Detection of a protein which binds specifically to the upstream region of the pcbAB gene in Penicillium chrysogenum

Sensing and transduction of nutritional and chemical signals in filamentous fungi: Impact on cell development and secondary metabolites biosynthesis

Filamentous fungi respond to hundreds of nutritional, chemical and environmental signals that affect expression of primary metabolism and biosynthesis of secondary metabolites. These signals are sensed at the membrane level by G protein coupled receptors (GPCRs). GPCRs contain usually seven transmembrane domains, an external amino terminal fragment that interacts with the ligand, and an internal carboxy terminal end interacting with the intracellular G protein. There is a great variety of GPCRs in filamentous fungi involved in sensing of sugars, amino acids, cellulose, cell-wall components, sex pheromones, oxylipins, calcium ions and other ligands. Mechanisms of signal transduction at the membrane level by GPCRs are discussed, including the internalization and compartmentalisation of these sensor proteins. We have identified and analysed the GPCRs in the genome of Penicillium chrysogenum and compared them with GPCRs of several other filamentous fungi. We have found 66 GPCRs classified into 14 classes, depending on the ligand recognized by these proteins, including most previously proposed classes of GPCRs. We have found 66 putative GPCRs, representatives of twelve of the fourteen previously proposed classes of GPCRs, depending on the ligand recognized by these proteins. A staggering fortytwo putative members of the new GPCR class XIV, the so-called Pth11 sensors of cellulosic material as reported for Neurospora crassa and some other fungi, were identified. Several GPCRs sensing sex pheromones, known in yeast and in several fungi, were also identified in P. chrysogenum, confirming the recent unravelling of the hidden sexual capacity of this species. Other sensing mechanisms do not involve GPCRs, including the two-component systems (HKRR), the HOG signalling system and the PalH mediated pH transduction sensor. GPCR sensor proteins transmit their signals by interacting with intracellular heterotrimeric G proteins, that are well known in several fungi, including P. chrysogenum. These G proteins are inactive in the GDP containing heterotrimeric state, and become active by nucleotide exchange, allowing the separation of the heterotrimeric protein in active Gα and Gβγ dimer subunits. The conversion of GTP in GDP is mediated by the endogenous GTPase activity of the G proteins. Downstream of the ligand interaction, the activated Gα protein and also the Gβ/Gγ dimer, transduce the signals through at least three different cascades: adenylate cyclase/cAMP, MAPK kinase, and phospholipase C mediated pathways.

The regulatory factor PcRFX1 controls the expression of the three genes of β-lactam biosynthesis in Penicillium chrysogenum

Penicillin biosynthesis is subjected to a complex regulatory network of signalling molecules that may serve as model for other secondary metabolites. The information provided by the new "omics" era about Penicillium chrysogenum and the advances in the knowledge of molecular mechanisms responsible for improved productivity, make this fungus an excellent model to decipher the mechanisms controlling the penicillin biosynthetic pathway. In this work, we have characterized a novel transcription factor PcRFX1, which is an ortholog of the Acremonium chrysogenum CPCR1 and Penicillium marneffei RfxA regulatory proteins. PcRFX1 DNA binding sequences were found in the promoter region of the pcbAB, pcbC and penDE genes. We show in this article that these motifs control the expression of the β-galactosidase lacZ reporter gene, indicating that they may direct the PcRFX1-mediated regulation of the penicillin biosynthetic genes. By means of Pcrfx1 gene knock-down and overexpression techniques we confirmed that PcRFX1 controls penicillin biosynthesis through the regulation of the pcbAB, pcbC and penDE transcription. Morphology and development seemed not to be controlled by this transcription factor under the conditions studied and only sporulation was slightly reduced after the silencing of the Pcrfx1 gene. A genome-wide analysis of processes putatively regulated by this transcription factor was carried out in P. chrysogenum. Results suggested that PcRFX1, in addition to regulate penicillin biosynthesis, is also involved in the control of several pathways of primary metabolism.

