Biosynthesis of 2-Deoxystreptamine by Three Crucial Enzymes in Streptomyces fradiae NBRC 12773

Biosynthesis of cyclitols

Review covering up to 2021Cyclitols derived from carbohydrates are naturally stable hydrophilic substances under ordinary physiological conditions, increasing the water solubility of whole molecules in cells. The stability of cyclitols is derived from their carbocyclic structures bearing no acetal groups, in contrast to sugar molecules. Therefore, carbocycle-forming reactions are critical for the biosynthesis of cyclitols. Herein, we review naturally occurring cyclitols that have been identified to date and categorize them according to the type of carbocycle-forming enzymatic reaction. Furthermore, the cyclitol-forming enzymatic reaction mechanisms and modification pathways of the initially generated cyclitols are reviewed.

Alteration of the Canine Metabolome After a 3-Week Supplementation of Cannabidiol (CBD) Containing Treats: An Exploratory Study of Healthy Animals

Despite the increased interest and widespread use of cannabidiol (CBD) in humans and companion animals, much remains to be learned about its effects on health and physiology. Metabolomics is a useful tool to evaluate changes in the health status of animals and to analyze metabolic alterations caused by diet, disease, or other factors. Thus, the purpose of this investigation was to evaluate the impact of CBD supplementation on the canine plasma metabolome. Sixteen dogs (18.2 ± 3.4 kg BW) were utilized in a completely randomized design with treatments consisting of control and 4.5 mg CBD/kg BW/d. After 21 d of treatment, blood was collected ~2 h after treat consumption. Plasma collected from samples was analyzed using CIL/LC-MS-based untargeted metabolomics to analyze amine/phenol- and carbonyl-containing metabolites. Metabolites that differed - fold change (FC) ≥ 1.2 or ≤ 0.83 and false discovery ratio (FDR) ≤ 0.05 - between the two treatments were identified using a volcano plot. Biomarker analysis based on receiver operating characteristic (ROC) curves was performed to identify biomarker candidates (area under ROC ≥ 0.90) of the effects of CBD supplementation. Volcano plot analysis revealed that 32 amine/phenol-containing metabolites and five carbonyl-containing metabolites were differentially altered (FC ≥ 1.2 or ≤ 0.83, FDR ≤ 0.05) by CBD; these metabolites are involved in the metabolism of amino acids, glucose, vitamins, nucleotides, and hydroxycinnamic acid derivatives. Biomarker analysis identified 24 amine/phenol-containing metabolites and 1 carbonyl-containing metabolite as candidate biomarkers of the effects of CBD (area under ROC ≥ 0.90; P < 0.01). Results of this study indicate that 3 weeks of 4.5 mg CBD/kg BW/d supplementation altered the canine metabolome. Additional work is warranted to investigate the physiological relevance of these changes.

Stepwise Post-glycosylation Modification of Sugar Moieties in Kanamycin Biosynthesis

Kanamycin A is the major 2-deoxystreptamine (2DOS)-containing aminoglycoside antibiotic produced by Streptomyces kanamyceticus. The 2DOS moiety is linked with 6-amino-6-deoxy-d-glucose (6ADG) at O-4 and 3-amino-3-deoxy-d-glucose at O-6. Because the 6ADG moiety is derived from d-glucosamine (GlcN), deamination at C-2 and introduction of C-6-NH₂ are required in the biosynthesis. A dehydrogenase, KanQ, and an aminotransferase, KanB, are presumed to be responsible for the introduction of C-6-NH₂ , although the substrates have not been identified. Here, we examined the substrate specificity of KanQ to better understand the biosynthetic pathway. It was found that KanQ oxidized kanamycin C more efficiently than the 3''-deamino derivative. Furthermore, the substrate specificity of an oxygenase, KanJ, that is responsible for deamination at C-2 of the GlcN moiety was examined, and the crystal structure of KanJ was determined. It was found that C-6-NH₂ is important for substrate recognition by KanJ. Thus, the modification of the GlcN moiety occurs after pseudo-trisaccharide formation, followed by the introduction of C-6-NH₂ by KanQ/KanB and deamination at C-2 by KanJ.

