Identification of a gene cluster encoding three high-molecular-weight proteins, which is required for synthesis of tolaasin by the mushroom pathogen Pseudomonas tolaasii

Exopolysaccharides from Pseudomonas tolaasii inhibit the growth of Pleurotus ostreatus mycelia

In the present study, the effect of exopolysaccharides (EPSs) extracted from Pseudomonas tolaasii on the growth of Pleurotus ostreatus mycelia was determined. P. ostreatus mycelia was cultivated with different concentrations of P. tolaasii EPSs, and their mycelial growth rate, protein content, and enzyme activity were measured and compared. The results showed that EPSs inhibited the growth of P. ostreatus. The proline and vitamin C contents of P. ostreatus increased at an EPS concentration of 40%. The cellulase, α-amylase, protein, and glucose utilisation rates of P. ostreatus gradually decreased with the increase in EPS concentration. Altogether, P. tolaasii EPSs had a significant inhibitory effect on mycelial growth. Therefore, we concluded that in addition to tolaasin, EPSs may also be the virulence factors responsible for the pathogenesis of P. tolaasii.

Comparative Studies on the Disease Prevalence and Population Dynamics of Ginger Blotch and Brown Blotch Pathogens of Button Mushrooms

Bacterial blotch is one of the most economically important diseases of button 'mushroom. Knowledge of mechanisms of disease expression, inoculum thresholds, and disease management is limited to the most well-known pathogen, Pseudomonas tolaasii. Recent outbreaks in Europe have been attributed to 'P. gingeri' and P. salomonii for ginger and brown blotch, respectively. Information about their identity, infection dynamics, and pathogenicity is largely lacking. The disease pressure in an experimental mushroom cultivation facility was evaluated for 'P. gingeri' and P. salomonii over varying inoculation densities, casing soil types, environmental humidity, and cultivation cycles. The pathogen population structures in the casing soils were simultaneously tracked across the cropping cycle using highly specific and sensitive TaqMan-quantitative PCR assays. 'P. gingeri' caused disease outbreaks at lower inoculum thresholds (104 CFU/g) in the soil than P. salomonii (105 CFU/g). Ginger blotch generically declined in later harvest cycles, although brown blotch did not. Casing soils were differentially suppressive to blotch diseases, based on their composition and supplementation. Endemic pathogen populations increased across the cultivation cycle although the inoculated pathogen populations were consistent between the first and second flush. In conclusion, 'P. gingeri' and P. salomonii have unique infection and population dynamics that vary over soil types. Their endemic populations are also differently abundant in peat-based casing soils. This knowledge is essential for interpreting diagnostic results from screening mushroom farms and designing localized disease control strategies.

Genomic Characterisation of Mushroom Pathogenic Pseudomonads and Their Interaction with Bacteriophages

Bacterial diseases of the edible white button mushroom Agaricus bisporus caused by Pseudomonas species cause a reduction in crop yield, resulting in considerable economic loss. We examined bacterial pathogens of mushrooms and bacteriophages that target them to understand the disease and opportunities for control. The Pseudomonastolaasii genome encoded a single type III protein secretion system (T3SS), but contained the largest number of non-ribosomal peptide synthase (NRPS) genes, multimodular enzymes that can play a role in pathogenicity, including a putative tolaasin-producing gene cluster, a toxin causing blotch disease symptom. However, Pseudomonasagarici encoded the lowest number of NRPS and three putative T3SS while non-pathogenic Pseudomonas sp. NS1 had intermediate numbers. Potential bacteriophage resistance mechanisms were identified in all three strains, but only P. agarici NCPPB 2472 was observed to have a single Type I-F CRISPR/Cas system predicted to be involved in phage resistance. Three novel bacteriophages, NV1, ϕNV3, and NV6, were isolated from environmental samples. Bacteriophage NV1 and ϕNV3 had a narrow host range for specific mushroom pathogens, whereas phage NV6 was able to infect both mushroom pathogens. ϕNV3 and NV6 genomes were almost identical and differentiated within their T7-like tail fiber protein, indicating this is likely the major host specificity determinant. Our findings provide the foundations for future comparative analyses to study mushroom disease and phage resistance.