Core promoters of the penicillin biosynthesis genes and quantitative RT-PCR analysis of these genes in high and low production strain of Penicillium chrysogenum

The transcription start points of the penicillin biosynthesis genes from Penicillium chrysogenum were mapped using the primer extension method. For each of the three genes consensus sequences of the core promoter elements were identified, supporting the notion that the basal transcription of these genes is mediated separately. Interestingly, transcription start of the pcbC gene is located within the potential Inr element with no TATA box-like sequence being found at expected position. This is in contrast to pcbAB and penDE genes with proposed TATA boxes or even to Aspergillus nidulans ipnA (pcbC) gene indicating possible differences in basal transcription regulation. Using the quantitative RT-PCR analysis the expression of all three biosynthesis genes was monitored in both the high and low production strain of P. chrysogenum during a 3-d cultivation under production conditions. The differences were found between the strains in time regulation and transcript levels of the biosynthesis genes. Furthermore, we showed that the effect of higher gene dosage on productivity in the production strain is amplified by more efficient transcription of the biosynthesis genes with the RNA levels approximately 37- and 12-times higher, respectively, than in a low production strain.

Recombinant microorganisms for industrial production of antibiotics

The enhancement of industrial antibiotic yield has been achieved through technological innovations and traditional strain improvement programs based on random mutation and screening. The development of recombinant DNA techniques and their application to antibiotic producing microorganisms has allowed yield increments and the design of biosynthetic pathways giving rise to new antibiotics. Genetic manipulations of the cephalosporin producing fungus Cephalosporium acremonium have included yield improvements, accomplished increasing biosynthetic gene dosage or enhancing oxygen uptake, and new biosynthetic capacities as 7-aminocephalosporanic acid (7-ACA) or penicillin G production. Similarly, in Penicillium chrysogenum, the industrial penicillin producing fungus, heterologous expression of cephalosporin biosynthetic genes has led to the biosynthesis of adipyl-7-aminodeacetoxycephalosporanic acid (adipyl-7-ADCA) and adipyl-7-ACA, compounds that can be transformed into the economically relevant 7-ADCA and 7-ACA intermediates. Escherichia coli expression of the genes encoding D-amino acid oxidase and cephalosporin acylase activities has simplified the bioconversion of cephalosporin C into 7-ACA, eliminating the use of organic solvents. The genetic manipulation of antibiotic producing actinomycetes has allowed productivity increments and the development of new hybrid antibiotics. A legal framework has been developed for the confined manipulation of genetically modified organisms.

Regulation and compartmentalization of β-lactam biosynthesis

Penicillins and cephalosporins are β‐lactam antibiotics widely used in human medicine. The biosynthesis of these compounds starts by the condensation of the amino acids l‐α‐aminoadipic acid, l‐cysteine and l‐valine to form the tripeptide δ‐l‐α‐aminoadipyl‐l‐cysteinyl‐d‐valine catalysed by the non‐ribosomal peptide ‘ACV synthetase’. Subsequently, this tripeptide is cyclized to isopenicillin N that in Penicillium is converted to hydrophobic penicillins, e.g. benzylpenicillin. In Acremonium and in streptomycetes, isopenicillin N is later isomerized to penicillin N and finally converted to cephalosporin. Expression of genes of the penicillin (pcbAB, pcbC, pendDE) and cephalosporin clusters (pcbAB, pcbC, cefD1, cefD2, cefEF, cefG) is controlled by pleitropic regulators including LaeA, a methylase involved in heterochromatin rearrangement. The enzymes catalysing the last two steps of penicillin biosynthesis (phenylacetyl‐CoA ligase and isopenicillin N acyltransferase) are located in microbodies, as shown by immunoelectron microscopy and microbodies proteome analyses. Similarly, the Acremonium two‐component CefD1–CefD2 epimerization system is also located in microbodies. This compartmentalization implies intracellular transport of isopenicillin N (in the penicillin pathway) or isopenicillin N and penicillin N in the cephalosporin route. Two transporters of the MFS family cefT and cefM are involved in transport of intermediates and/or secretion of cephalosporins. However, there is no known transporter of benzylpenicillin despite its large production in industrial strains.

