Fatty acid and beta-amino acid syntheses in strains of Bacillus subtilis producing iturinic antibiotics

Bacillus methylotrophicus DCS1: Production of Different Lipopeptide Families, In Vitro Antifungal Activity and Suppression of Fusarium Wilt in Tomato Plants

The present work aims to quantitatively and qualitatively monitor the production of lipopeptide mixtures by Bacillus methylotrophicus DCS1 strain in Landy medium and to investigate the antifungal activities of DCS1 strain and its produced lipopeptides. The in vitro activities were tested by the direct confrontation and agar well diffusion methods, while the in vivo study was carried out in order to test the efficiency of DCS1 bacterial suspension in the control of Fusarium wilt in tomato plants. Identification of lipopeptides by mass spectrometry (LC/MSD-TOF) showed that lipopeptide isoforms produced during the first 24 h and 48 h of fermentation are identical, belonging to bacillomycin D and fengycins A and B homologues with a difference in the yield of production. After 72 h of fermentation corresponding to the end of incubation period, B. methylotrophicus DCS1 is able to produce a mixture of surfactin, pumilacidin, iturin A/mycosubtilin, iturin C1, bacillomycin D and fengycins A and B isoforms. The results of in vitro antifungal experiments suggest that B. methylotrophicus DCS1 has a significant potential as a biocontrol agent, owing to lipopeptides produced, endowed with antifungal activity against several phytopathogenic fungi. The curative treatment of tomato plants with DCS1 bacterial suspension was more effective in the protection against Fusarium oxysporum f. sp. radicis-lycopersici (FORL) than the preventive treatment by comparing the average number of leaves remaining healthy after 30 days of each treatment and the appearance of tomato plants roots. The results indicate that B. methylotrophicus DCS1 exhibit a significant suppression of Fusarium wilt symptoms in tomato plants comparable to that of commercial fungicides and could be an alternative to chemically synthesized pesticides.

The elucidation of the multimodal action of the investigational anti-Candida lipopeptide (AF4) lead from Bacillus subtilis

Background: Candida species are the main etiological agents for candidiasis, and Candida albicans are the most common infectious species. Candida species' growing resistance to conventional therapies necessitates more research into novel antifungal agents. Antifungal peptides isolated from microorganisms have potential applications as novel therapeutics. AF4 a Bacillus-derived lipopeptide demonstrating broad-spectrum antifungal activity has been investigated for its ability to cause cell death in Candida species via membrane damage and oxidative stress. Methods: Using biophysical techniques, the secondary structure of the AF4 lipopeptide was identified. Scanning electron microscopy and confocal microscopy with fluorescent dyes were performed to visualise the effect of the lipopeptide. The membrane disruption and permeabilization were assessed using the 1,6-diphenyl hexatriene (DPH) fluorescence assay and flow cytometric (FC) assessment of propidium iodide (PI) uptake, respectively. The reactive oxygen species levels were estimated using the FC assessment. The induction of apoptosis and DNA damage were studied using Annexin V-FITC/PI and DAPI. Results: Bacillus-derived antifungal variant AF4 was found to have structural features typical of lipopeptides. Microscopy imaging revealed that AF4 damages the surface of treated cells and results in membrane permeabilization, facilitating the uptake of the fluorescent dyes. A loss of membrane integrity was observed in cells treated with AF4 due to a decrease in DPH fluorescence and a dose-dependent increase in PI uptake. Cell damage was also determined from the log reduction of viable cells treated with AF4. AF4 treatment also caused elevated ROS levels, induced phosphatidylserine externalisation, late-stage apoptosis, and alterations to nuclear morphology revealed by DAPI fluorescence. Conclusion: Collectively, the mode of action studies revealed that AF4 acts primarily on the cell membrane of C. albicans and has the potential to act as an antifungal drug candidate.