Six Multiplex TaqManTM-qPCR Assays for Quantitative Diagnostics of Pseudomonas Species Causative of Bacterial Blotch Diseases of Mushrooms

Bacterial blotch is a group of economically important diseases of the common button mushroom (Agaricus bisporus). Once the pathogens are introduced to a farm, mesophilic growing conditions (that are optimum for mushroom production) result in severe and widespread secondary infections. Efficient, timely and quantitative detection of the pathogens is hence critical for the design of localized control strategies and prediction of disease risk. This study describes the development of real-time TaqMan^TM assays that allow molecular diagnosis of three currently prevalent bacterial blotch pathogens: “Pseudomonas gingeri,” Pseudomonas tolaasii and (as yet uncharacterized) Pseudomonas strains (belonging to Pseudomonas salomonii and Pseudomonas edaphica). For each pathogen, assays targeting specific DNA markers on two different loci, were developed for primary detection and secondary verification. All six developed assays showed high diagnostic specificity and sensitivity when tested against a panel of 63 Pseudomonas strains and 40 other plant pathogenic bacteria. The assays demonstrated good analytical performance indicated by linearity across calibration curve (>0.95), amplification efficiency (>90%) and magnitude of amplification signal (>2.1). The limits of detection were optimized for efficient quantification in bacterial cultures, symptomatic tissue, infected casing soil and water samples from mushroom farms. Each target assay was multiplexed with two additional assays. Xanthomonas campestris was detected as an extraction control, to account for loss of DNA during sample processing. And the total Pseudomonas population was detected, to quantify the proportion of pathogenic to beneficial Pseudomonas in the soil. This ratio is speculated to be an indicator for blotch outbreaks. The multiplexed assays were successfully validated and applied by routine testing of diseased mushrooms, peat sources, casing soils, and water from commercial production units.

100 Years Since Tolaas: Bacterial Blotch of Mushrooms in the 21st Century

Among the biotic constraints of common mushroom (Agaricus bisporus) production, bacterial blotch is considered the most important mushroom disease in terms of global prevalence and economic impact. Etiology and management of bacterial blotch has been a major concern since its original description in 1915. Although Pseudomonas tolaasii is thought to be the main causal agent, various Pseudomonas species, as well as organisms from other genera have been reported to cause blotch symptoms on mushroom caps. In this review, we provide an updated overview on the etiology, epidemiology, and management strategies of bacterial blotch disease. First, diversity of the causal agent(s) and utility of high throughput sequencing-based approaches in the precise characterization and identification of blotch pathogen(s) is explained. Further, due to the limited options for use of conventional pesticides in mushroom farms against blotch pathogen(s), we highlight the role of balanced threshold of relative humidity and temperature in mushroom farms to combat the disease in organic and conventional production. Additionally, we discuss the possibility of the use of biological control agents (either antagonistic mushroom-associated bacterial strains or bacteriophages) for blotch management as one of the sustainable approaches for 21^st century agriculture. Finally, we aim to elucidate the association of mushroom microbiome in cap development and productivity on one hand, and blotch incidence/outbreaks on the other hand.

Draft Genome Sequence of Pseudomonas fluorescens Strain ITEM 17298, Associated with Cheese Spoilage

Pseudomonas fluorescens is a genetically and phenotypically heterogeneous species that is often reported as a spoiler of fresh foods, but it has recently been implicated in clinical infection. In this study, we sequenced the genome of P. fluorescens strain ITEM 17298, isolated from mozzarella cheese and able to cause several alterations under cold storage.

