Biosynthesis of phenazines. II. Incorporation of (6-14C)-D-shikimic acid into phenazine-1-carboxylic acid and iodinin

Burkholderia as a Source of Natural Products

Burkholderia bacteria are multifaceted organisms that are ecologically and metabolically diverse. The Burkholderia genus has gained prominence because it includes human pathogens; however, many strains are nonpathogenic and have desirable characteristics such as beneficial plant associations and degradation of pollutants. The diversity of the Burkholderia genus is reflected within the large genomes that feature multiple replicons. Burkholderia genomes encode a plethora of natural products with potential therapeutic relevance and biotechnological applications. This review highlights Burkholderia as an emerging source of natural products. An overview of the taxonomy of the Burkholderia genus, which is currently being revised, is provided. We then present a curated compilation of natural products isolated from Burkholderia sensu lato and analyze their characteristics in terms of biosynthetic class, discovery method, and bioactivity. Finally, we describe and discuss genome characteristics and highlight the biosynthesis of a select number of natural products that are encoded in unusual biosynthetic gene clusters. The availability of >1000 Burkholderia genomes in public databases provides an opportunity to realize the genetic potential of this underexplored taxon for natural product discovery.

Recent developments in the isolation, biological function, biosynthesis, and synthesis of phenazine natural products

Phenazines are natural products which are produced by bacteria or by archaeal Methanosarcina species. The tricyclic ring system enables redox processes, which producing organisms use for oxidation of NADH or for the generation of reactive oxygen species (ROS), giving them advantages over other microorganisms. In this review we summarize the progress in the field since 2005 regarding the isolation of new phenazine natural products, new insights in their biological function, and particularly the now almost completely understood biosynthesis. The review is complemented by a description of new synthetic methods and total syntheses of phenazines.

Isolation of phenazine 1,6-di-carboxylic acid from Pseudomonas aeruginosa strain HRW.1-S3 and its role in biofilm-mediated crude oil degradation and cytotoxicity against bacterial and cancer cells

Pseudomonas sp. has long been known for production of a wide range of secondary metabolites during late exponential and stationary phases of growth. Phenazine derivatives constitute a large group of secondary metabolites produced by microorganisms including Pseudomonas sp. Phenazine 1,6-di-carboxylic acid (PDC) is one of such metabolites and has been debated for its origin from Pseudomonas sp. The present study describes purification and characterization of PDC isolated from culture of a natural isolate of Pseudomonas sp. HRW.1-S3 while grown in presence of crude oil as sole carbon source. The isolated PDC was tested for its effect on biofilm formation by another environmental isolate of Pseudomonas sp. DSW.1-S4 which lacks the ability to produce any phenazine compound. PDC showed profound effect on both planktonic as well as biofilm mode of growth of DSW.1-S4 at concentrations between 5 and 20 μM. Interestingly, PDC showed substantial cytotoxicity against three cancer cell lines and against both Gram-positive and Gram-negative bacteria. Thus, the present study not only opens an avenue to understand interspecific cooperation between Pseudomonas species which may lead its applicability in bioremediation, but also it signifies the scope of future investigation on PDC for its therapeutic applications.

The structural biology of phenazine biosynthesis

The phenazines are a class of over 150 nitrogen-containing aromatic compounds of bacterial and archeal origin. Their redox properties not only explain their activity as broad-specificity antibiotics and virulence factors but also enable them to function as respiratory pigments, thus extending their importance to the primary metabolism of phenazine-producing species. Despite their discovery in the mid-19th century, the molecular mechanisms behind their biosynthesis have only been unraveled in the last decade. Here, we review the contribution of structural biology that has led to our current understanding of phenazine biosynthesis.

Pyocyanin: production, applications, challenges and new insights

Pseudomonas aeruginosa is an opportunistic, Gram-negative bacterium and is one of the most commercially and biotechnologically valuable microorganisms. Strains of P. aeruginosa secrete a variety of redox-active phenazine compounds, the most well studied being pyocyanin. Pyocyanin is responsible for the blue-green colour characteristic of Pseudomonas spp. It is considered both as a virulence factor and a quorum sensing signalling molecule for P. aeruginosa. Pyocyanin is an electrochemically active metabolite, involved in a variety of significant biological activities including gene expression, maintaining fitness of bacterial cells and biofilm formation. It is also recognised as an electron shuttle for bacterial respiration and as an antibacterial and antifungal agent. This review summarises recent advances of pyocyanin production from P. aeruginosa with special attention to antagonistic property and bio-control activity. The review also covers the challenges and new insights into pyocyanin from P. aeruginosa.