Bioinformatics Modelling and Metabolic Engineering of the Branched Chain Amino Acid Pathway for Specific Production of Mycosubtilin Isoforms in Bacillus subtilis

Mycosubtilin belongs to the family of lipopeptides. Different isoforms with various antifungal activities can be obtained according to the length and the isomery of the fatty acid. In this work, the activities of the mycosubtilin isoforms were first studied against the pathogen Aspergillus niger, revealing the high activity of the anteiso-C17 isoform. Modification of the mycosubtilin isoform patterns during cultures of the natural strain Bacillus subtilis ATCC 6633 was then investigated through amino acid feeding experiments. In parallel, single-gene knockouts and single-gene overexpression, leading to the overproduction of the anteiso-C15 fatty acid chains, were predicted using informatics tools which provide logical reasoning with formal models of reaction networks. In this way, it was in silico predicted that the single overexpression of the ilvA gene as well as the single knockout of the codY gene may lead to the overproduction of anteiso-C15 fatty acid chains. For the first time, it has been demonstrated that overexpression of ilvA helps to enhance the furniture of odd anteiso fatty acids leading to a favored mycosubtilin anteiso-C17 production pattern (+41%). Alternatively, a knock-out codY mutant led to a higher furniture of even iso fatty acids, leading to a favored mycosubtilin iso-C16 production pattern (+180%). These results showed that increased selective synthesis of particular isoforms of mycosubtilin through metabolic engineering is feasible, disclosing the interest of these approaches for future development of lipopeptide-producing strains.

Iturinic Lipopeptide Diversity in the Bacillus subtilis Species Group - Important Antifungals for Plant Disease Biocontrol Applications

Iturins and closely related lipopeptides constitute a family of antifungal compounds known as iturinic lipopeptides that are produced by species in the Bacillus subtilis group. The compounds that comprise the family are: iturin, bacillomycin D, bacillomycin F, bacillomycin L, mycosubtilin, and mojavensin. These lipopeptides are prominent in many Bacillus strains that have been commercialized as biological control agents against fungal plant pathogens and as plant growth promoters. The compounds are cyclic heptapeptides with a variable length alkyl sidechain, which confers surface activity properties resulting in an affinity for fungal membranes. Above a certain concentration, enough molecules enter the fungal cell membrane to create a pore in the cell wall, which leads to loss of cell contents and cell death. This study identified 330 iturinic lipopeptide clusters in publicly available genomes from the B. subtilis species group. The clusters were subsequently assigned into distinguishable types on the basis of their unique amino acid sequences and then verified by HPLC MS/MS analysis. The results show some lipopeptides are only produced by one species, whereas certain others can produce up to three. In addition, four species previously not known to produce iturinic lipopeptides were identified. The distribution of these compounds among the B. subtilis group species suggests that they play an important role in their speciation and evolution.

Antifungal Activity of Isolated Bacillus amyloliquefaciens SYBC H47 for the Biocontrol of Peach Gummosis

The gummosis disease is caused by Botryosphaeria dothidea (Moug. ex. Fr) Ces. et de Not., and it is one of the most important diseases of stone fruits worldwide. The use of biocontrol as an alternative approach to synthetic chemical fungicides has aroused general concern about how to control plant diseases that are caused by phytopathogens. The aim of this study is to isolate Bacillus strains from raw honeys with the capacity to inhibit B. dothidea and to explore the mechanisms by which they could be used in the biocontrol of peach gummosis. Bacillus amyloliquefaciens SYBC H47 was isolated and identified on the basis of its physiological and biochemical characteristics and its 16S rRNA and gyrB gene sequences. The cell suspension and the cell-free supernatant of its culture showed significant antifungal activity against Aspergillus niger, Mucor racemosus, Fusarium oxysporum, Penicillium citrinum, and Candida albicans by agar-diffusion assays. The primary antifungal substances were bacillomycin L, fengycin, and surfactin, which were analyzed by HPLC LC/ESI-MS/MS. Bacillomycin L showed the best inhibitory effect against conidial germination of B. dothidea, followed by fengycin and surfactin. Surfactin had limited effects on mycelial growth, contrary to those of bacillomycin L and fengycin. However, a mixture of the three lipopeptides had a synergistic effect that disrupted the structure of the conidia and mycelia. In order to reduce the production cost, the use of waste frying peanut oil and soy oil as the sole carbon source increased the lipopeptide yield levels by approximately 17% (2.42 g/L) and 110% (4.35 g/L), respectively. In a field trial, the decreases in the infected gummosis rate (IGR) and the disease severity index (DSI) through cell suspension treatments were 20% and 57.5% (in 2014), respectively, and 40% and 57.5% (in 2015), respectively, in comparison with the control. In conclusion, B. amyloliquefaciens SYBC H47 could inhibit the germination of conidia and the growth of mycelia from B. dothidea; therefore, this strain behaves as a potential biocontrol agent against the gummosis disease.