The global regulator LaeA controls penicillin biosynthesis, pigmentation and sporulation, but not roquefortine C synthesis in Penicillium chrysogenum

The biosynthesis of the beta-lactam antibiotic penicillin is an excellent model for the study of secondary metabolites produced by filamentous fungi due to the good background knowledge on the biochemistry and molecular genetics of the beta-lactam producing microorganisms. The three genes (pcbAB, pcbC, penDE) encoding enzymes of the penicillin pathway in Penicillium chrysogenum are clustered, but no penicillin pathway-specific regulators have been found in the genome region that contains the penicillin gene cluster. The biosynthesis of this beta-lactam is controlled by global regulators of secondary metabolism rather than by a pathway-specific regulator. In this work we have identified the gene encoding the secondary metabolism global regulator LaeA in P. chrysogenum (PcLaeA), a nuclear protein with a methyltransferase domain. The PclaeA gene is present as a single copy in the genome of low and high-penicillin producing strains and is not located in the 56.8-kb amplified region occurring in high-penicillin producing strains. Overexpression of the PclaeA gene gave rise to a 25% increase in penicillin production. PclaeA knock-down mutants exhibited drastically reduced levels of penicillin gene expression and antibiotic production and showed pigmentation and sporulation defects, but the levels of roquefortine C produced and the expression of the dmaW involved in roquefortine biosynthesis remained similar to those observed in the wild-type parental strain. The lack of effect on the synthesis of roquefortine is probably related to the chromatin arrangement in the low expression roquefortine promoters as compared to the bidirectional pbcAB-pcbC promoter region involved in penicillin biosynthesis. These results evidence that PcLaeA not only controls some secondary metabolism gene clusters, but also asexual differentiation in P. chrysogenum.

Binding of the PTA1 transcriptional activator to the divergent promoter region of the first two genes of the penicillin pathway in different Penicillium species

The aim of this work is to establish the correlation between the transcriptional activator PTA1 and the expression of the penicillin genes in different penicillin-producing strains. The level of expression of the first two genes of the penicillin pathway was clearly higher in Penicillium chrysogenum than in Penicillium notatum and Penicillium nalgiovense. The divergent promoter pcbAB-pcbC region contains binding sequences for several transcriptional factors that are conserved in P. notatum and P. chrysogenum, but not in P. nalgiovense. Binding of the purified P. chrysogenum transcriptional activator PTA1 to the palindromic heptamer TTAGTAA took place when the P. chrysogenum 35 bp DNA fragment containing the heptamer was used as a probe, but not when the sequence occurring in P. nalgiovense was used. P. nalgiovense protein fractions purified by heparin agarose chromatography did not bind to the 35-bp DNA fragment either from P. nalgiovense or P. chrysogenum, although some degree of binding was observed when crude extracts were used. This finding may explain the low expression of pcbC in P. nalgiovense. All the P. chrysogenum strains, including the industrial strain E1, showed the same nucleotide sequence, including the consensus PTA1 binding site.

Hybrid antibiotics

Hybrid antibiotics that do not occur in nature have been obtained by combining structural genes of antibiotic producers. Some of these substances were effective against pathogenic microorganisms resistant against antibiotics produced by the parent strains. The majority of hybrid antibiotics were obtained by combining genes encoding polyketide synthases. Hybrid peptides with new biological properties have also been synthesized.

