A heterologous expression platform in Aspergillus nidulans for the elucidation of cryptic secondary metabolism biosynthetic gene clusters: discovery of the Aspergillus fumigatus sartorypyrone biosynthetic pathway

Aspergillus fumigatus is a serious human pathogen causing life-threatening Aspergillosis in immunocompromised patients. Secondary metabolites (SMs) play an important role in pathogenesis, but the products of many SM biosynthetic gene clusters (BGCs) remain unknown. In this study, we have developed a heterologous expression platform in Aspergillus nidulans, using a newly created genetic dereplication strain, to express a previously unknown BGC from A. fumigatus and determine its products. The BGC produces sartorypyrones, and we have named it the spy BGC. Analysis of targeted gene deletions by HRESIMS, NMR, and microcrystal electron diffraction (MicroED) enabled us to identify 12 products from the spy BGC. Seven of the compounds have not been isolated previously. We also individually expressed the polyketide synthase (PKS) gene spyA and demonstrated that it produces the polyketide triacetic acid lactone (TAL), a potentially important biorenewable platform chemical. Our data have allowed us to propose a biosynthetic pathway for sartorypyrones and related natural products. This work highlights the potential of using the A. nidulans heterologous expression platform to uncover cryptic BGCs from A. fumigatus and other species, despite the complexity of their secondary metabolomes.

Branched-chain amino acid biosynthesis in fungi

Branched-chain amino acids (BCAAs)-isoleucine, leucine, and valine-are synthesized by fungi. These amino acids are important components of proteins and secondary metabolites. The biochemical pathway for BCAA biosynthesis is well-characterized in the yeast Saccharomyces cerevisiae. The biosynthesis of these three amino acids is interconnected. Different precursors are metabolized in multiple steps through shared enzymes to produce isoleucine and valine, and the valine biosynthesis pathway branches before the penultimate step to a series of leucine biosynthesis-specific steps to produce leucine. Recent efforts have made advances toward characterization of the BCAA biosynthesis pathway in several fungi, revealing diversity in gene duplication and functional divergence in the genes for these enzymatic steps in different fungi. The BCAA biosynthesis pathway is regulated by the transcription factor LEU3 in S. cerevisiae, and LeuB in Aspergillus nidulans and Aspergillus fumigatus, and the activity of these transcription factors is modulated by the leucine biosynthesis pathway intermediate α-isopropylmalate. Herein, we discuss recent advances in our understanding of the BCAA pathway and its regulation, focusing on filamentous ascomycete fungi and comparison with the well-established process in yeast.

Co-option of an extracellular protease for transcriptional control of nutrient degradation in the fungus Aspergillus nidulans

Nutrient acquisition is essential for all organisms. Fungi regulate their metabolism according to environmental nutrient availability through elaborate transcription regulatory programs. In filamentous fungi, a highly conserved GATA transcription factor AreA and its co-repressor NmrA govern expression of genes involved in extracellular breakdown, uptake, and metabolism of nitrogen nutrients. Here, we show that the Aspergillus nidulans PnmB protease is a moonlighting protein with extracellular and intracellular functions for nitrogen acquisition and metabolism. PnmB serves not only as a secreted protease to degrade extracellular nutrients, but also as an intracellular protease to control the turnover of the co-repressor NmrA, accelerating AreA transcriptional activation upon nitrogen starvation. PnmB expression is controlled by AreA, which activates a positive feedback regulatory loop. Hence, we uncover a regulatory mechanism in the well-established controls determining the response to nitrogen starvation, revealing functional evolution of a protease gene for transcriptional regulation and extracellular nutrient breakdown.

Biodegradable Drug-Delivery Peptide Nanocapsules

Branched amphiphilic peptide capsules (BAPCs) are an efficient transport system that can deliver nucleic acids, small proteins, and solutes. The ability of BAPCs to break down is essential to their adoption as a delivery vehicle for human and agricultural applications. Until now, however, BAPCs were shown to be inert to mammalian degradation systems. Here, we demonstrate, using BAPCs encapsulating the toxic urea analogue thiourea, that the common soil fungus Aspergillus nidulans can degrade BAPCs. We provide evidence that this degradation is extracellular through the action of secreted proteases. Our data indicate that BAPCs are likely biodegradable in the environment.

Hybrid Transcription Factor Engineering Activates the Silent Secondary Metabolite Gene Cluster for (+)-Asperlin in Aspergillus nidulans

Fungi are a major source of valuable bioactive secondary metabolites (SMs). These compounds are synthesized by enzymes encoded by genes that are clustered in the genome. The vast majority of SM biosynthetic gene clusters are not expressed under normal growth conditions, and their products are unknown. Developing methods for activation of these silent gene clusters offers the potential for discovering many valuable new fungal SMs. While a number of useful approaches have been developed, they each have limitations, and additional tools are needed. One approach, upregulation of SM gene cluster-specific transcription factors that are associated with many SM gene clusters, has worked extremely well in some cases, but it has failed more often than it has succeeded. Taking advantage of transcription factor domain modularity, we developed a new approach. We fused the DNA-binding domain of a transcription factor associated with a silent SM gene cluster with the activation domain of a robust SM transcription factor, AfoA. Expression of this hybrid transcription factor activated transcription of the genes in the target cluster and production of the antibiotic (+)-asperlin. Deletion of cluster genes confirmed that the cluster is responsible for (+)-asperlin production, and we designate it the aln cluster. Separately, coinduction of expression of two aln cluster genes revealed the pathway intermediate (2 Z,4 Z,6 E)-octa-2,4,6-trienoic acid, a compound with photoprotectant properties. Our findings demonstrate the potential of our novel synthetic hybrid transcription factor strategy to discover the products of other silent fungal SM gene clusters.