Immunostimulation by phospholipopeptide biosurfactant from Staphylococcus hominis in Oreochromis mossambicus

The immunostimulatory effect of phospholipopeptide biosurfactant from Staphylococcus hominis (GenBank Accession No: KJ564272) was assessed with Oreochromis mossambicus. The non-specific (serum lysozyme activity, serum antiprotease activity, serum peroxidase activity and serum bactericidal activity), specific (bacterial agglutination assay) immune responses and disease resistance activity against Aeromonas hydrophila were examined. Fish were intraperitonially injected with water soluble secondary metabolite (biosurfactant) of S. hominis at a dose of 2 mg, 20 mg and 200 mg kg(-1) body weight. Commercial surfactant surfactin (sigma) at 20 mg kg(-1) was used as standard and saline as negative control. All the doses of water soluble biosurfactant tested, significantly enhanced the specific, nonspecific immunity and disease resistance from the day of post administration of phospholipopeptide biosurfactant till the tail of the experimental period. These results clearly indicated that the secondary metabolite isolated from S. hominis stimulates the immunity of finfish thereby could enhance aquaculture production.

Isolation of anteiso-C17, iso-C17, iso-C16, and iso-C15 bacillomycin D from Bacillus amyloliquefaciens SD-32 and their antifungal activities against plant pathogens

Two new cyclic lipopeptides (3 and 4) were isolated from the culture filtrate of Bacillus amyloliquefaciens strain SD-32, together with two known metabolites, iso-C15 and iso-C16 bacillomycin D (1 and 2). Spectroscopic and chemical analyses identified the structures of the new compounds 3 and 4 as anteiso-C17 bacillomycin D, cyclic (l-Asn-d-Tyr-d-Asn-l-Pro-l-Glu-d-Ser-l-Thr-3-amino-14-methylhexadecanoic acid) and iso-C17 bacillomycin D, cyclic (l-Asn-d-Tyr-d-Asn-l-Pro-l-Glu-d-Ser-l-Thr-3-amino-15-methylhexadecanoic acid), respectively. The absolute configuration of C-3 in the β-amino fatty acid was determined to be R on the basis of the CD spectrum of its dinitrophenyl-p-methoxyaniline derivative. The activities of compounds 1-4 were evaluated against 13 plant pathogens: the activities of anteiso- and iso-C17 bacillomycin D (3 and 4) were almost the same and stronger than those of iso-C15 and iso-C16 bacillomycin D (1 and 2); iso-C15 bacillomycin D (1) was weakest. Compounds 2-4 inhibited the growth of all fungi tested; however, Pythium aphanidermatum was not inhibited at all by any of the compounds. Furthermore, compounds 1-4 at concentrations of 80, 40, 30, and 30 μM, respectively, inhibited completely the Botrytis cinerea infection in cucumber leaf.

Isolation and characterization of diverse antimicrobial lipopeptides produced by Citrobacter and Enterobacter

BACKGROUND

Increasing multidrug-resistance in bacteria resulted in a greater need to find alternative antimicrobial substances that can be used for clinical applications or preservation of food and dairy products. Research on antimicrobial peptides including lipopeptides exhibiting both narrow and broad spectrum inhibition activities is increasing in the recent past. Therefore, the present study was aimed at isolation and characterization of antimicrobial lipopeptide producing bacterial strains from fecal contaminated soil sample.

RESULTS

The phenotypic and 16S rRNA gene sequence analysis of all isolates identified them as different species of Gram-negative genera Citrobacter and Enterobacter. They exhibited common phenotypic traits like citrate utilization, oxidase negative and facultative anaerobic growth. The HPLC analysis of solvent extracts obtained from cell free fermented broth revealed the presence of multiple antimicrobial lipopeptides. The comprehensive mass spectral analysis (MALDI-TOF MS and GC-MS) of HPLC purified fractions of different isolates revealed that the lipopeptides varied in their molecular weight between (m/z) 607.21 to 1536.16 Da. Isomers of mass ion m/z 984/985 Da was produced by all strains. The 1495 Da lipopeptides produced by strains S-3 and S-11 were fengycin analogues and most active against all strains. While amino acid analysis of lipopeptides suggested most of them had similar composition as in iturins, fengycins, kurstakins and surfactins, differences in their β-hydroxy fatty acid content proposed them to be isoforms of these lipopeptides.