Phenazine compounds in fluorescent Pseudomonas spp. biosynthesis and regulation

The phenazines include upward of 50 pigmented, heterocyclic nitrogen-containing secondary metabolites synthesized by some strains of fluorescent Pseudomonas spp. and a few other bacterial genera. The antibiotic properties of these compounds have been known for over 150 years, but advances within the past two decades have provided significant new insights into the genetics, biochemistry, and regulation of phenazine synthesis, as well as the mode of action and functional roles of these compounds in the environment. This new knowledge reveals conservation of biosynthetic enzymes across genera but raises questions about conserved biosynthetic mechanisms, and sets the stage for improving the performance of phenazine producers used as biological control agents for soilborne plant pathogens.

The α-Effect in the Stereochemistry of Kinetic Ketonization of Enols1,2

Kinetic control of the stereoselectivity of protonation of enolates and other strongly delocalized anionic species is involved in a large number of organic reactions. Protonation occurring from the less hindered side of the, e.g., enolic system affords the less stable of two diastereomers. However, one apparent discrepancy has been in the synthesis of prostaglandins. The present research deals with the source of this behavior. A curious effect of the substituent at the enolic alpha carbon was uncovered. In certain instances an alpha substituent is forced to twist into a conformation blocking the proton donor from its side, thus reversing the stereochemistry of protonation. In the course of this research, a number of five-ring enols of varying structure were investigated. Finally, the ketonization reaction course has been studied theoretically.

Phenazines and their role in biocontrol by Pseudomonas bacteria

Various rhizosphere bacteria are potential (micro)biological pesticides which are able to protect plants against diseases and improve plant yield. Knowledge of the molecular mechanisms that govern these beneficial plant-microbe interactions enables optimization, enhancement and identification of potential synergistic effects in plant protection. The production of antifungal metabolites, induction of systemic resistance, and the ability to compete efficiently with other resident rhizobacteria are considered to be important prerequisites for the optimal performance of biocontrol agents. Intriguing aspects in the molecular mechanisms of these processes have been discovered recently. Phenazines and phloroglucinols are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. This review focuses on the current state of knowledge on biocontrol by phenazine-producing Pseudomonas strains and the action, biosynthesis, and regulation mechanisms of the production of microbial phenazines.

phzO, a gene for biosynthesis of 2-hydroxylated phenazine compounds in Pseudomonas aureofaciens 30-84

Certain strains of root-colonizing fluorescent Pseudomonas spp. produce phenazines, a class of antifungal metabolites that can provide protection against various soilborne root pathogens. Despite the fact that the phenazine biosynthetic locus is highly conserved among fluorescent Pseudomonas spp., individual strains differ in the range of phenazine compounds they produce. This study focuses on the ability of Pseudomonas aureofaciens 30-84 to produce 2-hydroxyphenazine-1-carboxylic acid (2-OH-PCA) and 2-hydroxyphenazine from the common phenazine metabolite phenazine-1-carboxylic acid (PCA). P. aureofaciens 30-84 contains a novel gene located downstream from the core phenazine operon that encodes a 55-kDa aromatic monooxygenase responsible for the hydroxylation of PCA to produce 2-OH-PCA. Knowledge of the genes responsible for phenazine product specificity could ultimately reveal ways to manipulate organisms to produce multiple phenazines or novel phenazines not previously described.