One-pot enzymatic synthesis of 2-deoxy-scyllo-inosose from d-glucose and polyphosphate

2-Deoxy-scyllo-inosose (2DOI, [2S,3R,4S,5R]-2,3,4,5-tetrahydroxycyclohexan-1-one) is a biosynthetic intermediate of 2-deoxystreptamine-containing aminoglycoside antibiotics, including butirosin, kanamycin, and neomycin. In producer microorganisms, 2DOI is constructed from d-glucose 6-phosphate (G6P) by 2-deoxy-scyllo-inosose synthase (DOIS) with the oxidized form of nicotinamide adenine dinucleotide (NAD+). 2DOI is also known as a sustainable biomaterial for production of aromatic compounds and a chiral cyclohexane synthon. In this study, a one-pot enzymatic synthesis of 2DOI from d-glucose and polyphosphate was investigated. First, 3 polyphosphate glucokinases (PPGKs) were examined to produce G6P from d-glucose and polyphosphate. A PPGK derived from Corynebacterium glutamicum (cgPPGK) was found to be suitable for G6P production under ordinary enzymatic conditions. Next, 7 DOISs were examined for the one-pot enzymatic reaction. As a result, cgPPGK and BtrC, the latter of which is a DOIS derived from the butirosin producer Bacillus circulans, achieved nearly full conversion of d-glucose to 2DOI in the presence of polyphosphate.

Exploration of Hygromycin B Biosynthesis Utilizing CRISPR-Cas9-Associated Base Editing

Hygromycin B is an aminoglycoside antibiotic widely used in industry and biological research. However, most of its biosynthetic pathway has not been completely identified due to the immense difficulty in genetic manipulation of the producing strain. To address this problem, we developed an efficient system that combines clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9-associated base editing and site-specific recombination instead of conventional double-crossover-based homologous recombination. This strategy was successfully applied to the in vivo inactivation of five candidate genes involved in the biosynthesis of hygromycin B by generating stop codons or mutating conserved residues within the encoding region. The results revealed that HygJ, HygL, and HygD are responsible for successive dehydrogenation, transamination, and transglycosylation of nucleoside diphosphate (NDP)-heptose. Notably, HygY acts as an unusual radical S-adenosylmethionine (SAM)-dependent epimerase for hydroxyl carbons, and HygM serves as a versatile methyltransferase in multiple parallel metabolic networks. Based on in vivo and in vitro evidence, the biosynthetic pathway for hygromycin B is proposed.

Comparison of Antibiotic Resistance Mechanisms in Antibiotic-Producing and Pathogenic Bacteria

Antibiotic resistance poses a tremendous threat to human health. To overcome this problem, it is essential to know the mechanism of antibiotic resistance in antibiotic-producing and pathogenic bacteria. This paper deals with this problem from four points of view. First, the antibiotic resistance genes in producers are discussed related to their biosynthesis. Most resistance genes are present within the biosynthetic gene clusters, but some genes such as paromomycin acetyltransferases are located far outside the gene cluster. Second, when the antibiotic resistance genes in pathogens are compared with those in the producers, resistance mechanisms have dependency on antibiotic classes, and, in addition, new types of resistance mechanisms such as Eis aminoglycoside acetyltransferase and self-sacrifice proteins in enediyne antibiotics emerge in pathogens. Third, the relationships of the resistance genes between producers and pathogens are reevaluated at their amino acid sequence as well as nucleotide sequence levels. Pathogenic bacteria possess other resistance mechanisms than those in antibiotic producers. In addition, resistance mechanisms are little different between early stage of antibiotic use and the present time, e.g., β-lactam resistance in Staphylococcus aureus. Lastly, guanine + cytosine (GC) barrier in gene transfer to pathogenic bacteria is considered. Now, the resistance genes constitute resistome composed of complicated mixture from divergent environments.

Direct Genetic and Enzymatic Evidence for Oxidative Cyclization in Hygromycin B Biosynthesis

Hygromycin B is an aminoglycoside antibiotic with a structurally distinctive orthoester linkage. Despite its long history of use in industry and in the laboratory, its biosynthesis remains poorly understood. We show here, by in-frame gene deletion in vivo and detailed enzyme characterization in vitro, that formation of the unique orthoester moiety is catalyzed by the α-ketoglutarate- and non-heme iron-dependent oxygenase HygX. In addition, we identify HygF as a glycosyltransferase adding UDP-hexose to 2-deoxystreptamine, HygM as a methyltransferase responsible for N-3 methylation, and HygK as an epimerase. These experimental results and bioinformatic analyses allow a detailed pathway for hygromycin B biosynthesis to be proposed, including the key oxidative cyclization reactions.