CONCLUSION

Although antimicrobial producing strains can be used as biocontrol agents in food preservation, strains with ability to produce multiple antimicrobial lipopeptides have potential applications in biotechnology sectors such as pharmaceutical and cosmetic industry. This is the first report on antibacterial lipopeptides production by strains of Citrobacter and Enterobacter.

Conformational analyses of bacillomycin D, a natural antimicrobial lipopeptide, alone or in interaction with lipid monolayers at the air-water interface

Bacillomycin D is a natural antimicrobial lipopeptide belonging to the iturin family. It is produced by Bacillus subtilis strains. Bacillomycin D is characterized by its strong antifungal and hemolytic properties, due to its interaction with the plasma membrane of sensitive cells. Until now, only few limited analyses were conducted to understand the biological activities of bacillomycin D at the molecular level. Our purpose was to analyze the conformation of bacillomycin D using IR spectroscopy and to model its interactions with cytoplasmic membranes using Langmuir interfacial monolayers. Our findings indicate that bacillomycin D contains turns and allow to model its three-dimensional structure. Bacillomycin D formed a monolayer film at the air-water interface and kept its turn conformation, as shown by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). To identify the membrane lipid target of bacillomycin D, its interactions with pure lipid monolayers were analyzed and an original behavior of the lipopeptide toward cholesterol-containing monolayers was shown. This original behavior was lost when bacillomycin D was interacting with pure cholesteryl acetate monolayers, suggesting the involvement of the alcohol group of cholesterol in the lipopeptide-cholesterol interaction.

Effects of different amino acids in culture media on surfactin variants produced by Bacillus subtilis TD7

Surfactin produced by Bacillus subtilis has different variants, which are affected by the composition of substrate available. To demonstrate the effects of amino acids on surfactin variants, B. subtilis TD7 was cultivated under the same conditions but with different amino acids supplied in media, respectively, and the type as well as the proportion of surfactin variants produced was analyzed with electrospray ionization mass spectrometry and gas chromatography-mass spectrometry. The result shows that the addition of different amino acids significantly influences the proportion of surfactin variants with different fatty acids. When Arg, Gln, or Val was added to the culture medium of B. subtilis TD7, the proportion of produced surfactin variants with even β-hydroxy fatty acids significantly increased, while the addition of Cys, His, Ile, Leu, Met, Ser, or Thr enhanced the proportion of surfactin variants with odd β-hydroxy fatty acids markedly. This result may be of some reference value in enhancing the production of specific surfactin variants as well as in the research on the relationship between culture media and the corresponding products of a certain bacterium.

Interactions of the natural antimicrobial mycosubtilin with phospholipid membrane models

Among the secondary metabolite lipopeptides produced by Bacillus subtilis, mycosubtilin is characterized by its strong antifungal activities. Even though its structure and its cellular target, the cytoplasmic membrane, have been determined, the molecular mechanisms of the biological activity of mycosubtilin have not been completely elucidated. In this work, the interactions between mycosubtilin and cytoplasmic membranes were modelled by using biomimetic systems such as Langmuir monolayers at the air-water interface and lipid multilamellar vesicles. The interactions of mycosubtilin with these biomimetic systems were examined, for the first time, by using specific techniques such as polarization modulation infrared reflection absorption spectroscopy, Brewster angle microscopy and high-resolution magic angle spinning NMR. Our findings indicate that mycosubtilin alone, at the air-water interface, forms a monolayer film and keeps its turn conformation. In the presence of DMPC, mycosubtilin binds to phospholipid monolayers, in a surface pressure-dependent manner. This binding results in the appearance of condensed domains which can be due to the formation of mycosubtilin clusters and/or to the lipopeptide aggregation with some phospholipid molecules and/or the formation of liquid-condensed domains of DMPC. Furthermore, in multilamellar vesicles, the mycosubtilin-DMPC interactions take place at the level of the aliphatic chains of the phospholipid because the phase transition temperature of DMPC decreased in the presence of mycosubtilin.

A novel lipopeptide produced by a Pacific Ocean deep-sea bacterium, Rhodococcus sp. TW53

AIMS

Our goal was to find a novel, biosurfactant-producing bacterium from Pacific Ocean deep-sea sediments.

