Gene disruption of the pcbAB gene encoding ACV synthetase in Cephalosporium acremonium

Diversity of Linear Non-Ribosomal Peptide in Biocontrol Fungi

Biocontrol fungi (BFs) play a key role in regulation of pest populations. BFs produce multiple non-ribosomal peptides (NRPs) and other secondary metabolites that interact with pests, plants and microorganisms. NRPs-including linear and cyclic peptides (L-NRPs and C-NRPs)-are small peptides frequently containing special amino acids and other organic acids. They are biosynthesized in fungi through non-ribosomal peptide synthases (NRPSs). Compared with C-NRPs, L-NRPs have simpler structures, with only a linear chain and biosynthesis without cyclization. BFs mainly include entomopathogenic and mycoparasitic fungi, that are used to control insect pests and phytopathogens in fields, respectively. NRPs play an important role of in the interactions of BFs with insects or phytopathogens. On the other hand, the residues of NRPs may contaminate food through BFs activities in the environment. In recent decades, C-NRPs in BFs have been thoroughly reviewed. However, L-NRPs are rarely investigated. In order to better understand the species and potential problems of L-NRPs in BFs, this review lists the L-NRPs from entomopathogenic and mycoparasitic fungi, summarizes their sources, structures, activities and biosynthesis, and details risks and utilization prospects.

RNA-silencing in Penicillium chrysogenum and Acremonium chrysogenum: validation studies using beta-lactam genes expression

In this work we report the development and validation of a new RNA interference vector (pJL43-RNAi) containing a double-stranded RNA expression cassette for gene silencing in the filamentous fungi Penicillium chrysogenum and Acremonium chrysogenum. Classical targeted gene disruption in these fungi is very laborious and inefficient due to the low frequency of homologous recombination. The RNAi vector has been validated by testing the attenuation of two different genes of the beta-lactam pathway; pcbC in P. chrysogenum and cefEF in A. chrysogenum. Quantification of mRNA transcript levels and antibiotic production showed knockdown of pcbC and cefEF genes in randomly isolated transformants of P. chrysogenum and A. chrysogenum, respectively. The process is efficient; 15 to 20% of the selected transformants were found to be knockdown mutants showing reduced penicillin or cephalosporin production. This new RNAi vector opens the way for exploring gene function in the genomes of P. chrysogenum and A. chrysogenum.

Isolation of Penicillium nalgiovense strains impaired in penicillin production by disruption of the pcbAB gene and application as starters on cured meat products

The presence of some fungi on a variety of food products, like cheeses or cured meat products, is beneficial for the ripening of the product and for the development of specific flavour features. The utilization of these fungi as starters, which are inoculated normally as asexual spores on the food products at the beginning of the ripening process, is becoming a usual procedure in the food industry. The starter culture also prevents undesirable fungi or bacteria from growing on the product. Penicillium nalgiovense is the most frequently used starter for cured and fermented meat products, but the fact that this fungus can secrete penicillin to the meat product makes it important to get strains unable to synthesize this antibiotic. In this work we report that P. nalgiovense strains impaired in penicillin production can be obtained by disruption of the pcbAB gene (the first gene of the penicillin biosynthetic pathway). When applied as starter on cecina (a salted, smoke-cured beef meat product from the region of León, Spain), the pcbAB-disrupted strain showed no differences with respect to the parental penicillin-producing strain in its ability to colonize the meat pieces and to control their normal mycoflora. Both strains exerted a similar control on the presence of bacteria in cecina. A similar proportion of penicillin-sensitive and penicillin-resistant bacteria were isolated from pieces inoculated with the penicillin-producing or the non-producing P. nalgiovense strains. The decrease of the bacterial population on the surface of cecina seems to be due to the higher competition for nutrients as a consequence of the inoculation and development of the P. nalgiovense mycelium and not due to the production of penicillin by this fungus. Penicillin production was less affected than growth in a solid medium with high NaCl concentrations; this suggests that the high salt concentration present in cecina is not a limiting factor for penicillin production by P. nalgiovense.