The three-dimensional structure of NeoB: An aminotransferase involved in the biosynthesis of neomycin

The aminoglycoside antibiotics, discovered as natural products in the 1940s, demonstrate a broad antimicrobial spectrum. Due to their nephrotoxic and ototoxic side effects, however, their widespread clinical usage has typically been limited to the treatment of serious infections. Neomycin B, first isolated from strains of Streptomyces in 1948, is one such drug that was approved for human use by the U.S. Food and Drug Administration in 1964. Only within the last 11 years has the biochemical pathway for its production been elaborated, however. Here we present the three-dimensional architecture of NeoB from Streptomyces fradiae, which is a pyridoxal 5'-phosphate or PLP-dependent aminotransferase that functions on two different substrates in neomycin B biosynthesis. For this investigation, four high resolution X-ray structures of NeoB were determined in various complexed states. The overall fold of NeoB is that typically observed for members of the "aspartate aminotransferase" family with the exception of an additional three-stranded antiparallel β-sheet that forms part of the subunit-subunit interface of the dimer. The manner in which the active site of NeoB accommodates quite different substrates has been defined by this investigation. In addition, during the course of this study, we also determined the structure of the aminotransferase GenB1 to high resolution. GenB1 functions as an aminotransferase in gentamicin biosynthesis. Taken together, the structures of NeoB and GenB1, presented here, provide the first detailed descriptions of aminotransferases that specifically function on aldehyde moieties in aminoglycoside biosynthesis.

Aminoglycoside Antibiotics: New Insights into the Biosynthetic Machinery of Old Drugs

2-Deoxystreptamine (2DOS) is the unique chemically stable aminocyclitol scaffold of clinically important aminoglycoside antibiotics such as neomycin, kanamycin, and gentamicin, which are produced by Actinomycetes. The 2DOS core can be decorated with various deoxyaminosugars to make structurally diverse pseudo-oligosaccharides. After the discovery of biosynthetic gene clusters for 2DOS-containing aminoglycoside antibiotics, the function of each biosynthetic enzyme has been extensively elucidated. The common biosynthetic intermediates 2DOS, paromamine and ribostamycin are constructed by conserved enzymes encoded in the gene clusters. The biosynthetic intermediates are then converted to characteristic architectures by unique enzymes encoded in each biosynthetic gene cluster. In this Personal Account, we summarize both common biosynthetic pathways and the pathways used for structural diversification.

Epimerization at C-3'' in butirosin biosynthesis by an NAD(+) -dependent dehydrogenase BtrE and an NADPH-dependent reductase BtrF

Butirosin is an aminoglycoside antibiotic consisting two epimers at C-3'' of ribostamycin/xylostasin with a unique 4-amino-2-hydroxybutyrate moiety at C-1 of the aminocyclitol 2-deoxystreptamine (2DOS). To date, most of the enzymes encoded in the biosynthetic gene cluster for butirosin, from the producing strain Bacillus circulans, have been characterized. A few unknown functional proteins, including nicotinamide adenine dinucleotide cofactor-dependent dehydrogenase/reductase (BtrE and BtrF), are supposed to be involved in the epimerization at C-3'' of butirosin B/ribostamycin but remain to be characterized. Herein, the conversion of ribostamycin to xylsostasin by BtrE and BtrF in the presence of NAD(+) and NADPH was demonstrated. BtrE oxidized the C-3'' of ribostamycin with NAD(+) to yield 3''-oxoribostamycin. BtrF then reduced the generated 3''-oxoribostamycin with NADPH to produce xylostasin. This reaction step was the last piece of butirosin biosynthesis to be described.

