Characterization of oleate-nonutilizing mutants of Aspergillus nidulans isolated by the 3-amino-1,2,4-triazole positive selection method

The versatility of peroxisome function in filamentous fungi

Peroxisomes are ubiquitous and versatile cell organelles. They consist of a single membrane that encloses a proteinaceous matrix. Conserved functions are fatty acid β-oxidation and hydrogen peroxide metabolism. In filamentous fungi, many other metabolic functions have been identified. Also, they contain highly specialized peroxisome-derived structures termed Woronin bodies, which have a structural function in plugging septal pores in order to prevent cytoplasmic bleeding of damaged hyphae.In filamentous fungi peroxisomes play key roles in the production of a range of secondary metabolites such as antibiotics. Most likely the atlas of fungal peroxisomal metabolic pathways is still far from complete. Relative recently discovered functions include their role in biotin biosynthesis as well as in the production of several toxins, among which polyketides. Finally, in filamentous fungi peroxisomes are important for development and pathogenesis.In this contribution we present an overview of our current knowledge on fungal peroxisome formation as well as on their functional diversity.

Peroxisomes are required for efficient penicillin biosynthesis in Penicillium chrysogenum

In the fungus Penicillium chrysogenum, penicillin (PEN) production is compartmentalized in the cytosol and in peroxisomes. Here we show that intact peroxisomes that contain the two final enzymes of PEN biosynthesis, acyl coenzyme A (CoA):6-amino penicillanic acid acyltransferase (AT) as well as the side-chain precursor activation enzyme phenylacetyl CoA ligase (PCL), are crucial for efficient PEN synthesis. Moreover, increasing PEN titers are associated with increasing peroxisome numbers. However, not all conditions that result in enhanced peroxisome numbers simultaneously stimulate PEN production. We find that conditions that lead to peroxisome proliferation but simultaneously interfere with the normal physiology of the cell may be detrimental to antibiotic production. We furthermore show that peroxisomes develop in germinating conidiospores from reticule-like structures. During subsequent hyphal growth, peroxisome proliferation occurs at the tip of the growing hyphae, after which the organelles are distributed over newly formed subapical cells. We observed that the organelle proliferation machinery requires the dynamin-like protein Dnm1.

Functional characterization of residues within the carnitine/acylcarnitine translocase RX2PANAAXF distinct motif

The mitochondrial carnitine/acylcarnitine carrier (CAC) is characterized by the presence of a distinct motif, RXXPANAAXF, within its sixth transmembrane alpha-helix. In this study, we analysed the role of the amino acids of this motif in the structure-function relationships of the human CAC by using two complementary approaches. First, we performed functional analysis in the model fungus Aspergillus nidulans of selected mutations with structural and functional relevance. Second, similar mutant human CACs were biochemically characterized after their reconstitution into liposomes. Both analyses have provided relevant information on the importance and role of the CAC motif residues in the activity and metabolic function of CAC. Only the two adjacent alanines, Ala281 and Ala282 in the human CAC, have been found not to be crucial for transport activity and in vivo function. Results obtained from amino acid substitutions of residues Arg275, Asn280 and Phe284 of human CAC together with structural analysis using molecular modelling of the carrier suggest that R275, N280 and F284 are involved in substrate binding during acylcarnitine/carnitine translocation. Furthermore, functional analysis of mutations of residues Pro278 and Ala279 in A. nidulans, together with kinetic data in reconstituted liposomes, suggest a predominant structural role for these amino acids.

Genetic analysis of the role of peroxisomes in the utilization of acetate and fatty acids in Aspergillus nidulans

Peroxisomes are organelles containing a diverse array of enzymes. In fungi they are important for carbon source utilization, pathogenesis, development, and secondary metabolism. We have studied Aspergillus nidulans peroxin (pex) mutants isolated by virtue of their inability to grow on butyrate or by the inactivation of specific pex genes. While all pex mutants are able to form colonies, those unable to import PTS1 proteins are partially defective in asexual and sexual development. The pex mutants are able to grow on acetate but are affected in growth on fatty acids, indicating a requirement for the peroxisomal localization of beta-oxidation enzymes. However, mislocalization of malate synthase does not prevent growth on either fatty acids or acetate, showing that the glyoxylate cycle does not require peroxisomal localization. Proliferation of peroxisomes is dependent on fatty acids, but not on acetate, and on PexK (Pex11), expression of which is activated by the FarA transcription factor. Proliferation was greatly reduced in a farADelta strain. A mutation affecting a mitochodrial ketoacyl-CoA thiolase and disruption of a mitochondrial hydroxy-acyl-CoA dehydrogenase gene prevented growth on short-chain but not long-chain fatty acids. Together with previous results, this is consistent with growth on even-numbered short-chain fatty acids requiring a mitochondrial as well as a peroxisomal beta-oxidation pathway. The mitochondrial pathway is not required for growth on valerate or for long-chain fatty acid utilization.