Peptide synthetase gene in Trichoderma virens

Trichoderma virens (synonym, Gliocladium virens), a deuteromycete fungus, suppresses soilborne plant diseases caused by a number of fungi and is used as a biocontrol agent. Several traits that may contribute to the antagonistic interactions of T. virens with disease-causing fungi involve the production of peptide metabolites (e.g., the antibiotic gliotoxin and siderophores used for iron acquisition). We cloned a 5,056-bp partial cDNA encoding a putative peptide synthetase (Psy1) from T. virens using conserved motifs found within the adenylate domain of peptide synthetases. Sequence similarities with conserved motifs of the adenylation domain, acyl transfer, and two condensation domains support identification of the Psy1 gene as a gene that encodes a peptide synthetase. Disruption of the native Psy1 gene through gene replacement was used to identify the function of this gene. Psy1 disruptants produced normal amounts of gliotoxin but grew poorly under low-iron conditions, suggesting that Psy1 plays a role in siderophore production. Psy1 disruptants cannot produce the major T. virens siderophore dimerum acid, a dipetide of acylated N(delta)-hydroxyornithine. Biocontrol activity against damping-off diseases caused by Pythium ultimum and Rhizoctonia solani was not reduced by the Psy1 disruption, suggesting that iron competition through dimerum acid production does not contribute significantly to disease suppression activity under the conditions used.

Targeted inactivation of the mecB gene, encoding cystathionine-gamma-lyase, shows that the reverse transsulfuration pathway is required for high-level cephalosporin biosynthesis in Acremonium chrysogenum C10 but not for methionine induction of the cephalosporin genes

Targeted gene disruption efficiency in Acremonium chrysogenum was increased 10-fold by applying the double-marker enrichment technique to this filamentous fungus. Disruption of the mecB gene by the double-marker technique was achieved in 5% of the transformants screened. Mutants T6 and T24, obtained by gene replacement, showed an inactive mecB gene by Southern blot analysis and no cystathionine-gamma-lyase activity. These mutants exhibited lower cephalosporin production than that of the control strain, A. chrysogenum C10, in MDFA medium supplemented with methionine. However, there was no difference in cephalosporin production between parental strain A. chrysogenum C10 and the mutants T6 and T24 in Shen's defined fermentation medium (MDFA) without methionine. These results indicate that the supply of cysteine through the transsulfuration pathway is required for high-level cephalosporin biosynthesis but not for low-level production of this antibiotic in methionine-unsupplemented medium. Therefore, cysteine for cephalosporin biosynthesis in A. chrysogenum derives from the autotrophic (SH(2)) and the reverse transsulfuration pathways. Levels of methionine induction of the cephalosporin biosynthesis gene pcbC were identical in the parental strain and the mecB mutants, indicating that the induction effect is not mediated by cystathionine-gamma-lyase.

Gene targeting in Penicillium chrysogenum: disruption of the lys2 gene leads to penicillin overproduction

Two strategies have been used for targeted integration at the lys2 locus of Penicillium chrysogenum. In the first strategy the disruption of lys2 was obtained by a single crossing over between the endogenous lys2 and a fragment of the same gene located in an integrative plasmid. lys2-disrupted mutants were obtained with 1.6% efficiency when the lys2 homologous region was 4.9 kb, but no homologous integration was observed with constructions containing a shorter homologous region. Similarly, lys2-disrupted mutants were obtained by a double crossing over (gene replacement) with an efficiency of 0.14% by using two lys2 homologous regions of 4.3 and 3.0 kb flanking the pyrG marker. No homologous recombination was observed when the selectable marker was flanked by short lys2 homologous DNA fragments. The disruption of lys2 was confirmed by Southern blot analysis of three different lysine auxotrophs obtained by a single crossing over or gene replacement. The lys2-disrupted mutants lacked alpha-aminoadipate reductase activity (encoded by lys2) and showed specific penicillin yields double those of the parental nondisrupted strain, Wis 54-1255. The alpha-aminoadipic acid precursor is channelled to penicillin biosynthesis by blocking the lysine biosynthesis branch at the alpha-aminoadipate reductase level.

