Cloning, expression, and characterization of Cryptococcus neoformans dihydrofolate reductase

Introns in Cryptococcus

In Cryptococcus neoformans, nearly all genes are interrupted by small introns. In recent years, genome annotation and genetic analysis have illuminated the major roles these introns play in the biology of this pathogenic yeast. Introns are necessary for gene expression and alternative splicing can regulate gene expression in response to environmental cues. In addition, recent studies have revealed that C. neoformans introns help to prevent transposon dissemination and protect genome integrity. These characteristics of cryptococcal introns are probably not unique to Cryptococcus, and this yeast likely can be considered as a model for intron-related studies in fungi.

Role of dihydrofolate reductase in tetrahydrobiopterin biosynthesis and lipid metabolism in the oleaginous fungus Mortierella alpina

Mortierella alpina is a well-known polyunsaturated fatty acid-producing oleaginous fungus. Analysis of the Mort. alpina genome suggests that there is a putative dihydrofolate reductase (DHFR) gene playing a role in the salvage pathway of tetrahydrobiopterin (BH4), which has never been explored in fungi before. DHFR is the sole source of tetrahydrofolate and plays a key role in maintaining BH4 levels. Transcriptome data analysis revealed that DHFR was up-regulated by nitrogen exhaustion, when Mort. alpina starts to accumulate lipids. Significant changes were found in the fatty acid profile in Mort. alpina grown on medium containing DHFR inhibitors compared to Mort. alpina grown on medium without inhibitors. To explore the role of DHFR in folate/BH4 metabolism and its relationship to lipid biosynthesis, we expressed heterologously the gene encoding DHFR from Mort. alpina in Escherichia coli and we purified the recombinant enzyme to homogeneity. The enzymatic activity was investigated by liquid chromatography and MS and VIS-UV spectroscopy. The kinetic parameters and the effects of temperature, pH, metal ions and inhibitors on the activity of DHFR were also investigated. The transcript level of cytosolic NADPH-producing gene involved in folate metabolism is down-regulated by DHFR inhibitors, which highlights the functional significance of DHFR in lipid biosynthesis. The relationship between DHFR and lipid metabolism is thus of major importance, and folate metabolism may be an alternative NADPH source in fatty acid synthesis. To our knowledge, this study is the first to report the comprehensive characterization of a BH4salvage pathway in a fungus.

Primary structure of dihydrofolate reductase and mitochondrial ribosomal protein L36 genes from the basidiomycete Coprinus cinereus

We amplified and sequenced the dihydrofolate reductase (DHFR) gene of the basidiomycete Coprinus cinereus. Downstream of the DHFR coding region, a mitochondrial (mt) ribosomal protein L36 (RPL36) gene was discovered in the opposite orientation to DHFR gene. Putative polyadenylation signals of the two genes overlapped, both containing the 8-bp palindrome 5'-aatatatt-3'. The finding that C. cinereus DHFR gene is closely clustered with a mt protein gene strongly suggests that C. cinereus DHFR is closely related to mt function and evolution. The amino acid sequence of C. cinereus DHFR is most homologous to eukaryotic proteins such as Cryptococcus neoformans and Pneumocystis carinii DHFRs. However, the sequence of C. cinereus mt RPL36 closely resembles RPL36 of bacteria and cyanobacteria such as Synechocystis sp. and Escherichia coli. This result strongly supports the serial endosymbiotic theory of the development of ancestral eukaryotes, and suggests that C. cinereus mt RPL36 gene originated from the ancestral eubacterial genome.

Genetic and biochemical studies establish that the fungicidal effect of a fully depeptidized inhibitor of Cryptococcus neoformans myristoyl-CoA:protein N-myristoyltransferase (Nmt) is Nmt-dependent

Cryptococcus neoformans is a fungal pathogen that causes chronic meningitis in 10% of patients with AIDS. Genetic and biochemical studies were conducted to determine whether myristoyl-CoA:protein N-myristoyltransferase (Nmt) is a target for development of a new class of fungicidal drugs. A single copy of a conditional lethal C. neoformans NMT allele was introduced into the fungal genome by homologous recombination. The allele (nmt487D) produces temperature-sensitive myristic acid auxotrophy. This phenotype is due, in part, to under-myristoylation of a cellular ADP ribosylation factor (Arf) and can be rescued by forced expression of human Nmt. Two isogenic strains with identical growth kinetics at 35 degreesC were used to test the biological effects of an Nmt inhibitor. CPA8 contained a single copy of wild type C. neoformans NMT. HMC1 contained nmt487D plus 10 copies of human NMT. Since a single copy of nmt487D will not support growth at 35 degreesC, survival of HMC1 depends upon its human Nmt. ALYASKLS-NH2, an inhibitor derived from an Arf, was fully depeptidized: p-[(2-methyl-1-imidazol-1-yl)butyl]phenyl-acetyl was used to represent the GLYA tetrapeptide, whereas SKLS was replaced with a chiral tyrosinol scaffold. Kinetic studies revealed Ki (app) values of 1.8 +/- 1 and 9 +/- 2.4 microM for purified fungal and human Nmts, respectively. The minimal inhibitory concentration of the compound was 2-fold lower for CPA8 compared with HMC1. A single dose of 100 microM produced a 5-fold greater inhibition of protein synthesis in CPA8 versus HMC1. The strain specificity of these responses indicates that the fungicidal effect was Nmt-dependent. These two strains may be useful for screening chemical libraries for Nmt-based fungicidal compounds with relatively little activity against the human enzyme.