Classification of Isolates from the Pseudomonas fluorescens Complex into Phylogenomic Groups Based in Group-Specific Markers

The Pseudomonas fluorescens complex of species includes plant-associated bacteria with potential biotechnological applications in agriculture and environmental protection. Many of these bacteria can promote plant growth by different means, including modification of plant hormonal balance and biocontrol. The P. fluorescens group is currently divided into eight major subgroups in which these properties and many other ecophysiological traits are phylogenetically distributed. Therefore, a rapid phylogroup assignment for a particular isolate could be useful to simplify the screening of putative inoculants. By using comparative genomics on 71 P. fluorescens genomes, we have identified nine markers which allow classification of any isolate into these eight subgroups, by a presence/absence PCR test. Nine primer pairs were developed for the amplification of these markers. The specificity and sensitivity of these primer pairs were assessed on 28 field isolates, environmental samples from soil and rhizosphere and tested by in silico PCR on 421 genomes. Phylogenomic analysis validated the results: the PCR-based system for classification of P. fluorescens isolates has a 98.34% of accuracy and it could be used as a rapid and simple assay to evaluate the potential of any P. fluorescens complex strain.

Pseudomonas protegens Pf-5 causes discoloration and pitting of mushroom caps due to the production of antifungal metabolites

Bacteria in the diverse Pseudomonas fluorescens group include rhizosphere inhabitants known for their antifungal metabolite production and biological control of plant disease, such as Pseudomonas protegens Pf-5, and mushroom pathogens, such as Pseudomonas tolaasii. Here, we report that strain Pf-5 causes brown, sunken lesions on peeled caps of the button mushroom (Agaricus bisporus) that resemble brown blotch symptoms caused by P. tolaasii. Strain Pf-5 produces six known antifungal metabolites under the control of the GacS/GacA signal transduction system. A gacA mutant produces none of these metabolites and did not cause lesions on mushroom caps. Mutants deficient in the biosynthesis of the antifungal metabolites 2,4-diacetylphloroglucinol and pyoluteorin caused less-severe symptoms than wild-type Pf-5 on peeled mushroom caps, whereas mutants deficient in the production of lipopeptide orfamide A caused similar symptoms to wild-type Pf-5. Purified pyoluteorin and 2,4-diacetylphloroglucinol mimicked the symptoms caused by Pf-5. Both compounds were isolated from mushroom tissue inoculated with Pf-5, providing direct evidence for their in situ production by the bacterium. Although the lipopeptide tolaasin is responsible for brown blotch of mushroom caused by P. tolaasii, P. protegens Pf-5 caused brown blotch-like symptoms on peeled mushroom caps through a lipopeptide-independent mechanism involving the production of 2,4-diacetylphloroglucinol and pyoluteorin.

Common virulence factors for Pseudomonas tolaasii pathogenesis in Agaricus and Arabidopsis

Brown blotch of cultivatable mushrooms is a disease caused by the small peptide toxin (tolaasin) secreted by Pseudomonas tolaasii. Here we found that the wild type tolassin-producing P. tolaasii stain 6264 was capable of infection in Arabidopsis thaliana cotyledons, causing chlorotic symptoms and growth arrest as a result of bacterial proliferation. Seven virulence-attenuated mutants of P. tolaasii were isolated from the Agaricus bisporus screen using 2512 mariner-based transposon insertion mutants, and all of them displayed reduced virulence and bacterial proliferation in Arabidopsis infection as well. The transposon was inserted within the genes for tolassin biosynthesis and amino acid biosynthesis, and within an intergenic region between the genes of unknown function. The finding that some virulence factors are commonly required for both Agaricus and Arabidopsis infections suggests that Arabidopsis could be exploited to study the host-pathogen interaction involving P. tolaasii.