Comparative genomics reveals high biological diversity and specific adaptations in the industrially and medically important fungal genus Aspergillus

BACKGROUND

The fungal genus Aspergillus is of critical importance to humankind. Species include those with industrial applications, important pathogens of humans, animals and crops, a source of potent carcinogenic contaminants of food, and an important genetic model. The genome sequences of eight aspergilli have already been explored to investigate aspects of fungal biology, raising questions about evolution and specialization within this genus.

RESULTS

We have generated genome sequences for ten novel, highly diverse Aspergillus species and compared these in detail to sister and more distant genera. Comparative studies of key aspects of fungal biology, including primary and secondary metabolism, stress response, biomass degradation, and signal transduction, revealed both conservation and diversity among the species. Observed genomic differences were validated with experimental studies. This revealed several highlights, such as the potential for sex in asexual species, organic acid production genes being a key feature of black aspergilli, alternative approaches for degrading plant biomass, and indications for the genetic basis of stress response. A genome-wide phylogenetic analysis demonstrated in detail the relationship of the newly genome sequenced species with other aspergilli.

CONCLUSIONS

Many aspects of biological differences between fungal species cannot be explained by current knowledge obtained from genome sequences. The comparative genomics and experimental study, presented here, allows for the first time a genus-wide view of the biological diversity of the aspergilli and in many, but not all, cases linked genome differences to phenotype. Insights gained could be exploited for biotechnological and medical applications of fungi.

Prevalence of transcription factors in ascomycete and basidiomycete fungi

Background

Gene regulation underlies fungal physiology and therefore is a major factor in fungal biodiversity. Analysis of genome sequences has revealed a large number of putative transcription factors in most fungal genomes. The presence of fungal orthologs for individual regulators has been analysed and appears to be highly variable with some regulators widely conserved and others showing narrow distribution. Although genome-scale transcription factor surveys have been performed before, no global study into the prevalence of specific regulators across the fungal kingdom has been presented.

Results

In this study we have analysed the number of members for 37 regulator classes in 77 ascomycete and 31 basidiomycete fungal genomes and revealed significant differences between ascomycetes and basidiomycetes. In addition, we determined the presence of 64 regulators characterised in ascomycetes across these 108 genomes. This demonstrated that overall the highest presence of orthologs is in the filamentous ascomycetes. A significant number of regulators lacked orthologs in the ascomycete yeasts and the basidiomycetes. Conversely, of seven basidiomycete regulators included in the study, only one had orthologs in ascomycetes.

Conclusions

This study demonstrates a significant difference in the regulatory repertoire of ascomycete and basidiomycete fungi, at the level of both regulator class and individual regulator. This suggests that the current regulatory systems of these fungi have been mainly developed after the two phyla diverged. Most regulators detected in both phyla are involved in central functions of fungal physiology and therefore were likely already present in the ancestor of the two phyla.

Deletion and overexpression of the Aspergillus nidulans GATA factor AreB reveals unexpected pleiotropy

The Aspergillus nidulans transcription factor AreA is a key regulator of nitrogen metabolic gene expression. AreA contains a C-terminal GATA zinc finger DNA-binding domain and activates expression of genes necessary for nitrogen acquisition. Previous studies identified AreB as a potential negative regulator of nitrogen catabolism showing similarity with Penicillium chrysogenum NreB and Neurospora crassa ASD4. The areB gene encodes multiple products containing an N-terminal GATA zinc finger and a leucine zipper motif. We deleted the areB gene and now show that AreB negatively regulates AreA-dependent nitrogen catabolic gene expression under nitrogen-limiting or nitrogen-starvation conditions. AreB also acts pleiotropically, with functions in growth, conidial germination and asexual development, though not in sexual development. AreB overexpression results in severe growth inhibition, aberrant cell morphology and reduced AreA-dependent gene expression. Deletion of either the DNA-binding domain or the leucine zipper domain results in loss of both nitrogen and developmental phenotypes.