X-ray analysis of butirosin biosynthetic enzyme BtrN redefines structural motifs for AdoMet radical chemistry

The 2-deoxy-scyllo-inosamine (DOIA) dehydrogenases are key enzymes in the biosynthesis of 2-deoxystreptamine-containing aminoglycoside antibiotics. In contrast to most DOIA dehydrogenases, which are NAD-dependent, the DOIA dehydrogenase from Bacillus circulans (BtrN) is an S-adenosyl-l-methionine (AdoMet) radical enzyme. To examine how BtrN employs AdoMet radical chemistry, we have determined its structure with AdoMet and substrate to 1.56 Å resolution. We find a previously undescribed modification to the core AdoMet radical fold: instead of the canonical (β/α)6 architecture, BtrN displays a (β5/α4) motif. We further find that an auxiliary [4Fe-4S] cluster in BtrN, thought to bind substrate, is instead implicated in substrate-radical oxidation. High structural homology in the auxiliary cluster binding region between BtrN, fellow AdoMet radical dehydrogenase anSME, and molybdenum cofactor biosynthetic enzyme MoaA provides support for the establishment of an AdoMet radical structural motif that is likely common to ~6,400 uncharacterized AdoMet radical enzymes.

Radical SAM enzymes in the biosynthesis of sugar-containing natural products

Carbohydrates play a key role in the biological activity of numerous natural products. In many instances their biosynthesis requires radical mediated rearrangements, some of which are catalyzed by radical SAM enzymes. BtrN is one such enzyme responsible for the dehydrogenation of a secondary alcohol in the biosynthesis of 2-deoxystreptamine. DesII is another example that catalyzes a deamination reaction necessary for the net C4 deoxygenation of a glucose derivative en route to desosamine formation. BtrN and DesII represent the two most extensively characterized radical SAM enzymes involved in carbohydrate biosynthesis. In this review, we summarize the biosynthetic roles of these two enzymes, their mechanisms of catalysis, the questions that have arisen during these investigations and the insight they can offer for furthering our understanding of radical SAM enzymology. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Engineering Streptomyces tenebrarius to synthesize single component of carbamoyl tobramycin

AIMS

To engineer Streptomyces tenebrarius for producing carbamoyl tobramycin as a main component.

METHODS AND RESULTS

The aprH-M gene fragment (apramycin biosynthetic gene from GenBank) in S. tenebrarius Tt49 was knocked out by genetic engineering to form S. tenebrarius T106 (ΔaprH-M). Compared to the wild-type strain, mutant strain T106 (ΔaprH-M) no longer produced apramycin, while mainly synthesize carbamoyl tobramycin. TLC and HPLC-MS analyses indicated that the mutant strain significantly increased the production of carbamoyl tobramycin.

CONCLUSIONS

The metabolic flow for the apramycin and its analogues biosynthesis was blocked by disrupting the aprH-M gene clusters. The aprH-M gene clusters might be essential for the biosynthesis of apramycin. The mutant strain T106 mainly synthesized carbamoyl tobramycin.

SIGNIFICANCE AND IMPACT OF STUDY

The mutant T106 mainly produces carbamoyl tobramycin without synthesizing apramycin, which will reduce cost of postextraction from fermentation products. Therefore, it has good prospects for industrial application.

Discovery of parallel pathways of kanamycin biosynthesis allows antibiotic manipulation

Kanamycin is one of the most widely used antibiotics, yet its biosynthetic pathway remains unclear. Current proposals suggest that the kanamycin biosynthetic products are linearly related via single enzymatic transformations. To explore this system, we have reconstructed the entire biosynthetic pathway through the heterologous expression of combinations of putative biosynthetic genes from Streptomyces kanamyceticus in the non-aminoglycoside-producing Streptomyces venezuelae. Unexpectedly, we discovered that the biosynthetic pathway contains an early branch point, governed by the substrate promiscuity of a glycosyltransferase, that leads to the formation of two parallel pathways in which early intermediates are further modified. Glycosyltransferase exchange can alter flux through these two parallel pathways, and the addition of other biosynthetic enzymes can be used to synthesize known and new highly active antibiotics. These results complete our understanding of kanamycin biosynthesis and demonstrate the potential of pathway engineering for direct in vivo production of clinically useful antibiotics and more robust aminoglycosides.