Genetic and functional characterization of cyclic lipopeptide white-line-inducing principle (WLIP) production by rice rhizosphere isolate Pseudomonas putida RW10S2

The secondary metabolite mediating the GacS-dependent growth-inhibitory effect exerted by the rice rhizosphere isolate Pseudomonas putida RW10S2 on phytopathogenic Xanthomonas species was identified as white-line-inducing principle (WLIP), a member of the viscosin group of cyclic lipononadepsipeptides. WLIP producers are commonly referred to by the taxonomically invalid name "Pseudomonas reactans," based on their capacity to reveal the presence of a nearby colony of Pseudomonas tolaasii by inducing the formation of a visible precipitate ("white line") in agar medium between both colonies. This phenomenon is attributed to the interaction of WLIP with a cyclic lipopeptide of a distinct structural group, the fungitoxic tolaasin, and has found application as a diagnostic tool to identify tolaasin-producing bacteria pathogenic to mushrooms. The genes encoding the WLIP nonribosomal peptide synthetases WlpA, WlpB, and WlpC were identified in two separate genomic clusters (wlpR-wlpA and wlpBC) with an operon organization similar to that of the viscosin, massetolide, and entolysin biosynthetic systems. Expression of wlpR is dependent on gacS, and the encoded regulator of the LuxR family (WlpR) activates transcription of the biosynthetic genes and the linked export genes, which is not controlled by the RW10S2 quorum-sensing system PmrR/PmrI. In addition to linking the known phenotypes of white line production and hemolytic activity of a WLIP producer with WLIP biosynthesis, additional properties of ecological relevance conferred by WLIP production were identified, namely, antagonism against Xanthomonas and involvement in swarming and biofilm formation.

Characterization of pseudomonads isolated from decaying sporocarps of oyster mushroom

Pleurotus ostreatus is one of the most extensively cultivated mushrooms in the world; however, the success of cultivation often depends on the proliferation of different bacterial pathogens. Pseudomonas tolaasii is thought as the major cause of brown blotch disease of Agaricus bisporus and yellowing of Pleurotus ostreatus. In this study we examined the pathogenicity and assessed the industrial damage causing effect of 41 Pseudomonas strains isolated from deformed, yellowing oyster mushroom (P. ostreatus) sporocarps. Identification of the isolates at species level by the partial sequence analysis of the hypervariable region of the rpoB gene revealed nine Pseudomonas sps. We analyzed the presence of the tolaasin gene-cluster, the production of fluorescent pigments, the oxidase- and nitrite reductase activities, the growth at restrictive temperatures and the carbon source utilizing abilities of each strain. Complex lipopeptide production (including tolaasin) was examined with thin layer chromatography and a novel in vitro necrosis-test was developed and evaluated for the investigation of the pathogenic effect of Pseudomonas strains. Our results underline the importance of extracellular enzyme production in the sporocarp decaying process. Strong correlations were found between the secretion of trypsin-like proteases and lipases and the necrotic effect of these bacteria. All the results clearly established that besides Ps. tolaasii, Ps. fluorescens biovar V strains were pathogenic to P. ostreatus and cause serious losses during mushroom production. Our results underline the importance of extracellular enzyme production in the sporocarp decaying process, especially the trypsin-like proteases and lipases.

Microbially induced diseases of Agaricus bisporus: biochemical mechanisms and impact on commercial mushroom production

The button mushroom, Agaricus bisporus (Lange) Imbach, the most common cultivated mushroom, is susceptible to a wide range of virus, bacterial, and fungal diseases. However, only some diseases were studied for the mechanisms involved in the host-microorganism interaction. This review deals with biochemical mechanisms related to cavity disease (Burkholderia gladioli) and to the interaction between A. bisporus and the causal agents responsible for the most severe diseases, namely the bacteria Pseudomonas tolaasii and Pseudomonas reactans and the fungi Trichoderma aggressivum and Lecanicillium fungicola.