Sumoylation in Aspergillus nidulans: sumO inactivation, overexpression and live-cell imaging

Sumoylation, the reversible covalent attachment of small ubiquitin-like modifier (SUMO) peptides has emerged as an important regulator of target protein function. In Saccharomyces cerevisiae, but not in Schizosaccharyomes pombe, deletion of the gene encoding SUMO peptides is lethal. We have characterized the SUMO-encoding gene, sumO, in the filamentous fungus Aspergillus nidulans. The sumO gene was deleted in a diploid and sumODelta haploids were recovered. The mutant was viable but exhibited impaired growth, reduced conidiation and self-sterility. Overexpression of epitope-tagged SumO peptides revealed multiple sumoylation targets in A. nidulans and SumO overexpression resulted in greatly increased levels of protein sumoylation without obvious phenotypic consequences. Using five-piece fusion PCR, we generated a gfp-sumO fusion gene expressed from the sumO promoter for live-cell imaging of GFP-SumO and GFP-SumO-conjugated proteins. Localization of GFP-SumO is dynamic, accumulating in punctate spots within the nucleus during interphase, lost at the onset of mitosis and re-accumulating during telophase.

Transcriptional control ofnmrAby the bZIP transcription factor MeaB reveals a new level of nitrogen regulation inAspergillus nidulans

Fungi can use a diverse range of nitrogen sources. Some nitrogen sources sustain a rapid growth rate and are used in preference to less readily metabolized nitrogen sources. The mechanisms involved in this control of nitrogen utilization have been studied in the model filamentous ascomycete, Aspergillus nidulans. The GATA transcription factor AreA is necessary for the expression of nitrogen-catabolic permeases and enzymes. AreA activity is controlled by multiple mechanisms including regulated areA transcript levels and regulated AreA nuclear export. During nitrogen sufficiency, AreA activation is also prevented by the co-repressor NmrA. We have investigated nitrogen signalling to NmrA. NmrA overexpression prevents AreA function irrespective of the nitrogen status. The mRNA levels of areA and nmrA are inversely regulated, suggesting that the relative levels of AreA and NmrA are critical in determining AreA activation. The bZIP transcription factor MeaB was found to activate nmrA expression and a conserved element, TTGCACCAT, bound by MeaB in vitro is present in the promoters of NmrA homologues in other filamentous ascomycetes. Expression of meaB was not strongly regulated suggesting that transcriptional activation by MeaB is modulated by the nitrogen status. This work highlights a new level of complexity in the regulation of nitrogen catabolism.

Genetic manipulation of Aspergillus nidulans: meiotic progeny for genetic analysis and strain construction

The multicellular microbial eukaryote Aspergillus nidulans is an excellent model for the study of a wide array of biological processes. Studies in this system contribute significantly to understanding fundamental biological principles and are relevant for biotechnology and industrial applications, as well as human, animal and plant fungal pathogenesis. A. nidulans is easily manipulated using classical and molecular genetics. Here, we describe the storage and handling of A. nidulans and procedures for genetic crossing, progeny analysis and growth testing. These procedures are used for Mendelian analysis of segregation of alleles to show whether a mutant phenotype segregates as a single gene and independent assortment of genes to determine the linkage relationship between genes. Meiotic crossing is used for construction of multiple mutant strains for genetic analysis. Genetic crossing and analysis of progeny can be undertaken in 2-3 weeks and growth testing takes 2-3 days.

Genetic manipulation of Aspergillus nidulans: heterokaryons and diploids for dominance, complementation and haploidization analyses

The haploid microbial eukaryote Aspergillus nidulans is a powerful genetic system, which allows analysis of a broad range of biological phenomena. In addition to conventional analysis of meiotic progeny in a single generation, parasexual analysis affords a rapid and convenient method for genetic analysis. We describe the construction of A. nidulans heterokaryons and diploids for use in genetic analysis to determine dominance and conduct complementation tests. We also describe the rapid mapping of mutations to chromosomes by haploidization of diploids carrying marked chromosomes. Balanced heterokaryons may be established within 10 days and diploids may be constructed in 2-3 weeks. Dominance tests and complementation tests using balanced heterokaryons or diploids may be completed in 2-3 days. Haploidization analysis of heterozygous diploids can be achieved within 10 days. These protocols should be adaptable for use in related Aspergilli and Penicillia, which lack a known meiotic cycle.

Characterization of regulatory non-catalytic hexokinases in Aspergillus nidulans

Hexokinases catalyse the first step in glucose metabolism and play a role in glucose sensing in mammals, plants and fungi. We describe a new class of hexokinases that appear to be solely regulatory in function. The Aspergillus nidulans hxkD gene (formerly named xprF) encodes a hexokinase-like protein. We constructed hxkDDelta gene disruption mutants which showed increased levels of extracellular protease in response to carbon starvation. The hxkDDelta mutations are not completely recessive, indicating that the level of the gene product is critical. Transcript levels of hxkD increase during carbon starvation and this response is not dependent on functional HxkD. A gene encoding a second atypical hexokinase (HxkC) was identified. The hxkCDelta gene disruption mutant exhibits a phenotype similar, but not identical, to hxkDDelta mutants. As with hxkD, mutations in hxkC are suppressed by loss-of-function mutations in xprG, which encodes a putative transcriptional activator involved in the response to nutrient limitation. We show that GFP-tagged HxkD was found only in nuclei suggesting a regulatory role for HxkD. GFP-tagged HxkC was associated with mitochondria. Homologs of hxkC and hxkD are conserved in multi-cellular fungi. Genes encoding atypical hexokinases are present in many genome sequence databases. Thus, non-catalytic hexokinases may be widespread.