Molecular regulation of beta-lactam biosynthesis in filamentous fungi

The most commonly used beta-lactam antibiotics for the therapy of infectious diseases are penicillin and cephalosporin. Penicillin is produced as an end product by some fungi, most notably by Aspergillus (Emericella) nidulans and Penicillium chrysogenum. Cephalosporins are synthesized by both bacteria and fungi, e.g., by the fungus Acremonium chrysogenum (Cephalosporium acremonium). The biosynthetic pathways leading to both secondary metabolites start from the same three amino acid precursors and have the first two enzymatic reactions in common. Penicillin biosynthesis is catalyzed by three enzymes encoded by acvA (pcbAB), ipnA (pcbC), and aatA (penDE). The genes are organized into a cluster. In A. chrysogenum, in addition to acvA and ipnA, a second cluster contains the genes encoding enzymes that catalyze the reactions of the later steps of the cephalosporin pathway (cefEF and cefG). Within the last few years, several studies have indicated that the fungal beta-lactam biosynthesis genes are controlled by a complex regulatory network, e. g., by the ambient pH, carbon source, and amino acids. A comparison with the regulatory mechanisms (regulatory proteins and DNA elements) involved in the regulation of genes of primary metabolism in lower eukaryotes is thus of great interest. This has already led to the elucidation of new regulatory mechanisms. Furthermore, such investigations have contributed to the elucidation of signals leading to the production of beta-lactams and their physiological meaning for the producing fungi, and they can be expected to have a major impact on rational strain improvement programs. The knowledge of biosynthesis genes has already been used to produce new compounds.

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.

Kinetics and Thermostability of NADP-Isocitrate Dehydrogenase from Cephalosporium acremonium

NADP-isocitrate dehydrogenase [isocitrate:NADP(sup+) oxidoreductase (decarboxylating); EC 1.1.1.42] was purified from Cephalosporium acremonium as a single species. The enzyme is a dimer of 140 kDa with identical subunits of 75 kDa. The existence of a monomer-dimer equilibrium is apparent as revealed by an enzyme dilution approach. The chelate complex of the tribasic form of isocitrate and Mg(sup2+) is the true substrate. The V(infmax) depends on a basic form of an ionizable group of the enzyme-substrate complex with a pK(infes) (pK of the enzyme-substrate complex) of 6.9 and a (Delta)H(infion) (activation enthalpy) of -2 (plusmn) 0.4 kcal mol(sup-1) (ca. 8 (plusmn) 2 kJ mol(sup-1)). The enzyme showed maximum activity at 60(deg)C, an unusually high temperature for a nonthermophilic fungus. The thermodynamic parameters for isocitrate oxidative decarboxylation and for the binding of isocitrate and NADP(sup+) were calculated. We analyzed the kinetic thermal stability of the enzyme at pH 6.5 and 7.6. It was inactivated above 40(deg)C following a first-order kinetics. The presence of 12 mM Mg(sup2+) plus 10 mM dl-isocitrate led to 100% protection of enzyme activity against inactivation at 60(deg)C for 120 min. Removal of either or both compounds led to activity loss. A greater stabilizing role for Mg(sup2+) was seen at pH 6.5 than at pH 7.6, whereas the stabilizing effect of isocitrate was not dependent on pH.