Bacterial mycophagy: definition and diagnosis of a unique bacterial-fungal interaction

This review analyses the phenomenon of bacterial mycophagy, which we define as a set of phenotypic behaviours that enable bacteria to obtain nutrients from living fungi and thus allow the conversion of fungal into bacterial biomass. We recognize three types of bacterial strategies to derive nutrition from fungi: necrotrophy, extracellular biotrophy and endocellular biotrophy. Each is characterized by a set of uniquely sequential and differently overlapping interactions with the fungal target. We offer a detailed analysis of the nature of these interactions, as well as a comprehensive overview of methodologies for assessing and quantifying their individual contributions to the mycophagy phenotype. Furthermore, we discuss future prospects for the study and exploitation of bacterial mycophagy, including the need for appropriate tools to detect bacterial mycophagy in situ in order to be able to understand, predict and possibly manipulate the way in which mycophagous bacteria affect fungal activity, turnover, and community structure in soils and other ecosystems.

Cyclic lipopeptide production by plant-associated Pseudomonas spp.: diversity, activity, biosynthesis, and regulation

Cyclic lipopeptides (CLPs) are versatile molecules produced by a variety of bacterial genera, including plant-associated Pseudomonas spp. CLPs are composed of a fatty acid tail linked to a short oligopeptide, which is cyclized to form a lactone ring between two amino acids in the peptide chain. CLPs are very diverse both structurally and in terms of their biological activity. The structural diversity is due to differences in the length and composition of the fatty acid tail and to variations in the number, type, and configuration of the amino acids in the peptide moiety. CLPs have received considerable attention for their antimicrobial, cytotoxic, and surfactant properties. For plant-pathogenic Pseudomonas spp., CLPs constitute important virulence factors, and pore formation, followed by cell lysis, is their main mode of action. For the antagonistic Pseudomonas sp., CLPs play a key role in antimicrobial activity, motility, and biofilm formation. CLPs are produced via nonribosomal synthesis on large, multifunctional peptide synthetases. Both the structural organization of the CLP synthetic templates and the presence of specific domains and signature sequences within peptide synthetase genes will be described for both pathogenic and antagonistic Pseudomonas spp. Finally, the role of various genes and regulatory mechanisms in CLP production by Pseudomonas spp., including two-component regulation and quorum sensing, will be discussed in detail.

Evidence for genotypic differences between the two siderovars of Pseudomonas tolaasii, cause of brown blotch disease of the cultivated mushroom Agaricus bisporus

Sixteen representative isolates of Pseudomonas tolaasii, the causal agent of brown blotch of the cultivated mushroom Agaricus bisporus, were previously assigned to two siderovars (sv1 and sv2) on the basis of pyoverdines synthesized. Each isolate was pathogenic and produced a typical white line precipitate when cultured adjacent to Pseudomonas "reactans" strain LMG 5329. These 16 isolates of P. tolaasii, representing sv1 and sv2, were further characterized using genotypic methods to examine the relationships between the isolates. Rep-PCR studies revealed two distinct patterns from these isolates, which were consistent with the siderovar grouping. Ribotyping differentiated P. tolaasii LMG 2342T (sv1) and PS 3a (sv2) into two distinct ribotypes. A pair of primers, targeted to a 2.1-kb fragment of tl1 (encoding a tolaasin peptide synthetase), yielded the same PCR product from P. tolaasii LMG 2342T (sv1) and PS 22.2 (sv1), but not from PS 3a (sv2). Southern blot analysis indicated that homologues of tl1 are present in PS 3a, but the pattern of hybridization differed from PS 22.2 and LMG 2342T. Sequence determination and analysis of the internally transcribed spacer region ITSI for P. tolaasii LMG 2342T, LMG 6641, and PS 3a strains further supported the presence of the two siderovars. It is concluded that considerable genotypic differences exist among Finnish isolates of P. tolaasii causing brown blotch disease on the cultivated mushroom, which is in agreement with the phenotypic diversity highlighted through previous siderotyping studies.