The Aspergillus nidulans rcoA gene is required for veA-dependent sexual development

The Aspergillus nidulans rcoADelta mutant exhibits growth and developmental defects. We show that the rcoADelta mutant lacks cleistothecia and is self-sterile. In crosses with wild-type strains, rcoADelta nuclei do not contribute to the cleistothecial walls. Furthermore, sexual development resulting from veA overexpression is rcoA dependent, indicating that rcoA lies downstream of veA in the sexual development pathway.

Nuclear accumulation of the GATA factor AreA in response to complete nitrogen starvation by regulation of nuclear export

Both the availability and the quality of nutrients affect cellular functions by controlling gene activity. AreA, a member of the GATA family of transcription factors, globally activates expression of genes involved in nitrogen source utilization in Aspergillus nidulans. The quality of the nitrogen source determines the level and activation capacity of AreA through controls at the level of areA mRNA stability and by interaction of AreA with the corepressor NmrA. The availability of potential nitrogen sources also affects the activation capacity of AreA. We show that the complete absence of a nitrogen source results in an enhanced level of AreA-dependent gene expression and that this response is independent of mechanisms regulating AreA activity in response to nitrogen source quality. During nitrogen starvation AreA accumulates in the nucleus, but the presence of a potential nitrogen source or carbon starvation prevents this accumulation. Furthermore, accumulated AreA is rapidly lost from the nuclei of nitrogen-starved cells when a nitrogen source is supplied or when a carbon source is absent, and this accompanies arrest of the AreA-dependent nitrogen starvation response on regulated gene expression. By the generation of a leptomycin B-sensitive mutant, we have been able to show that nuclear exit occurs via the CrmA exportin. We conclude that sensing mechanisms discriminate between starvation and the presence of potential nutrients that can signal to the AreA transcription factor. Nitrogen source availability, but not quality, affects nuclear accumulation by regulating nuclear exit of AreA, providing a rapid response to changes in the supply of nutrients.

TupA, the Penicillium marneffei Tup1p homologue, represses both yeast and spore development

Fungal pathogenesis is frequently associated with dimorphism - morphological changes between yeast and filamentous forms. Penicillium marneffei, an opportunistic human pathogen, exhibits temperature-dependent dimorphism, with growth at 25 degrees C as filamentous multinucleate hyphae switching at 37 degrees C to uninucleate yeast cells associated with intracellular pathogenesis. The filamentous hyphae also undergo asexual development generating uninucleate spores, the infectious propagules. Both processes require a switch to coupled nuclear and cell division. Homologous regulators, including Tup1p/GROUCHO-related WD40 repeat transcription factors, control dimorphism in Candida albicans and asexual development in Aspergillus nidulans. Unlike these fungi, P. marneffei has both developmental programmes allowing examination of common and programme-specific controls. We show that deletion of tupA, the P. marneffei TUP1 homologue, confers reduced filamentation and inappropriate yeast morphogenesis at 25 degrees C, in stark contrast to constitutive filamentation observed when C. albicans TUP1 is deleted. Deletion of tupA also confers premature brlA-dependent asexual development, unlike reduced asexual development in the corresponding A. nidulans rcoA deletion mutant. Furthermore, the A. nidulans rcoA deletion mutant is self-sterile, and we show that tupA from P. marneffei, which lacks an apparent sexual cycle, complements both the asexual and sexual development phenotypes. Therefore, TupA coordinates cell fate by promoting filamentation and repressing both spore and yeast morphogenetic programmes.

Detection of unpaired DNA at meiosis results in RNA-mediated silencing

During meiosis, homologous chromosomes must pair in order to permit recombination and correct chromosome segregation to occur. Two recent papers show that meiotic pairing is also important for correct gene expression during meiosis. They describe data for the filamentous fungus Neurospora crassa that show that a lack of pairing generated by ectopic integration of genes can result in silencing of genes expressed during meiosis. This can result in aberrant meioses whose defects are specific to the function of the unpaired gene. Furthermore, mutations affecting the silencing mechanism have been selected in a gene encoding a putative RNA-dependent RNA polymerase. This finding indicates the involvement of a meiotic specific post-transcriptional gene silencing mechanism (PTGS) similar to that observed in vegetative cells in N. crassa and other organisms. Finally, this gene product is essential for normal meiosis, suggesting that RNA-dependent processes are fundamental to the sexual cycle.