Construction of kanamycin B overproducing strain by genetic engineering of Streptomyces tenebrarius

Genetic engineering as an important approach to strain optimization has received wide recognition. Recent advances in the studies on the biosynthetic pathways and gene clusters of Streptomyces make stain optimization by genetic alteration possible. Kanamycin B is a key intermediate in the manufacture of the important medicines dibekacin and arbekacin, which belong to a class of antibiotics known as the aminoglycosides. Kanamycin could be prepared by carbamoylkanamycin B hydrolysis. However, carbamoylkanamycin B production in Streptomyces tenebrarius H6 is very low. Therefore, we tried to obtain high kanamycin B-producing strains that produced kanamycin B as a main component. In our work, aprD3 and aprD4 were clarified to be responsible for deoxygenation in apramycin and tobramycin biosynthesis. Based on this information, genes aprD3, aprQ (deduced apramycin biosynthetic gene), and aprD4 were disrupted to optimize the production of carbamoylkanamycin B. Compared with wild-type strain, mutant strain SPU313 (ΔaprD3, ΔaprQ, and ΔaprD4) produced carbamoylkanamycin B as a single antibiotic, whose production increased approximately fivefold. To construct a strain producing kanamycin B instead of carbamoylkanamycin B, the carbamoyl-transfer gene tacA was inactivated in strain SPU313. Mutant strain SPU314 (ΔaprD3, ΔaprQ, ΔaprD4, and ΔtacA) specifically produced kanamycin B, which was proven by LC-MS. This work demonstrated careful genetic engineering could significantly improve production and eliminate undesired products.

Heterologous production of ribostamycin derivatives in engineered Escherichia coli

Aminoglycosides are a class of important antibiotic compounds used for various therapeutic indications. In recent times, their efficacy has been curtailed due to the rapid development of bacterial resistance. There is a need to develop novel derivatives with an improved spectrum of activity and higher sensitivity against pathogenic bacteria. Although efforts have been focused on the development of newer therapeutic agents by chemical synthesis, to our knowledge, there has been no attempt to harness the potential of microorganisms for this purpose. Escherichia coli affords a widely studied cellular system that could be utilized not only for understanding but also for attempting to engineer the biosynthetic pathway of secondary metabolites. The primary metabolic pathway of E. coli can be engineered to divert the precursor pool required for the biosynthesis of secondary metabolites. Utilizing this approach previously, we engineered E. coli host and generated E. coli M1. Here, we produced a ribostamycin derivative in the engineered host by heterologous expression of the recombinants constructed from the genes encoding the biosynthetic pathway in aminoglycoside-producing strains. The products obtained from the transformants were isolated, analyzed and verified to be ribostamycin derivatives. The study further demonstrated the importance of E. coli as surrogate antibiotic producer and also offered future possibility for the production of other aminoglycoside derivatives through genetic engineering and expression in a heterologous background.

Biosynthetic genes for aminoglycoside antibiotics

Biosynthetic studies of aminoglycoside antibiotics have progressed remarkably during the last decade. Many biosynthetic gene clusters for aminoglycoside antibiotics including streptomycin, kanamycin, butirosin, neomycin and gentamicin have been identified to date. In addition, most butirosin and neomycin biosynthetic enzymes have been functionally characterized using recombinant proteins. Herein, we reanalyze biosynthetic genes for structurally related 2-deoxystreptamine (2DOS)-containing aminoglycosides, such as kanamycin, gentamicin and istamycin, based on genetic information including characterized biosynthetic enzymes in neomycin and butirosin biosynthetic pathways. These proposed enzymatic functions for uncharacterized enzymes are expected to support investigation of the complex biosynthetic pathways for this important class of antibiotics.

Progress in aminocyclitol biosynthesis

A stream of genetic and biochemical information available for the biosynthesis of aminocyclitols over the past few years has provided the foundation to study the modes of formation of this clinically important class of natural products. In addition to work on the identification and functional analysis of aminocyclitol biosynthetic gene clusters, a contingent of recent studies has focused on the detailed analysis of unique enzymatic and catalytic mechanisms inherent to these pathways. The results provide invaluable insights into the biochemical and molecular aspects of aminocyclitol biosynthesis and have revealed diverse and unique features of the pathways.