A physical map of the syringomycin and syringopeptin gene clusters localized to an approximately 145-kb DNA region of Pseudomonas syringae pv. syringae strain B301D

Genetic and phenotypic mapping of an approximately 145-kb DraI fragment of Pseudomonas syringae pv. syringae strain B301D determined that the syringomycin (syr) and syringopeptin (syp) gene clusters are localized to this fragment. The syr and syp gene clusters encompass approximately 55 kb and approximately 80 kb, respectively. Both phytotoxins are synthesized by a thiotemplate mechanism of biosynthesis, requiring large multienzymatic proteins called peptide synthetases. Genes encoding peptide synthetases were identified within the syr and syp gene clusters, accounting for 90% of the DraI fragment. In addition, genes encoding regulatory and secretion proteins were localized to the DraI fragment. In particular, the salA gene, encoding a regulatory element responsible for syringomycin production and lesion formation in P. syringae pv. syringae strain B728a, was localized to the syr gene cluster. A putative ATP-binding cassette (ABC) transporter homolog was determined to be physically located in the syp gene cluster, but phenotypically affects production of both phytotoxins. Preliminary size estimates of the syr and syp gene clusters indicate that they represent two of the largest nonribosomal peptide synthetase gene clusters. Together, the syr and syp gene clusters encompass approximately 135 kb of DNA and may represent a genomic island in P. syringae pv. syringae that contributes to virulence in plant hosts.

Genetic characterization of Pseudomonas 'NZI7'--a novel pathogen that results in a brown blotch disease of Agaricus bisporus

AIMS

To characterize a novel pseudomonad isolate capable of causing brown blotch disease of Agaricus bisporus.

METHODS AND RESULTS

Using the white-line-in-agar (WLA) assay, fluorescent pseudomonads isolated from a New Zealand mushroom farm were screened for the lipodepsipeptide tolaasin, a characteristic marker of Pseudomonas tolaasii. One isolate, NZI7, produced a positive WLA assay and caused brown lesions of A. bisporus comparable with those produced by Ps. tolaasii. However, genetic analysis suggested that Ps. tolaasii and NZI7 were genetically dissimilar, and that NZI7 is closely related to Pseudomonas syringae. Nucleotide sequence analyses of a gene involved in tolaasin production indicated that similar genes are present in both NZI7 and Ps. tolaasii.

CONCLUSION

NZI7 represents a novel Pseudomonas species capable of causing brown blotch disease of A. bisporus.

SIGNIFICANCE AND IMPACT OF THE STUDY

Phenotypic identification of Ps. tolaasii based on A. bisporus browning and positive WLA may have limited specificity.

Evidence and characterization of a gene cluster required for the production of viscosin, a lipopeptide biosurfactant, by a strain ofPseudomonas fluorescens

The genetic control of viscosin production was examined in a strain of Pseudomonas fluorescens (PfA7B) that causes broccoli head rot. Viscosin is a potent lipopeptide biosurfactant that enables the bacteria to come into intimate contact with the difficult-to-wet waxy heads of broccoli. Tn5 mutagenesis completely disrupted viscosin production as shown by HPLC analysis of the mutagenized cell lysates. The Vis–mutants retained their pectolytic capability and were able to decay potato slices. On broccoli, however, the Vis–mutants caused decay of wounded florets, but the decay failed to spread to adjacent nonwounded florets as had occurred with the wild-type PfA7B. Triparental matings of the Vis–mutants with their corresponding wild-type clones and the helper Escherichia coli HB101 carrying the mobilization plasmid pPK2013 resulted in three stable viscosin-producing transconjugants that caused typical decay of broccoli tissue. Linkage maps of clones and protein profiles showed that a 25-kb chromosomal DNA region of PfA7B affected the production of three high molecular mass proteins required for viscosin synthesis. These proteins, approximately 218, 215, and 137 kDa in size, likely compose a synthetase complex that assembles the nine amino acid peptide of viscosin and subsequently attaches this to the hydrophobic fatty acid component of the molecule. A probe made from this DNA region hybridized with DNA fragments of other phytopathogenic pseudomonads to varying degrees.Key words: virulence factor, head rot, broccoli.