Ability to grow on lipids accounts for the fully virulent phenotype in neutropenic mice of Aspergillus fumigatus null mutants in the key glyoxylate cycle enzymes

Incidence and mortality rates of invasive aspergillosis clearly indicate the need of novel antifungals to treat patients suffering from this disease. Fungal proteins playing a crucial role in pathogenesis and with no orthologue in human cells are considered as primary therapeutic targets for the development of new antifungals with a high therapeutic index, one of the major drawbacks of the standard antifungal therapy, so far. In this work, we have analyzed the role in pathogenesis of the key enzymes of the Aspergillus fumigatus glyxoxylate cycle, isocitrate lyase and malate synthase, two possible candidates to primary therapeutic targets in this fungus. Deletion strains lacking isocitrate lyase (DeltaacuD strains) or malate synthase (DeltaacuE mutants) were constructed in this work. The Neurospora crassa pyr-4 gene was used as the replacing marker in gene deletion experiments. The pathogenicities of DeltaacuD and DeltaacuE mutants were tested in neutropenic mice and compared with those of two reference wild-type isolates A. fumigatus 237 and A. fumigatus 293. Interestingly, virulence and cytological studies clearly indicated the dispensability of the A. fumigatus glyoxylate cycle for pathogenicity. In addition, these results suggested the suitability of the pyr-4 gene as a valuable replacing marker for virulence studies in this fungus, a fact that was further confirmed by gene expression analyses. Finally, growth tests were performed to investigate the germination and growth of the DeltaacuD and DeltaacuE strains in nutrient deprivation environments, resembling the conditions that A. fumigatus conidia face after phagocytosis. Results obtained in this work strongly suggest that the ability to grow on lipids (triglycerides) of A. fumigatus isocitrate lyase and malate synthase deletion strains accounts for their fully virulent phenotype.

Suppression of the acuH13 and acuH31 nonsense mutations in the carnitine/acylcarnitine translocase (acuH) gene of Aspergillus nidulans by the G265S substitution in the domain 2 of the release factor eRF1

A search for suppressors of the carnitine/acylcarnitine translocase (CACT) deficiency in Aspergillus nidulans permitted the identification of the suaE7 mutation, mapping at a new translational suppressor (suaE) gene. The suaE gene is essential in A. nidulans and encodes the eukaryotic release factor 1 (eRF1). The suaE7 mutation suppresses two acuH alleles (acuH13 and acuH31), both carrying nonsense mutations in the CACT encoding gene that involve the replacement of a CAG (Gln) codon with a premature TAG stop codon. In contrast, the suaE7 gene does not suppress the acuH20 amber nonsense mutation involving a TGG-->TAG change. The phenotype associated to the suaE7 mutation strictly resembles that of mutants at the suaA and suaC genes, two translational suppressor genes previously identified, suggesting that their gene products might functionally interact in translation termination. Sequencing of the suaE7 gene allowed the identification of a mutation in the domain 2 of the omnipotent class-1 eukaryotic release factor involving the Gly265Ser substitution in the A. nidulans eRF1. This mutation creates a structural context unfavourable for normal eRF binding that allows the misreading of stop codons by natural suppressor tRNAs, such as the tRNAs(Gln). Structural analysis using molecular modelling of A. nidulans eRF1 domain 2 bearing the G265S substitution and computer simulation results suggest that this mutation might impair the necessary conformational changes in the eRF1 to optimally recognize the stop codon and simultaneously interact with the peptidyl transferase centre of the 60S ribosomal subunit.

Synthesis of Penicillium chrysogenum acetyl-CoA:isopenicillin N acyltransferase in Hansenula polymorpha: first step towards the introduction of a new metabolic pathway

The enzyme acetyl-CoA:isopenicillin N acyltransferase (IAT) is a peroxisomal enzyme that mediates the final step of penicillin biosynthesis in the filamentous fungi Penicillium chrysogenum and Aspergillus nidulans. However, the precise role of peroxisomes in penicillin biosynthesis is still not clear. To be able to use the power of yeast genetics to solve the function of peroxisomes in penicillin biosynthesis, we introduced IAT in the yeast Hansenula polymorpha. To this purpose, the P. chrysogenum penDE gene, encoding IAT, was amplified from a cDNA library to eliminate the three introns and introduced in H. polymorpha. In this organism IAT protein was produced as a 40 kDa pre-protein and, as in P. chrysogenum, processed into an 11 and 29 kDa subunit, although the efficiency of processing seemed to be slightly reduced relative to P. chrysogenum. The P. chrysogenum IAT, produced in H. polymorpha, is normally localized in peroxisomes and in cell-free extracts IAT activity could be detected. This is a first step towards the introduction of the penicillin biosynthesis pathway in H. polymorpha.