Functional analysis of TamA, a coactivator of nitrogen-regulated gene expression in Aspergillus nidulans

The tam A gene of Aspergillus nidulans encodes a 739-amino acid protein with similarity to Uga35p/Dal81p/DurLp of Saccharomyces cerevisiae. It has been proposed that TamA functions as a co-activator of AreA, the major nitrogen regulatory protein in A. nidulans. Because AreA functions as a transcriptional activator under nitrogen-limiting conditions, we investigated whether TamA was also present in the nucleus. We found that a GFP-TamA fusion protein was predominantly localised to the nucleus in the presence and absence of ammonium, and that AreA was not required for this distribution. As the predicted DNA-binding domain of TamA is not essential for function, we have used a number of approaches to further define functionally important regions. We have cloned the tamA gene of A. oryzae and compared its functional and sequence characteristics with those of A. nidulans tamA and S. cerevisiae UGA35/DAL81/DURL. The Aspergillus homologues are highly conserved and functionally interchangeable, whereas the S. cerevisiae gene does not complement a tamA mutant when expressed in A. nidulans. Uga35p/Dal81p/DurLp was also found to be unable to recruit AreA. The sequence changes in a number of tamA mutant alleles were determined, and altered versions of TamA were tested for tamA complementation and interaction with AreA. Changes in most regions of TamA appeared to destroy its function, suggesting that the overall conformation of the protein may be critical for its activity.

PreproTRH(178-199) and two novel peptides (pFQ7 and pSE14) derived from its processing, which are produced in the paraventricular nucleus of the rat hypothalamus, are regulated during suckling

Suckling increases preproTRH messenger RNA in hypothalamic paraventricular neurons (PVN) and also markedly increases TRH release during the first period of lactation. Whether lactation alters preproTRH processing resulting in the generation of novel proTRH-derived peptides that may be involved in the regulation of PRL secretion lactation is not known. Therefore, in the present study we determine whether some other peptides derived from proTRH potentially contribute to lactation-induced PRL secretion. We have recently demonstrated that two members of the family of prohormone convertases PC1 and PC2 play a significant role in proTRH processing. PC1 is the major contributor in proTRH processing, whereas PC2 may have a specific role in cleaving TRH from its extended forms. In this study, we used a recombinant vaccinia virus system to coexpress rat preproTRH complementary DNA with PC1, PC2, and the neuropeptide 7B2 in GH4C1 cells (somatomammothophs, rat). We found that two novel peptides, preproTRH(178-184) (pFQ(7)), and preproTRH(186-199) (pSE(14)), were formed after the cleavage of their precursor preproTRH(178-199) (pFE(22)) by only PC2. Their formation was confirmed by microsequence analysis. Anatomical analyses revealed that these peptides are also found in the rat PVN. In addition, we found that pFE(22), pSE(14) and pFQ(7) produced a dose-dependent release of PRL from primary cultures of pituitary cells compared with one of the well studied secretagogues of PRL, TRH. To establish whether these peptides might play a role in vivo in the regulation of PRL release, we took rat litters on postnatal day 4, separated the pups from their mothers for 6 h, and then reunited the pups and mothers for 45 min. At the end of this period, the mothers were killed, acidic extracts of microdissected PVN were prepared and subjected to SDS-PAGE, followed by slicing and analysis by pFE(22) RIA. Forty-five minutes of suckling induced a marked 6-fold increase in serum levels of PRL. In addition, PVN levels of pFE(22) and pSE(14) increased approximately 5-fold during the same period in the acutely suckling females. Lactating animals that were separated from their litters and never reunited with their pups had low levels of PRL, and pFE(22) and pSE(14). These data provide the first evidence for alterations in proTRH processing in the PVN during lactation and suggest that the products of this altered processing may play a physiological role in the regulation of PRL secretion.

The Aspergillus nidulans creC gene involved in carbon catabolite repression encodes a WD40 repeat protein

Expression of many microbial genes required for the utilisation of less favoured carbon sources is carbon catabolite repressed in the presence of a preferred carbon source such as D-glucose. In Aspergillus nidulans, creC mutants show derepression in the presence of D-glucose of some, but not all, systems normally subject to carbon catabolite repression. These mutants also fail to grow on some carbon sources, and show minor morphological impairment and altered sensitivity to toxic compounds including molybdate and acriflavin. The pleiotropic nature of the phenotype suggests a role for the creC gene product in the carbon regulatory cascade. The creC gene was cloned and found to encode a protein which contains five WD40 motifs. The sequence changes in three mutant alleles were found to lead to production of truncated proteins which lack one or more of the WD40 repeats. The similarity of the phenotypes conferred by these alleles implies that these alleles represent loss of function alleles. Deletion analysis also showed that at least the most C-terminal WD40 motif is required for function. The CreC protein is highly conserved relative to the Schizosaccharomyces pombe protein Yde3--whose function is unknown--and human and mouse DMR-N9, which may be associated with myotonic dystrophy.