A novel heptameric sequence (TTAGTAA) is the binding site for a protein required for high level expression of pcbAB, the first gene of the penicillin biosynthesis in Penicillium chrysogenum

The first two genes pcbAB and pcbC of the penicillin biosynthesis pathway are expressed from a 1.01-kilobase bidirectional promoter region. A series of sequential deletions were made in the pcbAB promoter region, and the constructions with the modified promoters coupled to the lacZ reporter gene were introduced as single copies at the pyrG locus in Penicillium chrysogenum npe10. Three regions, boxes A, B, and C, produced a significant decrease in expression of the reporter gene when deleted. Protein-DNA complexes were observed by using the electrophoretic mobility shift assay with boxes A and B (complexes AG1, BG1, BG2, and BL1) but not with box C. Uracil interference assay showed that a protein in P. chrysogenum cell extracts interacts with the thymines in a palindromic heptanucleotide TTAGTAA. Point mutations and deletion of the entire TTAGTAA sequence supported the involvement of this sequence in the binding of a transcriptional activator named penicillin transcriptional activator 1 (PTA1). In vivo studies using constructions carrying point mutations in the TTAGTAA sequence (or a deletion of the complete heptanucleotide) confirmed that this intact sequence is required for high level expression of the pcbAB gene. The TTAGTAA sequence resembles the target sequence of BAS2 (PHO2), a factor required for expression of several genes in yeasts.

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.

Identification of a major cis-acting DNA element controlling the bidirectionally transcribed penicillin biosynthesis genes acvA (pcbAB) and ipnA (pcbC) of Aspergillus nidulans

The beta-lactam antibiotic penicillin is produced as a secondary metabolite by some filamentous fungi. In this study, the molecular regulation of the Aspergillus (Emericella) nidulans penicillin biosynthesis genes acvA (pcbAB) and ipnA (pcbC) was analyzed. acvA and ipnA are divergently oriented and separated by an intergenic region of 872 bp. Translational fusions of acvA and ipnA with the two Escherichia coli reporter genes lacZ and uidA enabled us to measure the regulation of both genes simultaneously. A moving-window analysis of the 872-bp intergenic region indicated that the divergently oriented promoters are, at least in part, overlapping and share common regulatory elements. Removal of nucleotides -353 to -432 upstream of the acvA gene led to a 10-fold increase of acvA-uidA expression and simultaneously to a reduction of ipnA-lacZ expression to about 30%. Band shift assays and methyl interference analysis using partially purified protein extracts revealed that a CCAAT-containing DNA element within this region was specifically bound by a protein (complex), which we designated PENR1, for penicillin regulator. Deletion of 4 bp within the identified protein binding site caused the same contrary effects on acvA and ipnA expression as observed for all of the deletion clones which lacked nucleotides -353 to -432. The PENR1 binding site thus represents a major cis-acting DNA element. The intergenic regions of the corresponding genes of the beta-lactam-producing fungi Penicillium chrysogenum and Acremonium chrysogenum also diluted the complex formed between the A. nidulans probe and PENR1 in vitro, suggesting that these beta-lactam biosynthesis genes are regulated by analogous DNA elements and proteins.

The Aspergillus nidulans penicillin-biosynthesis gene aat (penDE) is controlled by a CCAAT-containing DNA element

Analysis of the promoter of the penicillin biosynthesis aat (penDE) gene of Aspergillus nidulans using band-shift assays led to the identification of a CCAAT-containing DNA element which was specifically bound by a protein (complex). The identified DNA element was localised about 250 bp upstream of the transcriptional-start sites of aat. Substitution of the CCAAT core sequence by GATCC led to a fourfold reduction of expression of an aat-lacZ gene fusion. The identified binding site thus was functional in vivo and positively influenced at expression. Partial purification of the CCAAT binding protein and cross-competition experiments provided evidence that the binding protein is identical to the identified putative penicillin-regulatory protein PENR1, binding to the CCAAT element in the bidirectional intergenic promoter region between acvA (pcbAb) and ipnA (pcbC). Hence, PENR1 seems to be involved in the regulation of all three penicillin-biosynthesis genes. Cross-competition experiments demonstrated that the promoter region of the corresponding aat (penDE) gene of Penicillium chrysogenum was capable to dilute the shift of the A. nidulans probe with PENR1, suggesting the presence of a similar regulatory mechanism in this fungus. Taken together with previous data, CCAAT-containing DNA elements thus seem to represent major cis-acting sites in the promoters of beta-lactam-biosynthesis genes.