Assimilation of alkanes by yeasts: functions and biogenesis of peroxisomes

Alkane-grown Beauveria bassiana produce mycelial pellets displaying peroxisome proliferation, oxidative stress, and cell surface alterations

The entomopathogenic fungus Beauveria bassiana is able to grow on insect cuticle hydrocarbons, inducing alkane assimilation pathways and concomitantly increasing virulence against insect hosts. In this study, we describe some physiological and molecular processes implicated in growth, nutritional stress response, and cellular alterations found in alkane-grown fungi. The fungal cytology was investigated using light and transmission electron microscopy while the surface topography was examined using atomic force microscopy. Additionally, the expression pattern of several genes associated with oxidative stress, peroxisome biogenesis, and hydrophobicity were analysed by qPCR. We found a novel type of growth in alkane-cultured B. bassiana similar to mycelial pellets described in other alkane-free fungi, which were able to produce viable conidia and to be pathogenic against larvae of the beetles Tenebrio molitor and Tribolium castaneum. Mycelial pellets were formed by hyphae cumulates with high peroxidase activity, exhibiting peroxisome proliferation and an apparent surface thickening. Alkane-grown conidia appeared to be more hydrophobic and cell surfaces displayed different topography than glucose-grown cells. We also found a significant induction in several genes encoding for peroxins, catalases, superoxide dismutases, and hydrophobins. These results show that both morphological and metabolic changes are triggered in mycelial pellets derived from alkane-grown B. bassiana.

Molecular characterization and expression analysis of a suite of cytochrome P450 enzymes implicated in insect hydrocarbon degradation in the entomopathogenic fungus Beauveria bassiana

The insect epicuticle or waxy layer comprises a heterogeneous mixture of lipids that include abundant levels of long-chain alkanes, alkenes, wax esters and fatty acids. This structure represents the first barrier against microbial attack and for broad-host-range insect pathogens, such as Beauveria bassiana, it is the initial interface mediating the host–pathogen interaction, since these organisms do not require any specialized mode of entry and infect target hosts via the cuticle. B. bassiana is able to grow on straight chain alkanes up to n-C33 as a sole source of carbon and energy. The cDNA and genomic sequences, including putative regulatory elements, for eight cytochrome P450 enzymes, postulated to be involved in alkane and insect epicuticle degradation, were isolated and characterized. Expression studies using a range of alkanes as well as an insect-derived epicuticular extract from the blood-sucking bug Triatomas infestans revealed a differential expression pattern for the P450 genes examined, and suggest that B. bassiana contains a series of hydrocarbon-assimilating enzymes with overlapping specificity in order to target the surface lipids of insect hosts. Phylogenetic analysis of the translated ORFs of the sequences revealed that the enzyme which displayed the highest levels of induction on both alkanes and the insect epicuticular extract represents the founding member of a new cytochrome P450 family, with three of the other sequences assigned as the first members of new P450 subfamilies. The remaining four proteins clustered with known P450 families whose members include alkane monooxygenases.

Biochemistry of insect epicuticle degradation by entomopathogenic fungi

The biochemical interaction between fungal pathogens and their insect host epicuticle was studied by examining fungal hydrocarbon degrading ability. As a contact insecticide, entomopathogenic fungi invade their host through the cuticle, covered by an outermost lipid layer mainly composed of highly stable, very long chain structures. Strains of Beauveria bassiana and Metarhizium anisopliae (Deuteromycotina: Hyphomycetes), pathogenic both to the blood-sucking bug Triatoma infestans (Hemiptera: Reduviidae) and the bean-weevil Acanthoscelides obtectus (Coleoptera, Bruchidae), were grown on different carbon sources. Alkane-grown cells showed a lipid pattern different from that of glucose-grown cells, evidenced by a major switch in the triacylglycerol and sterol components. Radiolabelled hydrocarbons were used to investigate the catabolic pathway and the by-product incorporation into fungal cellular components. The first oxidation round is presumably carried out by a cytochrome P450 enzyme system, the metabolites will traverse the peroxisomal membrane, and after successive transformations will eventually provide the appropriate fatty acyl CoA for complete degradation in the peroxisomes, the site of beta-oxidation in fungi. In this review, we will show the relationship between fungal ability to catabolize very long chain hydrocarbons and virulence parameters.