METHODS AND RESULTS

An oil-degrading biosurfactant-producing bacterium TW53 was obtained from deep-sea sediment, and was identified through 16S rDNA analysis as belonging to the genus Rhodococcus. It lowered the surface tension of its culture to 34.4 mN m(-1). Thin layer chromatography (TLC) showed that the crude biosurfactants of TW53 were composed of lipopeptides and free fatty acids (FA). The lipopeptides were purified with column chromatography and then hydrolysed with 6 mol l(-1) HCl. Gas chromatography-mass spectrometry analysis showed that the hydrolyte in the hydrophobic fraction contained five kinds of FA with chain lengths of C(14)-C(19), and C(16)H(32)O(2) was a major component making up 59.18% of the total. However, 3-hydroxyl FA was not found, although it is usually found in lipopeptides. Silica gel TLC revealed that the hydrolyte in the hydrophilic fraction was composed of five kinds of amino acids; consistently, ESI-Q-TOF-MS analysis confirmed the composition results and provided their sequence tentatively as Ala-Ile-Asp-Met-Pro. Furthermore, the yield and CMC (critical micelle concentrations) of purified lipopeptides were examined. The purified product reduced the surface tension of water to 30.7 mN m(-1) with a CMC value of 23.7 mg l(-1). These results suggest that Rhodococcus sp. TW53 produces a novel lipopeptide that we have named rhodofactin.

CONCLUSION

The deep-sea isolate Rhodococcus sp. TW53 was the first reported lipopeptide-producing bacterium of this genus. The lipopeptides had novel chemical compositions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Rhodococcus sp. TW53 has potential in the exploration of new biosurfactants and could be used in bioremediation of marine oil pollution.

Importance of 3-hydroxy fatty acid composition of lipopeptides for biosurfactant activity

Biosurfactant production may be an economic approach to improving oil recovery. To obtain candidates most suitable for oil recovery, 207 strains, mostly belonging to the genus Bacillus, were tested for growth and biosurfactant production in medium with 5% NaCl under aerobic and anaerobic conditions. All strains grew aerobically with 5% NaCl, and 147 strains produced a biosurfactant. Thirty-five strains grew anaerobically with 5% NaCl, and two produced a biosurfactant. In order to relate structural differences to activity, eight lipopeptide biosurfactants with different specific activities produced by various Bacillus species were purified by a new protocol. The amino acid compositions of the eight lipopeptides were the same (Glu/Gln:Asp/Asn:Val:Leu, 1:1:1:4), but the fatty acid compositions differed. Multiple regression analysis showed that the specific biosurfactant activity depended on the ratios of both iso to normal even-numbered fatty acids and anteiso to iso odd-numbered fatty acids. A multiple regression model accurately predicted the specific biosurfactant activities of four newly purified biosurfactants (r2= 0.91). The fatty acid composition of the biosurfactant produced by Bacillus subtilis subsp. subtilis strain T89-42 was altered by the addition of branched-chain amino acids to the growth medium. The specific activities of biosurfactants produced in cultures with different amino acid additions were accurately predicted by the multiple regression model derived from the fatty acid compositions (r2= 0.95). Our work shows that many strains of Bacillus mojavensis and Bacillus subtilis produce biosurfactants and that the fatty acid composition is important for biosurfactant activity.

Matrix-assisted laser desorption ionization--time of flight mass spectrometry of lipopeptide biosurfactants in whole cells and culture filtrates of Bacillus subtilis C-1 isolated from petroleum sludge

An innovative method was developed for rapid sensitive detection and efficient structural characterization of lipopeptide biosurfactants by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry by using whole microbial cells and crude culture filtrates as targets in combination with surface tension measurements. This was done for a bacterial strain that was isolated from petroleum sludge and efficiently produces biosurfactants. This organism was identified by using biochemical, physiological, and genetic parameters as a Bacillus subtilis strain, designated B. subtilis C-1. This assignment was supported by a mass spectrometric investigation of the secondary metabolite spectrum determined by whole-cell MALDI-TOF mass spectrometry, which revealed three lipopeptide complexes, the surfactins, the iturins, and the fengycins, which are well-known biosurfactants produced by B. subtilis strains. These compounds were structurally characterized by in situ structure analysis by using postsource decay MALDI-TOF mass spectrometry. The isoforms were separated by miniaturized high-resolution reversed-phase high-performance liquid chromatography for mass spectrometric characterization. Iturin compounds which contain unusual fatty acid components were detected.