A seven-gene locus for synthesis of phenazine-1-carboxylic acid by Pseudomonas fluorescens 2-79

Pseudomonas fluorescens 2-79 produces the broad-spectrum antibiotic phenazine-1-carboxylic acid (PCA), which is active against a variety of fungal root pathogens. In this study, seven genes designated phzABCDEFG that are sufficient for synthesis of PCA were localized within a 6.8-kb BglII-XbaI fragment from the phenazine biosynthesis locus of strain 2-79. Polypeptides corresponding to all phz genes were identified by analysis of recombinant plasmids in a T7 promoter/polymerase expression system. Products of the phzC, phzD, and phzE genes have similarities to enzymes of shikimic acid and chorismic acid metabolism and, together with PhzF, are absolutely necessary for PCA production. PhzG is similar to pyridoxamine-5'-phosphate oxidases and probably is a source of cofactor for the PCA-synthesizing enzyme(s). Products of the phzA and phzB genes are highly homologous to each other and may be involved in stabilization of a putative PCA-synthesizing multienzyme complex. Two new genes, phzX and phzY, that are homologous to phzA and phzB, respectively, were cloned and sequenced from P. aureofaciens 30-84, which produces PCA, 2-hydroxyphenazine-1-carboxylic acid, and 2-hydroxyphenazine. Based on functional analysis of the phz genes from strains 2-79 and 30-84, we postulate that different species of fluorescent pseudomonads have similar genetic systems that confer the ability to synthesize PCA.

Cyclic voltammetry of phenazines and quinoxalines including mono- and di-N-oxides. Relation to structure and antimicrobial activity

Cyclic voltammetry data were obtained for eight phenazines and phenazine-N-oxides, and eleven quinoxalines and quinoxaline-N-oxides: 1,6-phenazine-diol-5,10-dioxide (iodinin), iodinin copper complex, 6-methoxy-1-phenazinol-5,10-dioxide 1,6-dimethoxyphenazine-5-oxide, 1,6-phenazinediol, 1,6-dimethoxyphenazine, quinoxaline-1,4-dioxide, 2-methylquinoxaline-1,4-dioxide, 2,3-diphenylquinoxaline-1,4-dioxide, 2-carboxyquinoxaline-1,4-dioxide, 5-hydroxyquinoxaline-1,4-dioxide, 5-hydroxy-8-methoxyquinoxaline-1,4-dioxide, 2-methylquinoxaline, 2,3-diphenylquinoxaline, 5-hydroxyquinoxaline, 5-hydroxy-8-methoxyquinoxaline and 2-(2-quinoxalinylmethylene)hydrazine carboxylic acid methyl ester-1,4-dioxide (Carbadox). The di-N-oxides exhibit the most positive E1/2 values within each class. Reversible first wave reductions were observed for iodinin, iodinin copper complex, 1,6-dimethoxyphenazine-5-oxide, 1,6-dimethoxyphenazine, quinoxaline-1,4-dioxide, 2-methylquinoxaline-1,4-oxide and 2,3-diphenylquinoxaline-1,4-dioxide. The results are correlated with structure. Some relationships exist between reduction potential and reported antimicrobial activity. A possible mechanism of drug action is addressed.

Incorporation of [14C]shikimate into plenazines and their further metabolism by Pseudomonas phenazinium

1. During growth of Pseudomonas phenazinium on l-threonine medium, phenazine pigment formation commenced early and 1,6-dihydroxyphenazine 5,10-dioxide (iodinin) was the major component. Growth on l-[U-(14)C]threonine showed that when growth was complete about 25% of the label had been incorporated into phenazines and 30% into cell substance. 2. The addition of d-[2,3,4,5(n)-(14)C]shikimate to cultures at different phases of growth showed that the greatest efficiency of incorporation (about 70%) occurred in the mid- to late-exponential phase. Phenazines accounting for most of the (14)C supplied were iodinin and 9-hydroxyphenazine-1-carboxylate plus 2,9-dihydroxyphenazine-1-carboxylate. Radioactivity incorporated into cell substance was about one-third of the amount found in phenazines. 3. Kinetic studies showed that radioactivity from a pulse of [(14)C]-shikimate was incorporated into phenazines immediately, without a discernible lag, and into all detectable phenazines simultaneously rather than sequentially. 4. Radioactive phenazines isolated from culture media were fed to growing cultures and their metabolism was studied. The results supported a scheme for the biosynthesis of iodinin and 1,8-dihydroxyphenazine 10-monoxide by a branched pathway. 5. It is proposed that phenazine-1,6-dicarboxylate is the common precursor of all naturally occurring phenazines.