Functional characterization of kanB by complementing in engineered Streptomyces fradiae Deltaneo6::tsr

A putative aminotransferase gene, kanB, lies in the biosynthetic gene cluster of Streptomyces kanamyceticus ATCC 12853 and has 66% identity with neo6 in neomycin biosynthesis. Streptomyces fradiae Deltaneo6::tsr was generated by disrupting neo6 in the neomycin producer Streptomyces fradiae. Neomycin production was completely abolished in the disruptant mutant but was restored through self-complementation of neo6. S. fradiae HN4 was generated through complementation with kanB in Streptomyces fradiae neo6::tsr. Based on metabolite analysis by ESI/MS and LC/MS, neomycin production was restored in Streptomyces fradiae HN4. Thus, like neo6, kanB also functions as a 2-deoxy-scyllo-inosose aminotransferase that has dual functions in the formation of 2-deoxy-scyllo-inosose (DOS).

Medium- and short-chain dehydrogenase/reductase gene and protein families : the MDR superfamily

The MDR superfamily with ~350-residue subunits contains the classical liver alcohol dehydrogenase (ADH), quinone reductase, leukotriene B4 dehydrogenase and many more forms. ADH is a dimeric zinc metalloprotein and occurs as five different classes in humans, resulting from gene duplications during vertebrate evolution, the first one traced to ~500 MYA (million years ago) from an ancestral formaldehyde dehydrogenase line. Like many duplications at that time, it correlates with enzymogenesis of new activities, contributing to conditions for emergence of vertebrate land life from osseous fish. The speed of changes correlates with function, as do differential evolutionary patterns in separate segments. Subsequent recognitions now define at least 40 human MDR members in the Uniprot database (corresponding to 25 genes when excluding close homologues), and in all species at least 10888 entries. Overall, variability is large, but like for many dehydrogenases, subdivided into constant and variable forms, corresponding to household and emerging enzyme activities, respectively. This review covers basic facts and describes eight large MDR families and nine smaller families. Combined, they have specific substrates in metabolic pathways, some with wide substrate specificity, and several with little known functions.

Biosynthetic enzymes for the aminoglycosides butirosin and neomycin

Butirosin and neomycin belong to a family of clinically valuable 2-deoxystreptamine (2DOS)-containing aminoglycoside antibiotics. The biosynthetic gene clusters for butirosin and neomycin were identified in 2000 and in 2005, respectively. In recent years, most of the enzymes encoded in the gene clusters have been characterized, and thus almost all the biosynthetic steps leading to the final antibiotics have been understood. This knowledge could shed light on the complex biosynthetic pathways for other related structurally diverse aminoglycoside antibiotics. In this chapter, the enzymatic reactions in the biosynthesis of butirosin and neomycin are reviewed step by step.

Enzymology of aminoglycoside biosynthesis-deduction from gene clusters

The classical aminoglycosides are, with very few exceptions, typically actinobacterial secondary metabolites with antimicrobial activities all mediated by inhibiting translation on the 30S subunit of the bacterial ribosome. Some chemically related natural products inhibit glucosidases by mimicking oligo-alpha-1,4-glucosides. The biochemistry of the aminoglycoside biosynthetic pathways is still a developing field since none of the pathways has been analyzed to completeness as yet. In this chapter we treat the enzymology of aminoglycoside biosyntheses as far as it becomes apparent from recent investigations based on the availability of DNA sequence data of biosynthetic gene clusters for all major structural classes of these bacterial metabolites. We give a more general overview of the field, including descriptions of some key enzymes in various aminoglycoside pathways, whereas in Chapter 20 provides a detailed account of the better-studied enzymology thus far known for the neomycin and butirosin pathways.

Biosynthesis of 3'-deoxy-carbamoylkanamycin C in a Streptomyces tenebrarius mutant strain by tacB gene disruption

Streptomyces tenebrarius H6 mainly produces three kinds of antibiotics: apramycin, carbamoyltobramycin and some carbamoylkanamycin B. In our present study, a dehydrogenase gene tacB in the tobramycin biosynthetic gene cluster was disrupted by in-frame deletion. The result of TLC bio-autograph analysis demonstrated the disruption mutant strain produced apramycin and a new antibiotic. The new antibiotic was identified as 3'-deoxy-carbamoylkanamycin C by MS and NMR analysis after isolation and purification. The disruption mutant was restored to produce carbamoyltobramycin in a complementation experiment by the intact tacB gene. Our studies suggested that the tacB gene encodes a 6'-dehydrogenase, which reduces the 6'-hydroxyl group of paromamine to a keto group, thus facilitating the transfer of an aminogroup to form neamine. This study is the first report on the generation of a tobramycin derivative by gene engineering, and will contribute to clarify the complete biosynthetic pathway of tobramycin.