Subcellular localization of glyoxylate cycle key enzymes involved in oxalate biosynthesis of wood-destroying basidiomycete Fomitopsis palustris grown on glucose

This study investigated the subcellular localization of key enzymes of the glyoxylate cycle, i.e. isocitrate lyase (ICL; EC 4.1.3.1) and malate synthase (EC 2.3.3.9), that function constitutively in coordination with oxalate biosynthesis of glucose-grownFomitopsis palustris. The ICL purified previously fromF. palustrisis termed FPICL1. Subcellular fractionation analysis of the cell homogenate by the sucrose density-gradient method showed that both key enzymes were present in peroxisomes, whereas acetyl-CoA synthase (EC 6.2.1.1) and oxalate-producing oxaloacetate acetylhydrolase (EC 3.7.1.1) were cytosolic. The peroxisomal localization of FPICL1 was further confirmed by electron microscopic and immunocytochemical analysis with anti-FPICL1 antibody. In addition, the peroxisomal target signal, composed of SKL at the C terminus of the cDNA encoding FPICL1, was found, which also suggests that FPICL1 is peroxisomal. Accordingly, it is postulated that transportation of succinate from peroxisomes to mitochondria, and vice versa, for the transportation of isocitrate or citrate, occurs in glucose-grownF. palustrisfor the constitutive metabolic coordination of the TCA and glyoxylate cycles with oxalate biosynthesis.

Candida tropicalis expresses two mitochondrial 2-enoyl thioester reductases that are able to form both homodimers and heterodimers

Here we report on the cloning of a Candida tropicalis gene, ETR2, that is closely related to ETR1. Both genes encode enzymatically active 2-enoyl thioester reductases involved in mitochondrial synthesis of fatty acids (fatty acid synthesis type II) and respiratory competence. The 5'- and 3'-flanking (coding) regions of ETR2 and ETR1 are about 90% (97%) identical, indicating that the genes have evolved via gene duplication. The gene products differ in three amino acid residues: Ile67 (Val), Ala92 (Thr), and Lys251 (Arg) in Etr2p (Etr1p). Quantitative PCR analysis and reverse transcriptase-PCR indicated that both genes were expressed about equally in fermenting and ETR1 predominantly respiring yeast cells. Like the situation with ETR1, expression of ETR2 in respiration-deficient Saccharomyces cerevisiae mutant cells devoid of Ybr026p/Etr1p was able to restore growth on glycerol. Triclosan that is used as an antibacterial agent against fatty acid synthesis type II 2-enoyl thioester reductases inhibited growth of FabI overexpressing mutant yeast cells but was not able to inhibit respiratory growth of the ETR2- or ETR1-complemented mutant yeast cells. Resolving of crystal structures obtained via Etr2p and Etr1p co-crystallization indicated that all possible dimer variants occur in the same asymmetric unit, suggesting that similar dimer formation also takes place in vivo.

Construction of an autonomously replicating plasmid in n-alkane-assimilating yeast, Candida tropicalis

A transformation system using the autonomously replicating plasmid in the n-alkane-assimilating and asporogenic diploid yeast, Candida tropicalis, was developed. For the cloning of a DNA fragment containing a potential autonomously replicating sequence (ARS) from the genomic DNA of C. tropicalis, the ura3 mutant obtained using ethylmethane sulfonate as the host and the URA3 gene amplified by PCR using the C. tropicalis genomic DNA as a selectable marker were prepared. Comparison of ARSs among yeasts revealed that the consensus sequence found in S. cerevisiae was also present in C. tropicalis. The autonomously replicating plasmid containing the putative ARS as the shuttle vector, capable of replicating in both E. coli and C. tropicalis, was first constructed. The transformation system using this plasmid, in addition to the integrative transformation system, will be applicable to genetic studies of C. tropicalis.

Isocitrate lyase of Ashbya gossypii--transcriptional regulation and peroxisomal localization

The isocitrate lyase-encoding gene AgICL1 from the filamentous hemiascomycete Ashbya gossypii was isolated by heterologous complementation of a Saccharomyces cerevisiae icl1d mutant. The open reading frame of 1680 bp encoded a protein of 560 amino acids with a calculated molecular weight of 62584. Disruption of the AgICL1 gene led to complete loss of AgIcl1p activity and inability to grow on oleic acid as sole carbon source. Compartmentation of AgIcl1p in peroxisomes was demonstrated both by Percoll density gradient centrifugation and by immunogold labeling of ultrathin sections using specific antibodies. This fitted with the peroxisomal targeting signal AKL predicted from the C-terminal DNA sequence. Northern blot analysis with mycelium grown on different carbon sources as well as AgICL1 promoter replacement with the constitutive AgTEF promoter revealed a regulation at the transcriptional level. AgICL1 was subject to glucose repression, derepressed by glycerol, partially induced by the C2 compounds ethanol and acetate, and fully induced by soybean oil.