FacB, the Aspergillus nidulans activator of acetate utilization genes, binds dissimilar DNA sequences

The facB gene is required for acetate induction of acetamidase (amdS) and the acetate utilization enzymes acetyl-CoA synthase (facA), isocitrate lyase (acuD) and malate synthase (acuE) in Aspergillus nidulans. The facB gene encodes a transcriptional activator with a GAL4-type Zn(II)2Cys6 zinc binuclear cluster DNA-binding domain which is shown to be required for DNA binding. In vitro DNA-binding sites for FacB in the 5' regions of the amdS, facA, acuD and acuE genes have been identified. Mutations in amdS FacB DNA-binding sites affected expression of an amdS-lacZ reporter in vivo and altered the affinity of in vitro DNA binding. This study shows that the FacB Zn(II)2Cys6 cluster binds to dissimilar sites which show similarity in form but not sequence with DNA-binding sites of other Zn(II)2Cys6 proteins. Sequences with homology to FacB sites are found in the 5' regions of genes regulated by the closely related yeast Zn(II)2Cys6 protein CAT8.

Molecular characterization of mutants of the acetate regulatory gene facB of Aspergillus nidulans

The facB gene of Aspergillus nidulans encodes a DNA binding transcriptional activator required for growth on acetate as a sole carbon source. FacB contains N-terminal GAL4-like Zn(II)2Cys6 (or C6 zinc) binuclear cluster DNA binding and leucine zipper-like heptad repeat motifs and central and C-terminal acidic alpha-helical regions. facB recessive loss of function mutants are deficient in acetate induction of acetyl-CoA synthase, isocitrate lyase, malate synthase, acetamidase, and NADP-isocitrate dehydrogenase. Characterization of lesions in facB mutant alleles has localized important functional regions of the FacB protein. Two extreme mutants are shown to lack the C-terminal region of the protein. Two temperature sensitive mutants contain amino acid substitutions in the DNA binding domain and are shown to affect acetate induction of amdS-lacZ expression and confer temperature sensitive in vitro DNA binding. Two temperature sensitive facB mutations result in thermolability of acetyl-CoA synthase, isocitrate lyase, and malate synthase but not acetamidase or NADP-isocitrate dehydrogenase in crude extracts. This suggests that FacB may have a structural role in acetate metabolism in addition to its regulatory function.

Processing of prothyrotropin-releasing hormone by the family of prohormone convertases

The post-translational processing of prothyrotropin-releasing hormone (pro-TRH25-255) has been extensively studied in our laboratory, and the processing pathway to mature TRH has been elucidated. We have also demonstrated that recombinant PC1 and PC2 process partially purified pro-TRH to cryptic peptides in vitro and that pro-TRH and PC1 mRNAs are coexpressed in primary cultures of hypothalamic neurons. To further define the role of each convertase, and particularly PC1 and PC2, in pro-TRH processing, recombinant vaccinia viruses were used to coexpress the prohormone convertases PC1, PC2, PACE4, PC5-B, furin, or control dynorphin together with rat prepro-TRH in constitutively secreting LoVo cells or in the regulated endocrine GH4C1 cell line. Radioimmunoassays from LoVo-derived secreted products indicated that furin cleaves the precursor to generate both N- and C-terminal intermediates. PC1, PC2, and PACE4 only produced N-terminal intermediates, but less efficiently than furin. In GH4C1 cells, PC1, PC2, furin, PC5-B, and PACE4 produced both N-terminal and C-terminal forms. Significantly, TRH-Gly and TRH were mostly produced by PC1, PC2, and furin. Utilizing gel electrophoresis to further analyze the cleavage specificities of PC1 and PC2, we found that PC1 seems primarily responsible for cleavage to both intermediates and mature TRH, since it generated all products at significantly higher levels than PC2. The addition of 7B2 to the coinfection did not augment the ability of PC2 to cleave pro-TRH to either N- or C-terminal forms.

Evolution of a fungal regulatory gene family: the Zn(II)2Cys6 binuclear cluster DNA binding motif

The coevolution of DNA binding proteins and their cognate binding sites is essential for the maintenance of function. As a result, comparison of DNA binding proteins of unknown function in one species with characterized DNA binding proteins in another can identify potential targets and functions. The Zn(II)2Cys6 (or C6 zinc) binuclear cluster DNA binding domain has thus far been identified exclusively in fungal proteins, generally transcriptional regulators, and there are more than 80 known or predicted proteins which contain this motif, the best characterized of which are GAL4, PPR1, LEU3, HAP1, LAC9, and PUT3. Here we review all known proteins containing the Zn(II)2Cys6 motif, along with their function, DNA binding, dimerization, and zinc(II) coordination properties and DNA binding sites. In addition, we have identified all of the Zn(II)2Cys6 motif-containing proteins in the sequence databases, including a large number with unknown function from the completed Saccharomyces cerevisiae and ongoing Schizosaccharomyces pombe genome projects, and examined the phylogenetic relationships of all the Zn(II)2Cys6 motifs from these proteins. Based on these relationships, we have assigned potential functions to a number of these unknown proteins.