Genes for beta-lactam antibiotic biosynthesis

The genes pcbAB, pcbC and penDE encoding enzymes involved in the biosynthesis of penicillin have been cloned from Penicillium chrysogenum and Aspergillus nidulans. They are clustered in chromosome I (10.4 Mb) of P. chrysogenum, but they are located in chromosome II of Penicillium notatum (9.6 Mb) and in chromosome VI (3.0 Mb) of A. nidulans. Expression studies have shown that each gene is expressed as a single transcript from separate promoters. Enzyme regulation studies and gene expression analysis have provided useful information to understand the control of gene expression leading to overexpression of the genes involved in penicillin biosynthesis. Cephalosporin genes have been studied in Cephalosporium acremonium and also in cephalosporin-producing bacteria. In C. acremonium the genes involved in cephalosporin biosynthesis are separated in at least two clusters. Cluster I (pcbAB-pcbC) encodes the first two enzymes of the cephalosporin pathway which are very similar to those involved in penicillin biosynthesis. Cluster II (cefEF-cefG), encodes the last three enzymatic activities of the cephalosporin pathway. It is unknown, at this time, if the cefD gene encoding isopenicillin epimerase is linked to any of the two clusters. In cephamycin producing bacteria the genes encoding the entire biosynthetic pathway are located in a single cluster extending for about 30 kb in Nocardia lactamdurans, and in Streptomyces clavuligerus. The cephamycin clusters of N. lactamdurans and S. clavuligerus include a gene lat which encodes lysine-6-aminotransferase an enzyme involved in formation of the precursor alpha-aminoadipic acid. The N. lactamdurans cephamycin cluster includes, in addition, a beta-lactamase (bla) gene, a penicillin binding protein (pbp), and a transmembrane protein gene (cmcT) that is probably involved in secretion of the cephamycin. Little is known however about the mechanism of control of gene expression in the different beta-lactam producers. The availability of most of the structural genes provides a good basis for further studies on gene expression. This knowledge should lead in the next decade to a rational design of strain improvement procedures. The origin and evolution of beta-lactam genes is intriguing since their nucleotide sequences are extremely conserved despite their restricted distribution in the microbial world.

Disruption of the cyclosporin synthetase gene of Tolypocladium niveum

Cyclosporin A is a potent and clinically-important immunosuppressive drug (SandimmunR). It is produced by the fungus Tolypocladium niveum. A transformation system for T. niveum ATCC34921 based on hygromycin selection was established. In order to obtain a T. niveum promoter, the cyclophilin gene was isolated using the Neurospora crassa gene as probe. A plasmid vector was constructed in which the promoter region of the T. niveum cyclophilin gene was fused to a bacterial hygromycin phosphotransferase gene. Protoplasts were transformed with this plasmid and hygromycin-resistant transformants were isolated. Using this transformation system, mutants of T. niveum with disrupted versions of the cyclosporin synthetase gene (simA) were engineered by DNA-mediated transformation. Disruption of the gene resulted in loss of the ability to produce cyclosporins.

Analysis of the regulation of penicillin biosynthesis in Aspergillus nidulans by targeted disruption of the acvA gene

To analyse the regulation of the biosynthesis of the secondary metabolite penicillin in Aspergillus nidulans, a strain with an inactivated acvA gene produced by targeted disruption was used. acvA encodes delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase (ACVS), which catalyses the first step in the penicillin biosynthetic pathway. To study the effect of the inactivated acvA gene on the expression of acvA and the second gene, ipnA, which encodes isopenicillin N synthase (IPNS), A. nidulans strain XEPD, with the acvA disruption, was crossed with strain AXB4A carrying acvA-uidA and ipnA-lacZ fusion genes. Ascospores with the predicted non-penicillin producing phenotype and a hybridization pattern indicating the presence of the disrupted acvA gene, and the fusion genes integrated in single copy at the chromosomal argB locus were identified. Both fusion genes were expressed at the same level as in the non-disrupted strain. Western blot analysis (immunoblotting) revealed that similar amounts of IPNS enzyme were present in both strains from 24 to 68 h of a fermentation run. In the acvA disrupted strain, IPNS and acyl-CoA: 6-aminopenicillanic acid acyltransferase (ACT) specific activities were detected, excluding a sequential induction mechanism of regulation of the penicillin biosynthesis gene ipnA and the third gene aat.

Targeted integration into the Acremonium chrysogenum genome: disruption of the pcbC gene

The cephalosporin C-producing fungus Acremonium chrysogenum was transformed to hygromycin B resistance using different vector constructs. These constructs contain sequences of the pcbC gene from A. chrysogenum, encoding isopenicillin N synthetase. Detailed analysis of transformants, including pulsed-field gel electrophoresis (PFGE), suggests that integration of multiple vector copies takes place predominantly via non-homologous integration. By increasing the length of vector-DNA homologous to genomic DNA, integration occurs more frequently into chromosome VI, carrying the endogenous pcbC gene copy. In gene disruption experiments, the length of vector homology required to obtain cephalosporin C-minus transformants was investigated. Inactivation of the pcbC gene was observed only when homologous fragments of more than 3.0 kb were used on both sites of the resistance cassette. Southern analysis indicated homologous, as well as heterologous, integration of recombinant DNA. The integration of multiple vector copies leads to the appearance of truncated pcbC transcripts.