beta-Tubulin genes and the basis for benzimidazole sensitivity of the opportunistic fungus Cryptococcus neoformans

The basidiomycete Cryptococcus neoformans causes life-threatening infections in immunocompromised patients, and available chemotherapeutic agents are potentially toxic or have limited efficacy. In vitro, C. neoformans is very sensitive to selected benzimidazole compounds (e.g. albendazole), which act by disrupting microtubules through binding to the beta-tubulin subunit. To understand the basis for this benzimidazole sensitivity, we have characterized C. neoformans beta-tubulin genes and their expression. Analysis of PCR amplification products, genomic and cDNA clones and Southern blots identified two beta-tubulin genes. TUB1 contains seven introns, including one that splits the start codon, and encodes a 447 amino acid protein with > 80% identity to most other beta-tubulins. A partial sequence of TUB2 revealed a higher density of introns and a considerably more divergent beta-tubulin. The relative expression of TUB1 to TUB2 determined by reverse-transcription PCR was about 3:1, consistent with a more limited role for the TUB2 product. Comparisons of beta-tubulin sequences from C. neoformans and from various benzimidazole-sensitive and -resistant organisms strongly suggest that the TUB1 product represents the primary benzimidazole target. This was supported by the identification of a His6 to Gln change in TUB1 from three independently isolated albendazole-resistant mutants.

Molecular targets for new antifungal drugs

Fungal infections in man and animals have a significant impact on health. However, there are only a few antifungal agents available for treatment of invasive mycoses. Further understanding of fungal molecular pathogenesis in collaboration with biochemistry and molecular modeling strategies should be able to develop new selective fungicidal agents. An example of this approach is Cryptococcus neoformans, which is reviewed in this discussion, as a model system for identification of antifungal molecular targets. Key words: antifungals, fungi, treatment, cryptococcosis, molecular biology, targets.

The dihydrofolate reductase-encoding gene dyrA of the hyperthermophilic bacterium Thermotoga maritima

The structural gene (dyrA) encoding dihydrofolate reductase (DHFR) of Thermotoga maritima has been cloned, sequenced and expressed in Escherichia coli. The dyrA gene, located immediately upstream from the gene encoding aspartate carbamoyltransferase (pyrB), encodes a highly thermostable enzyme with a distinct thermophilic activity profile. Important structural features are conserved among all bacterial DHFR, yet the DHFR of T. maritima appears unique in a number of insertions and deletions, some of which are reminiscent of eukaryotic DHFR.

Cloning, expression and characterization of thymidylate synthase from Cryptococcus neoformans

The thymidylate synthase (TS)-encoding gene from Cryptococcus neoformans (Cn) has been isolated from cDNA and genomic libraries. The 1127-bp gene contains three introns and a 951-bp open reading frame encoding a 35,844-Da protein. The cDNA clones lack 324 bp of the 5' coding region of the gene. The complete coding sequence was assembled as an expression cassette in pUC19 using parts of the coding sequence from the cDNA and genomic DNA and completing the sequence using synthetic DNA. Production of active TS from Cn (CnTS) was first demonstrated by complementation of a thymine(Thy)-requiring Escherichia coli strain. The expression cassette was subsequently subcloned into the T7 polymerase vector pET15-b. In this construct, CnTS is produced as approximately 10% of the total soluble protein in E. coli. Homogeneous enzyme was obtained at a 36% yield after consecutive chromatography on DEAE-cellulose, Q-Sepharose, phenyl-Sepharose and Affi-Gel Blue. Steady-state kinetic analysis showed that the Km values for dUMP and CH2H4.folate were 2.7 +/- 0.5 microM and 38.2 +/- 2.5 microM, respectively, and the kcat was 5.1 s-1. The enzyme was stable upon storage at -80 degrees C in Tris.HCl pH 7.4 and thiol.

Biochemical and molecular characterization of the diphenol oxidase of Cryptococcus neoformans: identification as a laccase

Melanin production is a major virulence factor for Cryptococcus neoformans, an organism causing life-threatening infections in an estimated 10% of AIDS patients. In order to characterize the events involved in melanin synthesis, an enzyme having diphenol oxidase activity was purified and its gene was cloned. The enzyme was purified as a glycosylated 75-kDa protein which migrated at 66 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis after deglycosylation by endoglycosidase F. Substrate specificity resembled that of a laccase in that it oxidized multiple diphenolic and diamino compounds. Dopamine was shown by mass spectroscopy to be oxidized to decarboxy dopachrome, an intermediate of melanin synthesis. The enzyme contained 4.1 +/- 0.1 mol of copper per mol. It resembled a laccase in its absorbance spectrum, containing a peak of 610 nm and the shoulder at 320 nm, corresponding to the absorbance of a type I and type III copper, respectively. The cloned gene of C. neoformans laccase (CNLAC1) contained a single open reading frame encoding a polypeptide 624 amino acids in length. The encoded polypeptide contained a presumptive leader sequence, on the basis of its relative hydrophobicity and by comparison of the sequence to that of the N-terminal sequence of the purified enzyme. CNLAC1 also contained 14 introns ranging from 52 to 340 bases long. Transcriptional activity of CNLAC1 was found to be derepressed in the absence of glucose and to correspond to an increase in enzymatic activity.