[New antifungal antibiotics, bacillopeptins and fusaricidins]

We isolated four strains of bacteria producing antifungal antibiotics from the rhizosphere of garlic with basal rot caused by the plant pathogenic fungal strain Fusarium oxysporum. Among them, Bacillus subtilis FR-2 was found to produce new antifungal antibiotics, named bacillopeptins A, B, and C. Their structures have been determined by 1D and 2D NMR and MS experiments, and amino acid analysis coupled with chiral HPLC, to be cyclic lipopeptides each containing a long-chain beta-amino acid. Another bacterial strain, Bacillus polymyxa KT-8, was shown to produce new antifungal antibiotics named fusaricidins A, B, C, and D which are more potent than bacillopeptins in their antimicrobial activity. The structures of the fusaricidins have been elucidated similarly as bacillopeptins to be cyclic hexadepsipeptides all containing 15-guanidino-3-hydroxypentadecanoic acid as a side chain. Fusaricidins strongly inhibit the growth of various kinds of fungi and moreover surprisingly show strong inhibitory activity against Gram-positive bacteria such as Staphylococcus aureus or Micrococcus luteus.

NMR structure of antibiotics plipastatins A and B from Bacillus subtilis inhibitors of phospholipase A(2)

Plipastatins A and B are antifungal antibiotics belonging to a family of lipopeptides capable of inhibiting phospholipase A(2) (PLA(2)) and are biosynthesised under certain circumstances by Bacillus subtilis. U-(15)N plipastatins A and B were obtained from cultures of the strain NCIB 8872 on a Landy medium modified for stable-isotope labelling by the substitution of the L-glutamic acid used as the sole nitrogen source, by (15)NH(4)Cl. These two lipo-decapeptides, lactonised by esterification of the Ile10 C-terminus with the phenolic hydroxyl of Tyr3, differ only by a D-Ala (plipastatin A)/D-Val (plipastatin B) substitution at the position 6. The (1)H- and (15)N-nuclear magnetic resonance (NMR) signals of a 4:6 mixture of plipastatins A and B were unambiguously assigned and their structures in dimethylsulfoxide solution were calculated on the basis of a set of NMR-derived restraints. Plipastatins A and B are well-defined structures in solution stabilised by a type 1 beta-turn comprising residues 6-9 and several other specific hydrogen bonds. The structures afford a first molecular basis for the future studies of their biological activities both in lipidic layers or on PLA(2).

NMR structure of active and inactive forms of the sterol-dependent antifungal antibiotic bacillomycin L

The antifungal antibiotic lipopeptide bacillomycin L [cyclo-(L-Asp1-D-Tyr2-D-Asn3-L-Ser4-L-Gln5-D-Ser6++ +-L-Thr7-beta-amino fatty acid)] from Bacillus subtilis belongs to the iturinic family of antifungal agents and acts with a strict sterol-phospholipid dependence on biomembranes. This antibiotic has been analysed using solution NMR spectroscopy in its native active form and its inactive (L-Asp1, D-Tyr2) di-O-methylated form. The structures were calculated under NMR-derived restraints using molecular-dynamic simulated-annealing protocols starting from a random array of atoms. The structure of the native antibiotic is spread over different conformers in which two families are recognized. It was found that most structures have dihedral phi and psi angles defining a type-II' beta-turn including amino acids 5-8, in certain cases stabilized by a 8HN-5CO hydrogen bond, whereas a minority of structures adopt an inverse gamma-turn including amino acids 6-8, stabilized in all cases by an 8HN-6CO hydrogen bond. The di-O-methylation of L-Asp1 and D-Tyr2, an amino acid strictly conserved within the iturinic group of antibiotics, does not induce major differences in the NMR spectra and in the NMR structures. The results are discussed in relation to the specific loss of interaction with sterols when the native antifungal bacillomycin L is methylated on the conserved D-Tyr2 position.

Studies on lipopeptide biosynthesis by Bacillus subtilis: isolation and characterization of iturin-, surfactin+ mutants

Two mutant strains, M35 and M89, were obtained by UV irradiation from a wild-type Bacillus subtilis producing iturin and surfactin. Sporulation and surfactin production were similar in both mutants and in the parent strain, while the iturin production of M35 was 300-fold less than that of the wild-type strain; M89 did not produce any iturin. The analysis of the incorporation of sodium [1-14C]acetate into cellular lipids and lipopeptides showed that M89 still synthesized beta-amino fatty acids, the lipid moiety of iturin.