Characterization of Arabidopsis Fluoroacetate-resistant Mutants Reveals the Principal Mechanism of Acetate Activation for Entry into the Glyoxylate Cycle

The toxic acetate analogue monofluoroacetic acid was employed to isolate Arabidopsis tDNA-tagged plants deficient in their ability to utilize or sense acetate. Several tDNA-tagged lines were isolated, including two that were determined to be allelic to an EMS-mutagenized line denoted acn1 for ac non-utilizing. Following conventions, the tDNA-tagged mutants were designated acn1-2 and acn1-3. Both mutants displayed identical behavior to acn1-1 on a variety of fluorinated and nonfluorinated organic acids, indicating that resistance was specific to fluoroacetate. Thermal asymmetric interlaced PCR identified the sites of tDNA insertion in both mutants to be within different exons in a gene, which encoded a protein containing an AMP-binding motif. Reverse transcription-PCR confirmed that the gene was not expressed in the mutants, and quantitative reverse transcription-PCR showed that the gene is expressed in imbibed seeds and increases in amount during establishment. The wild type AMP-binding protein cDNA was cloned and expressed in Escherichia coli, and the expressed protein was purified by nickel chelate chromatography. The enzyme was identified as an acyl-CoA synthetase that was more active with acetate than butyrate and was not active with fatty acids longer than C-4. The enzyme was localized to peroxisomes by enzymatic analysis of organellar fractions isolated by sucrose density gradient centrifugation. Labeling studies with [(14)C]acetate showed that acn1 seedlings, like those of the isocitrate lyase mutant icl-1 (isocitrate lyase), are compromised in carbohydrate synthesis, indicating that this enzyme is responsible for activating exogenous acetate to the coenzyme A form for entry into the glyoxylate cycle.

Mitochondrial beta-oxidation in Aspergillus nidulans

Beta-oxidation (beta-ox) occurs exclusively in the peroxisomes of Saccharomyces cerevisiae and other yeasts, leading to the supposition that fungi lack mitochondrial beta-ox. Here we present unequivocal evidence that the filamentous fungus Aspergillus nidulans houses both peroxisomal and mitochondrial beta-ox. While growth of a peroxisomal beta-ox disruption mutant (DeltafoxA) was eliminated on a very long-chain fatty acid (C(22:1)), growth was only partially impeded on a long-chain fatty acid (C(18:1)) and was not affected at all on short chain (C4-C6) fatty acids. In contrast, growth of a putative enoyl-CoA hydratase mutant (DeltaechA) was abolished on short-chain and severely restricted on long- and very long-chain fatty acids. Furthermore fatty acids inhibited growth of the DeltaechA mutant but not the DeltafoxA mutant in the presence of an alternate carbon source (lactose). Disruption of echA led to a 28-fold reduction in 2-butenoyl-CoA hydratase activity in a preparation of organelles. EchA was also required for growth on isoleucine and valine. The subcellular localization of the FoxA and EchA proteins was confirmed through the use of red and green fluorescent protein fusions.

The Aspergillus nidulans alcA promoter drives tightly regulated conditional gene expression in Aspergillus fumigatus permitting validation of essential genes in this human pathogen

Aspergillus fumigatus causes invasive aspergillosis, a mycosis that is usually fatal in immunocompromised patients. Functional genomics in this fungus will aid the discovery of novel antifungal drugs to treat invasive aspergillosis. However, there is still a need for appropriate molecular genetic tools to facilitate such functional studies. Here, we describe the use of a conditional gene expression system allowing the identification of novel therapeutic targets through validation of essential genes in A. fumigatus. This system is based on the capacity of the Aspergillus nidulans alcA promoter (alcA(p)) to tightly regulate gene expression in this fungus. Conditionally regulated gene expression in A. fumigatus was demonstrated by transcriptional and phenotypic analyses of strains expressing a nuclear migration gene with a terminal phenotype, the A. fumigatus nudC gene, under control of this promoter. This conditional expression system, the first one described in A. fumigatus, will also be useful for investigating the function of essential genes by altering the threonine/glucose ratio in the growth medium.

Functional analysis of mutations in the human carnitine/acylcarnitine translocase in Aspergillus nidulans

Deficiency of the carnitine/acylcarnitine translocase (CACT), the most severe disorder of fatty acid beta-oxidation, is usually lethal in both humans and animals, precluding the development of animal models of the disease. In contrast, CACT deficiency is conditionally lethal in the fungus Aspergillus nidulans, since loss-of-function mutations in acuH, the translocase structural gene, do not prevent growth on carbon sources other than ketogenic compounds, such as fatty acids. Here, we describe the molecular characterization of extant acuH alleles and the development of a fungal model for CACT deficiency based on the ability of human CACT to fully complement, when expressed at physiological levels, the growth defect of an A. nidulans DeltaacuH strain on acetate and long-chain fatty acids. By using growth tests and in vitro assays this model enabled us to carry out a functional characterization of human CACT mutations showing that it may be useful for distinguishing potentially pathogenic human CACT missense mutations from neutral, single residue substitution-causing polymorphisms.

The Aspergillus nidulans carnitine carrier encoded by the acuH gene is exclusively located in the mitochondria

The location of the Aspergillus nidulans carnitine/acyl-carnitine carrier (ACUH) was studied. ACUH with a His-tag at its N-terminus was over-expressed in Escherichia coli and purified by Ni(2+) affinity chromatography. The purified protein was utilised to raise polyclonal antibodies which were characterised by Western blotting. For localisation studies A. nidulans T1 strain, that contains the acuH gene under control of the strong promoter alcA(p), was derived. Results obtained demonstrate the exclusively mitochondrial localisation of ACUH and therefore exclude the targeting of the acuH gene product to the peroxisomal membrane.