A regulatory mutant of Hansenula polymorpha exhibiting methanol utilization metabolism and peroxisome proliferation in glucose

Mutant LGM-128 of Hansenula polymorpha harbors the recessive mutation glr2-1 which confers a complex pleiotropic phenotype, the major feature of which is the metabolically unnecessary induction of methanol utilization metabolism (C1 metabolism) during growth on glucose, whether or not methanol is in the medium. Therefore, in this mutant, peroxisomes are formed and proliferate upon cultivation in glucose-containing media. In these media, LGM-128 shows induction levels of C1 metabolism that are similar to those observed in methanol-containing media. This indicates that GLR2 controls the repression-derepression process stimulated by glucose and that the induction process triggered by methanol plays only a minor role in activating C1 metabolism. Cultivating LGM-128 in methanol and then transferring it to glucose media revealed that active degradative processes occur, leading to the disappearance of C1 metabolism. This observation suggests that, although stimulated by glucose, the two processes are controlled by elements which are, at least in part, distinct. Finally, glr2-1 does not affect ethanol repression, suggesting that in H. polymorpha the two repressing circuits are separated.

Entomopathogenous fungi degrade epicuticular hydrocarbons of Triatoma infestans

Studies were undertaken to analyze the ability of entomopathogenous fungi to degrade insect hydrocarbons. Strains of Beauveria bassiana and Metarhizium anisopliae pathogenic to the blood-sucking bug Triatoma infestans were grown on hydrocarbon and non-hydrocarbon insect lipid extracts and on synthetic hydrocarbon-enriched media as the sole carbon source. Entomopathogenous fungi were shown to utilize hydrocarbons as the only carbon source for their growth. Insect-derived hydrocarbons served more efficiently as metabolic fuel rather than synthetic compounds of similar structure. [3H]n-Pentacosane, [11,12-3H]3,11-dimethylnonacosane, and [14C]n-hexadecane were catabolized into different amounts of polar lipids, free fatty acids, and acylglycerols. In experiments using the branched alkane, labeled hydrocarbons of different chain length than the precursor were also synthesized. Evidence of complete catabolism was obtained by a significant release of 14CO2 from [1-14C]n-hexadecane. 14CO2 production might be used as a simple method to compare hydrocarbon utilization by fungal strains. These data demonstrate that entomopathogenous fungi are able to transform a variety of hydrocarbon structures into different lipid products, part of which may be subsequently utilized for energy production and for the biosynthesis of cellular components. These data are the first evidence of hydrocarbon catabolism and synthesis in entomopathogenous fungi.

Peroxisomal and mitochondrial carnitine acetyltransferases of the n-alkane-assimilating yeast Candida Tropicalis. Analysis of gene structure and translation products

A genomic DNA clone encoding carnitine acetyltransferases (EC 2.3.1.7), localized in two subcellular organelles, peroxisomes and mitochondria of an n-alkane-assimilating yeast Candida tropicalis, was isolated from the yeast lambda EMBL library using a carnitine acetyltransferase CDNA probe. Nucleotide sequence analysis disclosed that the open reading frame was 1881 bp, corresponding to 627 amino acids with a molecular mass of 70760 Da. Comparison of the predicted amino acid sequence of the C. tropicalis enzyme with that of Saccharomyces cerevisiae mitochondrial matrix carnitine acetyltransferase revealed 46.3% identity. It was noticeable that the C. tropicalis enzymes had amino acid sequences similar to both proposed mitochondrial and peroxisomal targeting signals. When the C. tropicalis gene was expressed in S. cerevisiae using its own 5'-upstream region, a 12-fold increase in activity was observed. Western blot analysis revealed the presence of two major proteins whose sizes corresponded to the peroxisomal and mitochondrial proteins detected in C. tropicalis. This suggested that peroxisomal and mitochondrial carnitine acetyltransferases were encoded by one gene, as suggested for the S. cerevisiae enzyme. Furthermore, we have separated and purified these enzymes from peroxisomes and mitochondria of C. tropicalis, and analyzed the amino-terminal amino acid sequences of each. The amino-terminal sequence of the mitochondrial enzyme suggested that a signal sequence had been cleaved during translocation into mitochondria. Concerning the peroxisomal enzyme, the evidence obtained indicated that in vivo the translation was initiated at the second methionine of the open reading frame.

Study of the delta 12-desaturase system of Lipomyces starkeyi

The specific activity of the microsomal delta 12-desaturase system, which transforms oleic acid into linoleic acid, was about 16 pmol/min/mg protein. However, most of the total activity was nonsedimentable even after a 200000 x g centrifugation for 100 min. The study of various physicochemical parameters showed that this enzymatic complex, functioning optimally between pH 7 and 8, had low thermal stability. Ca2+ which may cause an aggregation of the microsomes, and Hg2+ completely inhibited the activity, whereas Mg2+, Mn2+, and Zn2+ were activators. The delta 12-desaturase system was relatively specific toward oleic acid, though isomers of this fatty acid also had an action, either as substrates or as competitive inhibitors, on the activity of the system. The study of the effect of the exogenous oleoyl-CoA and elaidoyl-CoA on the specific activity of the delta 12-desaturase system showed a preference toward oleoyl-CoA.