The acetate regulatory gene facB of Aspergillus nidulans encodes a Zn(II)2Cys6 transcriptional activator

Genetic studies have indicated that the facB gene of Aspergillus nidulans is a major regulatory gene involved in acetamide and acetate utilisation. Sequencing of the facB gene revealed that it encodes a protein that contains an N-terminal GAL4-like Zn(II)2Cys6 (or C6 zinc) binuclear cluster for DNA binding, leucine zipper-like heptad repeat motifs and central and C-terminal acidic alpha-helical regions, consistent with a function as a DNA-binding transcriptional activator. The Zn(II)2Cys6 cluster shows strong similarity with those of the Saccharomyces cerevisiae carbon metabolism regulatory proteins CAT8 and SIP4. A significant level of similarity with CAT8 is found throughout the length of the protein, suggesting at least partial functional homology. The facB genes of Aspergillus oryzae and Aspergillus niger were also sequenced and found to be highly conserved. Deletion of the facB gene confirmed that it is required for growth on acetate as a sole carbon source. Functional dissection using deletion and fusion constructs and in vitro mutagenesis indicated that the Zn(II)2Cys6 cluster and the C-terminal end of the protein are required for function.

Pro-thyrotropin-releasing hormone processing by recombinant PC1

Pro-thyrotropin-releasing hormone (proTRH) is the precursor to thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2), the hypothalamic releasing factor that stimulates synthesis and release of thyrotropin from the pituitary gland. Five copies of the TRH progenitor sequence (Gln-His-Pro-Gly) and seven cryptic peptides are formed following posttranslational proteolytic cleavage of the 26-kDa rat proTRH precursor. The endopeptidase(s) responsible for the physiological conversion of proTRH to the TRH progenitor form is currently unknown. We examined the in vitro processing of [3H]leucine-labeled or unlabeled proTRH by partially purified recombinant PC1. Recombinant PC1 processed the 26-kDa TRH precursor by initially cleaving the prohormone after the basic amino acid at either position 153 or 159. Based on the use of our well-established antibodies, we propose that the initial cleavage gave rise to the formation of a 15-kDa N-terminal peptide (preproTRH25-152 or pre-proTRH25-158) and a 10-kDa C-terminal peptide (pre-proTRH154-255 or preproTRH160-255). Some initial cleavage occurred after amino acid 108 to generate a 16.5-kDa C-terminal peptide. The 15-kDa N-terminal intermediate was further processed to a 6-kDa peptide (prepro-TRH25-76 or preproTRH25-82) and a 3.8-kDa peptide (preproTRH83-108), whereas the 10-kDa C-terminal intermediate was processed to a 5.4-kDa peptide (prepro-TRH206-255). The optimal pH for these cleavages was 5.5. ZnCl2, EDTA, EGTA, and the omission of Ca2+ inhibited the formation of pYE27 (preproTRH25-50), one of the proTRH N-terminal products, by 48, 82, 72, and 45%, respectively. This study provides evidence, for the first time, that recombinant PC 1 enzyme can process proTRH to its predicted peptide intermediates.

Acute and chronic effects of diethylstilbestrol on serum levels of alpha melanocyte stimulating hormone and prolactin in the hamster

Continuous exposure to diethylstilbestrol (DES) induces renal tumors in male hamsters. The tumor formation is preceded by an increase in pituitary weight and elevation of the pituitary hormones-alpha melanocyte stimulating hormone (aMSH) and prolactin (Pr1). A decline in Pr1 (to normal levels) but not aMSH then accompanies the development of tumors and the enlargement of the intermediate lobe of the pituitary. Hypophysectomy, castration and thymectomy reduced serum levels of aMSH. DES administered for one week increased the serum levels of both hormones in normal and castrated animals, but not in hypophysectomized hamsters.

Effects of chronic hyperprolactinemia on sexual arousal and erectile function in male rats

Studies were conducted to determine if the inhibitory effects of chronic hyperprolactinemia on sexual behavior in male rats occur through reduced sexual arousal as opposed to reduced erectile function. Pituitary-grafted (hyperprolactinemic) and sham-operated, gonadally intact male rats were given standard tests of copulatory behavior, mounting behavior tests after genital anesthetization and penile reflex tests while restrained in a supine position. Beginning 7 days after pituitary transplantation, the number of erections displayed in penile reflex tests was significantly reduced in the pituitary-grafted animals. Increased intromission latencies and reduced intromission rates in tests of copulatory behavior were also observed at this time. Beginning 3-4 weeks after surgery, mounting rates were inhibited in hyperprolactinemic animals in both the copulatory behavior tests and tests of mounting behavior after genital anesthetization. Prolactin levels were significantly elevated in the pituitary-grafted animals, but serum testosterone levels were unaffected. To determine if the effects of hyperprolactinemia on erectile function occurred through changes in supraspinal input to neurons in the spinal cord controlling erectile function, pituitary-grafted and sham-operated male rats were subjected to spinal transection (between vertebral levels T6 and T9). Beginning 7 days later, penile reflex performance was re-examined. Hyperprolactinemic animals displayed significantly fewer erections during the initial test, but not in tests performed 10 and 13 days after spinal transection. The pituitary-grafted animals also showed a significant reduction in latency to the first erection with each successive test, suggesting a delayed recovery from increased supraspinal inhibitory input.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenal corticoids in hamsters: role in circadian timing