Evidence for a gene cluster involving trichothecene-pathway biosynthetic genes in Fusarium sporotrichioides

Two overlapping cosmid clones (Cos1-1 and Cos9-1) carrying the Tox5 gene were isolated from a library of F. sporotrichioides strain NRRL 3299 genomic DNA. These cosmids were used to transform three T-2 toxin-deficient mutants that are blocked at different steps in the trichothecene pathway. Both cosmids restored T-2 toxin production to Tox3-1- or Tox4-1- mutants but neither restored T-2 toxin production to a Tox1-2- mutant. The production of T-2 toxin by the complemented Tox3-1- and Tox4-1- mutants, as well as the production of diacetoxycirpenol by the cosmid-transformed Tox1-2- mutant, were 2- to 10-fold higher than in strain NRRL 3299. In addition, those transformants carrying Cos9-1 produced significantly higher levels of trichothecenes than transformants carrying Cos1-1. Two different DNA fragments (FSC13-9 and FSC14-5), representing the region of overlap between the cosmid clones, were isolated. These fragments specifically complemented either the Tox3-1- mutant (FSC14-5) or the Tox4-1- mutant (FSC13-9). The trichothecene-production phenotype of these transformants was similar to NRRL 3299. These results suggest that two or more genes involved in the biosynthesis of trichothecenes are closely linked to Tox5.

Trichothecene biosynthesis in Fusarium species: chemistry, genetics, and significance

Several species of the genus Fusarium and related fungi produce trichothecenes which are sesquiterpenoid epoxides that act as potent inhibitors of eukaryotic protein synthesis. Interest in the trichothecenes is due primarily to their widespread contamination of agricultural commodities and their adverse effects on human and animal health. In this review, we describe the trichothecene biosynthetic pathway in Fusarium species and discuss genetic evidence that several trichothecene biosynthetic genes are organized in a gene cluster. Trichothecenes are highly toxic to a wide range of eukaryotes, but their specific function, if any, in the survival of the fungi that produce them is not obvious. Trichothecene gene disruption experiments indicate that production of trichothecenes can enhance the severity of disease caused by Fusarium species on some plant hosts. Understanding the regulation and function of trichothecene biosynthesis may aid in development of new strategies for controlling their production in food and feed products.

Delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase, the multienzyme integrating the four primary reactions in beta-lactam biosynthesis, as a model peptide synthetase

ACV synthetase forms the tripeptide precursor of penicillins and cephalosporins from alpha-aminoadipate, cysteine, and valine. Catalytic sites for substrate carboxyl activation as adenylates, peptide bond formations, epimerization and release of the tripeptide-thioester are integrated in multifunctional enzymes of 405 to 425 kD. These have been characterized from several pro- and eukaryotic beta-lactam producers. Implications of these results for the thio-template mechanism of peptide formation are discussed, as well as the use of this multienzyme as a model system for enzymatic peptide synthesis.

Cloning of a Streptomyces clavuligerus DNA fragment encoding the cephalosporin 7 alpha-hydroxylase and its expression in Streptomyces lividans

A 26-mer DNA probe was designed from N-terminal sequence data for the cephalosporin 7 alpha-hydroxylase (CH) of Streptomyces clavuligerus NRRL 3585 and used to screen a DNA library from this organism. The library was constructed in the lambda GEM-11 phage system. After plaque purification and reprobing, positive recombinant phages were chosen for further analysis. Characterization of the cloned DNA by restriction mapping and Southern hybridization showed that a 1.5-kb SalI fragment hybridized to the probe. Polymerase chain reaction assays using this fragment as a template and the probe as a primer indicated that the fragment carries the entire putative CH gene (cmcI). This was confirmed through the expression of CH enzymatic activity when the fragment was introduced into Streptomyces lividans. A putative beta-lactamase activity was detected in S. lividans.