Genetic dissection of the biosynthetic route to gentamicin A2 by heterologous expression of its minimal gene set

Since the first use of streptomycin as an effective antibiotic drug in the treatment of tuberculosis, aminoglycoside antibiotics have been widely used against a variety of bacterial infections for over six decades. However, the pathways for aminoglycoside biosynthesis still remain unclear, mainly because of difficulty in genetic manipulation of actinomycetes producing this class of antibiotics. Gentamicin belongs to the group of 4,6-disubstituted aminoglycosides containing a characteristic core aminocyclitol moiety, 2-deoxystreptamine (2-DOS), and the recent discovery of its biosynthetic gene cluster in Micromonospora echinospora has enabled us to decipher its biosynthetic pathway. To determine the minimal set of genes and their functions for the generation of gentamicin A(2), the first pseudotrisaccharide intermediate in the biosynthetic pathway for the gentamicin complex, various sets of candidate genes from M. echinospora and other related aminoglycoside-producing strains were introduced into a nonaminoglycoside producing strain of Streptomyces venezuelae. Heterologous expression of different combinations of putative 2-DOS biosynthetic genes revealed that a subset, gtmB-gtmA-gacH, is responsible for the biosynthesis of this core aminocyclitol moiety of gentamicin. Expression of gtmG together with gtmB-gtmA-gacH led to production of 2'-N-acetylparomamine, demonstrating that GtmG acts as a glycosyltransferase that adds N-acetyl-d-glucosamine (GLcNA) to 2-DOS. Expression of gtmM in a 2'-N-acetylparomamine-producing recombinant S. venezuelae strain generated paromamine. Expression of gtmE in an engineered paromamine-producing strain of S. venezuelae successfully generated gentamicin A(2), indicating that GtmE is another glycosyltransferase that attaches d-xylose to paromamine. These results represent in vivo evidence elucidating the complete biosynthetic pathway of the pseudotrisaccharide aminoglycoside.

Gene inactivation study of gntE reveals its role in the first step of pseudotrisaccharide modifications in gentamicin biosynthesis

A gene inactivation study was performed on gntE, a member of the gentamicin biosynthetic gene cluster in Micromonospora echinospora. Computer-aided homology analysis predicts a methyltransferase-related cobalamin-binding domain and a radical S-adenosylmethionine domain in GntE. It is also found that there is no gntE homolog within other aminoglycoside biosynthetic gene clusters. Inactivation of gntE was achieved in both M. echinospora ATCC 15835 and a gentamicin high-producer GMC106. High-performance liquid chromatographic analysis, coupled with mass spectrometry, revealed that gntE mutants accumulated gentamicin A2 and its derivative with a methyl group installed on the glucoamine moiety. This result substantiated that GntE participates in the first step of pseudotrisaccharide modifications in gentamicin biosynthesis, though the catalytic nature of this unusual oxidoreductase/methyltransferase candidate is not resolved. The present gene inactivation study also demonstrates that targeted genetic engineering can be applied to produce specific gentamicin structures and potentially new gentamicin derivatives in M. echinospora.

Unique O-ribosylation in the biosynthesis of butirosin

Using a comparative genetics approach, one or more of the BtrA, BtrL, BtrP, and BtrV proteins encoded in the butirosin biosynthetic gene cluster (btr) from Bacillus circulans SANK72073 were identified as being responsible for an O-ribosylation process leading to the formation of ribostamycin, a key intermediate in this, and related antibiotic biosynthetic pathways. Functional analysis of the recombinantly expressed proteins revealed that both BtrL and BtrP were responsible for the ribosylation of neamine, using 5-phosphoribosyl-1-diphosphate (PRPP) as the ribosyl donor. Further detailed analysis indicated that this process occurs via two discrete steps: with BtrL first catalyzing the phosphoribosylaion of neamine to form 5''-phosphoribostamycin, followed by a BtrP-catalyzed dephosphorylation to generate ribostamycin. To the best of our knowledge, this is the first time that the functional characterization of a glycosyltransferase from an aminoglycoside biosynthetic gene cluster has been reported.

