or constructed palindrome upstream sequences, indicating that PdhR plays no physiological role in regulation of lipA expression.

The β-galactosidase (BgaC) of the zoonotic pathogen Streptococcus suis is a surface protein without the involvement of bacterial virulence

Streptococcal pathogens have evolved to express exoglycosidases, one of which is BgaC β-galactosidase, to deglycosidate host surface glycolconjucates with exposure of the polysaccharide receptor for bacterial adherence. The paradigm BgaC protein is the bgaC product of Streptococcus, a bacterial surface-exposed β-galactosidase. Here we report the functional definition of the BgaC homologue from an epidemic Chinese strain 05ZYH33 of the zoonotic pathogen Streptococcus suis. Bioinformatics analyses revealed that S. suis BgaC shared the conserved active sites (W240, W243 and Y454). The recombinant BgaC protein of S. suis was purified to homogeneity. Enzymatic assays confirmed its activity of β-galactosidase. Also, the hydrolysis activity was found to be region-specific and sugar-specific for the Gal β-1,3-GlcNAc moiety of oligosaccharides. Flow cytometry analyses combined with immune electron microscopy demonstrated that S. suis BgaC is an atypical surface-anchored protein in that it lacks the “LPXTG” motif for typical surface proteins. Integrative evidence from cell lines and mice-based experiments showed that an inactivation of bgaC does not significantly impair the ability of neither adherence nor anti-phagocytosis, and consequently failed to attenuate bacterial virulence, which is somewhat similar to the scenario seen with S. pneumoniae. Therefore we concluded that S. suis BgaC is an atypical surface-exposed protein without the involvement of bacterial virulence.

A Francisella virulence factor catalyses an essential reaction of biotin synthesis

We recently identified a gene (FTN_0818) required for Francisella virulence that seemed likely involved in biotin metabolism. However, the molecular function of this virulence determinant was unclear. Here we show that this protein named BioJ is the enzyme of the biotin biosynthesis pathway that determines the chain length of the biotin valeryl side-chain. Expression of bioJ allows growth of an Escherichia coli bioH strain on biotin-free medium, indicating functional equivalence of BioJ to the paradigm pimeloyl-ACP methyl ester carboxyl-esterase, BioH. BioJ was purified to homogeneity, shown to be monomeric and capable of hydrolysis of its physiological substrate methyl pimeloyl-ACP to pimeloyl-ACP, the precursor required to begin formation of the fused heterocyclic rings of biotin. Phylogenetic analyses confirmed that distinct from BioH, BioJ represents a novel subclade of the α/β-hydrolase family. Structure-guided mapping combined with site-directed mutagenesis revealed that the BioJ catalytic triad consists of Ser151, Asp248 and His278, all of which are essential for activity and virulence. The biotin synthesis pathway was reconstituted reaction in vitro and the physiological role of BioJ directly assayed. To the best of our knowledge, these data represent further evidence linking biotin synthesis to bacterial virulence.