The cefG gene of Cephalosporium acremonium is linked to the cefEF gene and encodes a deacetylcephalosporin C acetyltransferase closely related to homoserine O-acetyltransferase

The gene (cefG) encoding the acetyl coenzyme A:deacetylcephalosporin C acetyltransferase of Cephalosporium acremonium (synonym Acremonium chrysogenum) C10 has been cloned. It contains two introns and encodes a protein of 444 amino acids with an M(r) of 49,269 that correlates well with the M(r) deduced by gel filtration. The cefG gene is linked to the cefEF gene (encoding the bifunctional deacetoxycephalosporin C synthase/hydroxylase), but it is expressed in an orientation opposite that of the cefEF gene. Two transcripts of 1.2 and 1.4 kb were found in C. acremonium that correspond to the cefEF and cefG genes, respectively; the degree of expression of the cefG gene was clearly lower than that of the cefEF gene in 48-h cultures. The cloned cefG complemented the deficiency of deacetylcephalosporin acetyltransferase in the nonproducer mutant C. acremonium ATCC 20371 and restored cephalosporin biosynthesis in this strain. Heterologous expression of the cefG genes took place in Penicillium chrysogenum. The deacetylcephalosporin acetyltransferase showed a much higher degree of homology with the O-acetylhomoserine acetyltransferases of Saccharomyces cerevisiae and Ascobolus immersus than with other O-acetyltransferases. The cefEF-cefG cluster of genes encodes the enzymes that carry out the three late steps of the cephalosporin biosynthetic pathway and is not linked to the pcbAB-pcbC gene cluster that encodes the first two steps of the pathway.

Genetic engineering of ß-lactam antibiotic biosynthetic pathways in filamentous fungi

Recombinant DNA technology has facilitated a rapid increase in our knowledge of beta-lactam antibiotic biosynthesis. Using the tools of this technology, beta-lactam biosynthetic genes and proteins have been characterized at the molecular level, cephalosporin-C production has been improved, new beta-lactams produced, and novel beta-lactam biosynthetic pathways have been constructed.

ACV synthetase

ACV synthetase (ACVS) is the first enzyme and plays a key role in the biosynthesis of all natural penicillins and cephalosporins. The enzyme is extremely unstable and little had been known about it until recently. This article summarizes the progress in research on this enzyme, including the establishment of a cell-free assay system, stabilization, purification, characterization, and gene cloning. A possible reaction sequence for ACVS catalysis is suggested.

Localization of the lysine epsilon-aminotransferase (lat) and delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase (pcbAB) genes from Streptomyces clavuligerus and production of lysine epsilon-aminotransferase activity in Escherichia coli

Lysine epsilon-aminotransferase (LAT) in the beta-lactam-producing actinomycetes is considered to be the first step in the antibiotic biosynthetic pathway. Cloning of restriction fragments from Streptomyces clavuligerus, a beta-lactam producer, into Streptomyces lividans, a nonproducer that lacks LAT activity, led to the production of LAT in the host. DNA sequencing of restriction fragments containing the putative lat gene revealed a single open reading frame encoding a polypeptide with an approximately Mr 49,000. Expression of this coding sequence in Escherichia coli led to the production of LAT activity. Hence, LAT activity in S. clavuligerus is derived from a single polypeptide. A second open reading frame began immediately downstream from lat. Comparison of this partial sequence with the sequences of delta-(L-alpha-aminoadipyl)-L-cysteinyl-D valine (ACV) synthetases from Penicillium chrysogenum and Cephalosporium acremonium and with nonribosomal peptide synthetases (gramicidin S and tyrocidine synthetases) found similarities among the open reading frames. Since mapping of the putative N and C termini of S. clavuligerus pcbAB suggests that the coding region occupies approximately 12 kbp and codes for a polypeptide related in size to the fungal ACV synthetases, the molecular characterization of the beta-lactam biosynthetic cluster between pcbC and cefE (approximately 25 kbp) is nearly complete.