Biochemical and physiological aspects of brown blotch disease of Agaricus bisporus

Pseudomonas tolaasii is a bacterium endemic to the compost beds where common mushroom (Agaricus bisporus) is cultivated. Under some environmental conditions still not well-determined, but influenced by temperature and relative humidity, the bacterium can become pathogenic and provoke the brown blotch disease. This review describes the interaction between P. tolaasii and A. bisporus that results in the appearance of brown spots on the mushroom caps, typical symptoms of the disease. Firstly, P. tolaasii is studied, the changes in pathogenicity are explained, the compounds that provoke the damage are enumerated as well as various experimental methods to identify the pathogenic form of the bacteria. Secondly, mechanisms involved in the formation of the brown colour on the A. bisporus caps upon infection are briefly mentioned, taking into account the enzymes that catalyse the reaction, their mechanism, substrates and reaction products. Afterwards, a detailed description of the infection process is presented step by step, starting by the chemotactical attraction, fixation, secretion of the toxins, membrane breakdown, effect of the toxin on mushroom polyphenol oxidases and on the discolouration reaction. A possible mechanism of infection is hypothesised at the molecular level. Finally, the strategies tested until now to control the disease are discussed.

Phenotypic variation and survival of genetically marked Pseudomonas tolaasii in mushroom compost

The role of Pseudomonas tolaasii as an important pathogen of the common mushroom Agaricus bisporus is difficult to study in the microbially complex growth medium used for mushroom production. Two strains of P. tolaasii that had been marked with kanamycin resistance and xylE genes were introduced individually into casing soil over mushroom compost. Survival studies revealed that P. tolaasii numbers in casing soil over mushroom compost decreased a 1000-fold in the first 9 days and then remained relatively stable over the rest of the monitoring period. The presence of the pathogenic colony form and the nonpathogenic colony variant was monitored on mushroom caps and in mushroom compost to detect any phenotypic variation while incubated in these environments. Reversion from the nonpathogenic to pathogenic form was detected following isolation and culture from diseased mushroom caps. Inoculation of the marked strains directly onto the cap or into compost beds seeded with A. bisporus resulted in the appearance of brown blotch symptoms.Key words: Pseudomonas tolaasii, Agaricus bisporus, phenotypic variation, mushroom disease.

Pseudomonas tolaasii: extra-genomic factor mediates toxin production and efficiency

Pseudomonas tolaasii is responsible for brown blotch symptoms on Agaricus bisporus. Investigations among 15 P. tolaasii wild-type strains revealed the existence of an extra-genomic factor. Pseudomonas tolaasii CNBP2152D, a spontaneous derivative of the wild-type strain CNBP2152 which has lost the factor, exhibited a significant decline in pathogenicity. Comparison of blotch symptoms induced by toxins extracted from CNBP2152 and CNBP2152D showed that the extragenomic factor mediates toxin production and efficiency.

Engineered biosynthesis of peptide antibiotics

In certain bacteria and filamentous fungi, a wide variety of bioactive peptides are produced non-ribosomally on large protein templates, called peptide synthetases. Recently, significant progress has been made towards understanding the modular arrangement of these complex multifunctional enzymes and the mechanisms by which they generate their corresponding peptide products. It has now been established that the synthesis of bioactive peptides and the specification of their sequence are brought about by a protein template that contains the appropriate number and the correct order of activating units (domains). These advances have enabled the development of a technique that permits the construction of hybrid genes encoding peptide synthetases with specifically altered substrate specificities. A programmed alteration within the primary structure of a peptide antibiotic is achieved by the substitution of an amino acid-activating domain in the corresponding protein template at the genetic level by a two-step recombination method. It utilizes successive gene disruption and reconstitution and demonstrates, for the first time, the potential of genetic engineering in the biosynthesis of novel peptide antibiotics. Many organisms, for instance those that cause diseases like tuberculosis and pneumonia, have evolved potent mechanisms of drug resistance. Therefore, the targeted engineering of peptide antibiotics could be one potential strategy for the development of novel drugs that overcome this resistance.