Production of aminoglycosides in non-aminoglycoside producing Streptomyces lividans TK24

The pRBM4 cosmid, which harbors a putative cluster of genes spanning a 31.8-kb chromosomal region of the ribostamycin producer Streptomyces ribosidificus ATCC 21294, was heterologously expressed in Streptomyces lividans TK24. ESI-MS/MS, HPLC, and LC-ESI MS analyses showed that the transformation gave rise to ribostamycin production in various culture broths. This is the first report of heterologous aminoglycoside production.

Elaboration of Neosamine Rings in the Biosynthesis of Neomycin and Butirosin

The proteins Neo-11 and Neo-18 encoded in the neomycin gene cluster (neo) of Streptomyces fradiae NCIMB 8233 have been characterized as glucosaminyl-6'-oxidase and 6'-oxoglucosaminyl:L-glutamate aminotransferase, respectively. The joint activity of Neo-11 and Neo-18 is responsible for the conversion of paromamine to neamine in the biosynthetic pathway of neomycin through a mechanism of FAD-dependent dehydrogenation followed by a pyridoxal-5'-phosphate-mediated transamination. Neo-18 is also shown to catalyze deamination at C-6''' of neomycin, thus suggesting bifunctional roles of the two enzymes in the formation of both neosamine rings of neomycin. The product of the btrB gene, a homologue of neo-18 in the butirosin biosynthetic gene cluster (btr) in Bacillus circulans, exhibits the same activity as Neo-18; this indicates that there is a similar reaction sequence in both butirosin and neomycin biosynthesis.

Role of glutamate 243 in the active site of 2-deoxy-scyllo-inosose synthase from Bacillus circulans

2-Deoxy-scyllo-inosose (DOI) synthase is involved in the biosynthesis of 2-deoxystreptamine-containing aminoglycoside antibiotics and catalyzes the carbocyclic formation from d-glucose-6-phosphate (G-6-P) into DOI. The reaction mechanism is proposed to be similar to that of dehydroquinate (DHQ) synthase in the shikimate pathway, and includes oxidation of C-4, beta-elimination of phosphate, reduction of C-4, ring opening, and intramolecular aldol cyclization. To investigate the reaction mechanism of DOI synthase, site-directed mutational analysis of three presumable catalytically important amino acids of DOI synthase derived from the butirosin producer Bacillus circulans (BtrC) was carried out. Steady state and pre-steady state kinetic analysis suggested that E243 of BtrC is catalytically involved in the phosphate elimination step. Further analysis of the mutant E243Q of BtrC using substrate analogue, glucose-6-phosphonate, clearly confirmed that E243 was responsible to abstract a proton at C-5 in G-6-P and set off phosphate elimination. This glutamate residue is completely conserved in all DOI synthases identified so far and the corresponding amino acid of DHQ synthase is completely conserved as asparagine. Therefore, this characteristic glutamate residue of DOI synthase is a key determinant to distinguish the reaction mechanism between DOI synthase and DHQ synthase as well as primary sequence.

Biosynthesis of aminocyclitol-aminoglycoside antibiotics and related compounds

This review covers the biosynthesis of aminocyclitol-aminoglycoside antibiotics and related compounds, particularly from the molecular genetic perspectives. 195 references are cited.

Biosynthesis of 2-Deoxystreptamine-containing Antibiotics in Streptoalloteichus hindustanus JCM 3268: Characterization of 2-Deoxy-scyllo-inosose Synthase

A part of the new biosynthetic gene cluster for 2-deoxystreptamine-containing antibiotics was identified from Streptoalloteichus hindustanus. The alloH gene in the gene cluster was deduced to encode 2-deoxy-scyllo-inosose synthase and the expressed protein AlloH was confirmed to have this enzyme activity. Furthermore, biochemical properties of AlloH were studied.

Biosynthesis of 2-deoxystreptamine-containing aminoglycoside antibiotics

The 2-deoxystreptamine-containing aminoglycosides are an important class of clinically valuable antibiotics. A deep understanding of the biosynthesis of these natural products is required to enable efforts to rationally manipulate and engineer the biological production of novel aminoglycosides. This review discusses the development of our biosynthetic knowledge over the past half-century, with emphasis on the relatively recent contributions of molecular biology to the elucidation of these biosynthetic pathways.