Peptides

If we include beta-lactam antibiotics on the grounds that they have the same biosynthetic origin, peptides remain commercially the most important group of pharmaceuticals. However, our increasing knowledge of the genetic and enzymic background to biosynthesis, and of the regulation of metabolite production, will eventually bring a more unified approach to bioactive compounds. Mixing of structural types will become important, and we will be able to use our knowledge of biosynthetic genes and their regulatory networks. We will also benefit from an appreciation of the modular organization of catalytic functions, substrate transfer mechanisms and signalling between interacting enzymes. Since all of this is, in fact, the basis for enzymic synthesis of complex natural products in vivo, the exploitation of living cells requires mastery of a formidable network of cellular controls and compartments. For the present we are able to see fascinating connections emerging between genes in a variety of reaction sequences, not only in biosynthetic but also in degradative pathways. Peptide synthetases show surprising similarities to acylcoenzyme A synthetases, which are key enzymes in forming polyketides as well as in generating the CoA-derivatives that serve as substrates in degradative pathways. 4'-Phosphopantetheine, the functional half of CoA, plays a key role as the intrinsic transfer cofactor in various multienzyme systems. The comparatively small catalogue of reactions modifying natural products, notably epimerization, methylation, hydroxylation, decarboxylation (of peptides) and reduction/dehydration (of polyketides) can be found within or amongst biosynthetic proteins, generally as modules and organized in a specified order. The biochemist is coming close to the synthetic chemist's recipes, and may soon be recruiting proteins to carry them out.

Biosynthesis of bacillomycin D by Bacillus subtilis. Evidence for amino acid-activating enzymes by the use of affinity chromatography

Bacillomycin D is an antifungal lipopeptide produced by B. subtilis. The formation of the peptidyl bonds of bacillomycin D occurs non-ribosomally, as demonstrated by the use of chloramphenicol, an inhibitor of protein biosynthesis. Amino acid-activating enzymes were found in B. subtilis cell lysates purified by affinity chromatography on a gel containing L-Pro, an amino acid of bacillomycin D. Presence of ATP during this purification increases the binding of enzymatic proteins and their activity. An enzyme, with an apparent molecular weight of 230 kDa, catalyzed ATP-PPi exchange reactions, which were mediated by specific amino acids, corresponding to a partial sequence of bacillomycin D.

Synthesis of beta-hydroxy fatty acids and beta-amino fatty acids by the strains of Bacillus subtilis producing iturinic antibiotics

The iturinic antibiotics, which contain long chain beta-amino acids, are produced by Bacillus subtilis. Screening these strains for the presence of a possible precursor of the iturinic antibiotics, we isolated a lipopeptide containing beta-hydroxy fatty acids. The structure of this compound was studied and it appears to be identical or structurally very similar to surfactin. The carbon chain of its beta-hydroxy fatty acids was n C16, iso C16, iso C15 or anteiso C15. The percentages of each beta-hydroxy fatty acids varied according to the strain producing iturinic antibiotics and were influenced by addition of branched-chain alpha-amino acids to the culture medium. These results demonstrate for the first time that iso C14 beta-hydroxy fatty acid is a constituent present in such a surfactin like lipopeptide. Besides, the presence of radioactive beta-hydroxy fatty acids in the phospholipids when the strains were grown in the presence of sodium [14C]acetate seems also characterize the different strains producing iturinic antibiotics.

Studies on the biosynthesis of iturin, an antibiotic of Bacillus subtilis, and a lipopeptide containing beta-hydroxy fatty acids

The biosynthesis of iturin, an antibiotic containing a beta-amino fatty acid, was studied by incubating Bacillus subtilis in the presence of various 14C-labelled precursors. Sodium acetate or palmitic acid were incorporated into the beta-amino acids of iturin. Among the alpha-amino acids (asparagine, glutamine, serine, proline and tyrosine) in the peptidic part of iturin, asparagine appears to be the best precursor. In the presence of sodium [14C]acetate or [14C]asparagine, there was a synthesis of radioactive compound (compound X) before the synthesis of radioactive iturin. Compound X contained asparagine and/or aspartic acid, glutamine and/or glutamic acid and beta-hydroxy fatty acids.