A nonribosomal system of peptide biosynthesis

This review covers peptide structures originating from the concerted action of enzyme systems without the direct participation of nucleic acids. Biosynthesis proceeds by formation of linear peptidyl intermediates which may be enzymatically modified as well as transformed into specific cyclic structures. The respective enzyme systems are constructed of biosynthetic modules integrated into multienzyme structures. Genetic and DNA-sequence analysis of biosynthetic gene clusters have revealed extensive similarities between prokaryotic and eukaryotic systems, conserved principles of organisation, and a unique mechanism of transport of intermediates during elongation and modification steps involving 4'-phospho-pantetheine. These similarities permit the identification of peptide synthetases and related aminoacyl-ligases and acyl-ligases from sequence data. Similarities to other biosynthetic systems involved in the assembly of polyketide metabolites are discussed.

The nonribosomal peptide biosynthetic system--on the origins of structural diversity of peptides, cyclopeptides and related compounds

A variety of peptides have been detected in microorganisms. Some have found applications in various fields, for example the classical beta-lactam antibiotics, immunosuppressors like cyclosporin, promising new antibacterials like teichoplanin or daptomycin and antifungals like echinocandin. For none of these has it been established how their complicated biosynthetic pathways have evolved or what functions they fulfill within or for their producers. So it is unclear what selection processes limit the range of their structural analogues within various groups of microorganisms. We here consider recent data in the field of biosynthesis and how they may suggest mechanisms of genetic diversity. These may illustrate the complexity of genetic and intracellular organization of biosynthetic pathways and indicate the cellular context of some metabolites related to the complex background of the production of each metabolite. Research focusing on various targets like the increase of productivity of fermentations or the spread of resistances to antibacterials is slowly being understood.

Identification and characterization of a locus which regulates multiple functions in Pseudomonas tolaasii, the cause of brown blotch disease of Agaricus bisporus

Pseudomonas tolaasii, the causal agent of brown blotch disease of Agaricus bisporus, spontaneously gives rise to morphologically distinct stable sectors, referred to as the phenotypic variant form, at the margins of the wild-type colonies. The phenotypic variant form is nonpathogenic and differs from the wild type in a range of biochemical and physiological characteristics. A genomic cosmid clone (pSISG29) from a wild-type P. tolaasii library was shown to be capable of restoring a range of characteristics of the phenotypic variant to those of the wild-type form, when present in trans. Subcloning and saturation mutagenesis analysis with Tn5lacZ localized a 3.0-kb region from pSISG29, designated the pheN locus, required for complementation of the phenotypic variant to the wild-type form. Marker exchange of the Tn5lacZ-mutagenized copy of the pheN locus into the wild-type strain demonstrated that a functional copy of the pheN gene is required to maintain the wild-type pathogenic phenotype and that loss of the pheN gene or its function results in conversion of the wild-type form to the phenotypic variant form. The pheN locus contained a 2,727-bp open reading frame encoding an 83-kDa protein. The predicted amino acid sequence of the PheN protein showed homology to the sensor and regulator domains of the conserved family of two component bacterial sensor regulator proteins. Southern hybridization analysis of pheN genes from the wild type and the phenotypic variant form revealed that DNA rearrangement occurs within the pheN locus during phenotypic variation. Analysis of pheN expression with a pheN::lacZ fusion demonstrated that expression is regulated by environmental factors. These results are related to a model for control for phenotypic variation in P. tolaasii.

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.