The 24-h patterns of circulating cortisol and corticosterone were determined in male hamsters housed under a 14:10 light-dark cycle. Corticoid levels varied significantly over the 24-h sampling period with peak levels of both hormones occurring near the onset of the daily dark phase. The ratio of cortisol to corticosterone changed dramatically during the day. Corticosterone levels were significantly higher than cortisol during the early part of the light phase; however, cortisol levels became significantly higher than corticosterone when both hormones began their daily rise. To examine whether the circadian rhythm of cortisol secretion could be involved in the physiological control of hamster circadian organization, cortisol was infused at approximately physiological levels into adrenalectomized hamsters either continuously or in a 24-h rhythm. No significant differences were observed in the timing of circadian wheel-running rhythms in hamsters housed in LD 16:8, LD 14:10, or LL when cortisol was infused continuously, in a 24-h rhythm that mimicked the cortisol rhythm of intact hamsters, or in a 24-h rhythm several hours out of phase with the rhythm of intact hamsters. Provision of cortisol in a 24-h rhythm appeared to promote the survival of adrenalectomized hamsters since hamsters receiving a 24-h pattern of cortisol survived the experimental protocol significantly longer than those receiving the same dose of cortisol continuously.

Serum concentrations of testosterone throughout pregnancy in rats

Testosterone concentrations in serum of rats bled throughout pregnancy and post partum were measured using Celite microcolumn chromatography and a radioimmunoassay for testosterone. Mean serum levels of testosterone ranged from about 170 to 340 pmol/l during the first 10 days of pregnancy. Significant increases in concentrations of testosterone in serum of pregnant rats were found on days 12, 15 and 18 of gestation. The highest testosterone concentrations occurred on days 18 and 20 of pregnancy when mean levels were 3228 and 3685 pmol/l respectively. Testosterone levels declined before parturition on day 22 (mean = 1449 pmol/l and declined further after parturition (mean = 315 pmol/l). In order to determine whether serum testosterone concentrations varied during the day in the pregnant rat, samples were collected at 6-h intervals on days 6–7 and 14–15 of gestation. Diurnal variations in serum testosterone concentrations were not evident during early or late pregnancy, unlike the rhythmic changes in serum prolactin levels found at these times during early pregnancy. The possible sources of the increased titres of serum testosterone during the second part of gestation in rats are discussed.

Response of serum prolactin to catechol estrogen in the immature rat

The response of serum prolactin to the catechol estrogens, 2-hydroxyestrone (2-OH E1) and 2-hydroxyestradiol (2-OH E2) and their primary estrogens, estrone (E1) and estradiol (E2), was studied in 35-day-old male rats. The subcutaneous administration of 50 microgram of 2-OH E1 or 2-OH E2 significantly suppressed serum prolactin concentrations, but they were not significantly altered by the administration of 50 microgram of E1 or E2.

Hormonally induced tumors of the reproductive system of parabiosed male rats

Parabiosis of intact male rats to castrated males or oophorectomized females for a period of approximately 20 months resulted in three interstitial cell tumors of the testis. When unilateral nephrectomy was added to the parabiotic procedure in ten pairs, eight interstitial cell tumors of the testis and four adenocarcinomas of the prostate occurred in the target male parabionts. These changes were preceded by elevations in luteinizing and follicle-stimulating hormone levels in the serum of the castrates and high levels of testosterone and, to a lesser degree, of androstenedion in the target partners developing the tumors.

Pituitary gonadotropin responsiveness with danazol

Danazol (17alpha-pregn-4-en-20-yno-[2,3-d]isoxazol-17-ol) was administered daily for 4 days to castrated female rats. As previously demonstrated, danazol lowered serum levels of luteinizing hormone (LH) in an apparent dose-dependent fashion. Animals which received danazol in a dose sufficient to lower serum LH responded to administered LH-releasing hormone (LHRH) with increases in serum LH levels which were not diminished as compared with those of control animals. Although these experiments do not preclude an effect of danazol directly on the pituitary, the results indicate that this agent probably lowers serum LH primarily by inhibition of hypothalamic LHRH secretion.

Danazol inhibits steroidogenesis

Danazol was found to inhibit multiple enzymes of steroidogenesis directly in the pregnant mare serum (PMS)-treated hamster ovary and the rat testis and adrenal in vitro. In the PMS-treated hamster ovary, danazol inhibited 17alpha-hydroxylase, 17,20-lyase, and 3beta-hydroxysteroid dehydrogenase. In the rat testis, danazol inhibited 17alpha-hydroxylase, 17,20-lyase, 3beta-hydroxysteroid dehydrogenase, and 17beta-hydroxysteroid dehydrogenase. In the rat adrenal, danazol inhibited 3beta-hydroxysteroid dehydrogenase, 21-hydroxylase, and 11beta-hydroxylase. Two hours after a subcutaneous injection of 5 mg/kg of danazol to adult male rats, serum luteinizing hormone levels were significantly increased and serum testosterone levels were significantly suppressed. These findings suggest that in the rodent one of danazol's major pharmacologic effects is the direct inhibition of steroidogenesis.