The regulation of NADP-linked isocitrate dehydrogenase in Aspergillus nidulans

Generation and phenotypic characterization of Aspergillus nidulans methylisocitrate lyase deletion mutants: methylisocitrate inhibits growth and conidiation

Propionate is a very abundant carbon source in soil, and many microorganisms are able to use this as the sole carbon source. Nevertheless, propionate not only serves as a carbon source for filamentous fungi but also acts as a preservative when added to glucose containing media. To solve this contradiction between carbon source and preservative effect, propionate metabolism of Aspergillus nidulans was studied and revealed the methylcitrate cycle as the responsible pathway. Methylisocitrate lyase is one of the key enzymes of that cycle. It catalyzes the cleavage of methylisocitrate into succinate and pyruvate and completes the alpha-oxidation of propionate. Previously, methylisocitrate lyase was shown to be highly specific for the substrate (2R,3S)-2-methylisocitrate. Here, the identification of the genomic sequence of the corresponding gene and the generation of deletion mutants is reported. Deletion mutants did not grow on propionate as sole carbon and energy source and were severely inhibited during growth on alternative carbon sources, when propionate was present. The strongest inhibitory effect was observed, when glycerol was the main carbon source, followed by glucose and acetate. In addition, asexual conidiation was strongly impaired in the presence of propionate. These effects might be caused by competitive inhibition of the NADP-dependent isocitrate dehydrogenase, because the K(i) of (2R,3S)-2-methylisocitrate, the product of the methylcitrate cycle, on NADP-dependent isocitrate dehydrogenase was determined as 1.55 microM. Other isomers had no effect on enzymatic activity. Therefore, methylisocitrate was identified as a potential toxic compound for cellular metabolism.

Transcriptional autoregulation and inhibition of mRNA translation of amino acid regulator gene cpcA of filamentous fungus Aspergillus nidulans

The CPCA protein of the filamentous fungus Aspergillus nidulans is a member of the c-Jun-like transcriptional activator family. It acts as central transcription factor of the cross-pathway regulatory network of amino acid biosynthesis and is functionally exchangeable for the general control transcriptional activator Gcn4p of Saccharomyces cerevisiae. In contrast to GCN4, expression of cpcA is strongly regulated by two equally important mechanisms with additive effects that lead to a fivefold increased CPCA protein amount under amino acid starvation conditions. One component of cpcA regulation involves a transcriptional autoregulatory mechanism via a CPCA recognition element (CPRE) in the cpcA promoter that causes a sevenfold increased cpcA mRNA level when cells are starved for amino acids. Point mutations in the CPRE cause a constitutively low mRNA level of cpcA and a halved protein level when amino acids are limited. Moreover, two upstream open reading frames (uORFs) in the 5' region of the cpcA mRNA are important for a translational regulatory mechanism. Destruction of both short uORFs results in a sixfold increased CPCA protein level under nonstarvation conditions and a 10-fold increase under starvation conditions. Mutations in both the CPRE and uORF regulatory elements lead to an intermediate effect, with a low cpcA mRNA level but a threefold increased CPCA protein level independent of amino acid availability. These data argue for a combined regulation of cpcA that includes a translational regulation like that of yeast GCN4 as well as a transcriptional regulation like that of the mammalian jun and fos genes.

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.

Fungal catabolic gene regulation: molecular genetic analysis of the amdS gene of Aspergillus nidulans

Aspergillus nidulans is an excellent experimental organism for the study of gene regulation. Genetic and molecular analyses of trans-acting and cis-acting mutations have revealed a complex pattern of regulation involving multiple independent controls. Expression of the amdS gene is regulated by the facB and amdA genes which encode positively acting regulatory proteins mediating a major and a minor form of acetate induction respectively. The product of the amdR gene mediates omega amino acid induction of amdS. The binding sites for each of these proteins have been localised through amdS cis-acting mutations which specifically affect the interaction with the regulatory protein. The global controls of nitrogen metabolite repression and carbon catabolite repression regulate the expression of many catabolic genes, including amdS. Nitrogen control is exerted through the positively acting areA gene product and carbon control is dependent on the creA gene product. Each of the characterized regulatory genes encodes a DNA-binding protein which recognises particular sequences in the amdS promoter to activate or repress gene expression. In addition, there is evidence for other genetically uncharacterized proteins, including a CCAAT-binding complex, which interact with the 5' region of the amdS gene.

Isolation and analysis of the acetate regulatory gene, facB, from Aspergillus nidulans

The facB gene of Aspergillus nidulans is thought to be involved in acetate induction of enzymes required for acetate utilization and of the acetamidase encoded by the multiply regulated amdS gene. In addition, some evidence suggests that the facB gene has a structural as well as a regulatory role in acetate metabolism. The facB gene was cloned from a cosmid library by complementation of the facB101 loss-of-function mutation. Transformants receiving multiple copies of facB displayed stronger growth on acetamide media, indicating increased amdS expression, while growth on acetate was inhibited in these multicopy transformants. A 3.1-kilobase acetate-inducible facB transcript was detected by Northern (RNA) blot analysis. Examination of message levels in wild-type and mutant strains indicated that the facB gene is subject to carbon catabolite repression. Previous work has indicated that the presence of multiple copies of the 5' end of the amdS gene can result in titration of regulatory proteins. Additional copies of the facB gene were shown to specifically overcome the effect of facB product titration.

Isolation and analysis of the acetate regulatory gene, facB, from Aspergillus nidulans

The facB gene of Aspergillus nidulans is thought to be involved in acetate induction of enzymes required for acetate utilization and of the acetamidase encoded by the multiply regulated amdS gene. In addition, some evidence suggests that the facB gene has a structural as well as a regulatory role in acetate metabolism. The facB gene was cloned from a cosmid library by complementation of the facB101 loss-of-function mutation. Transformants receiving multiple copies of facB displayed stronger growth on acetamide media, indicating increased amdS expression, while growth on acetate was inhibited in these multicopy transformants. A 3.1-kilobase acetate-inducible facB transcript was detected by Northern (RNA) blot analysis. Examination of message levels in wild-type and mutant strains indicated that the facB gene is subject to carbon catabolite repression. Previous work has indicated that the presence of multiple copies of the 5' end of the amdS gene can result in titration of regulatory proteins. Additional copies of the facB gene were shown to specifically overcome the effect of facB product titration.

Isolation of the facA (acetyl-coenzyme A synthetase) and acuE (malate synthase) genes of Aspergillus nidulans

Acetate inducible genes of Aspergillus nidulans were cloned via differential hybridization to cDNA probes. Using transformation of mutant strains the genes were identified as facA (acetyl-Coenzyme A synthetase) and acuE (malate synthase). The levels of RNA encoded by these genes were shown to be acetate inducible and subject to carbon catabolite repression. Induction is abolished in a facB mutant and carbon catabolite repression is relieved in a creA mutant.

NADP-specific isocitrate dehydrogenase from the citric acid-accumulating fungus Aspergillus niger

NADP-specific isocitrate dehydrogenase [threo-DS-isocitrate: NADP+ oxidoreductase (decarboxylating), EC 1.1.1.42] was purified 200-300-fold from the citric acid-accumulating fungus Aspergillus niger. The enzyme consists of a single polypeptide chain with a molecular mass of 60 +/- 4 kDa and has a pI of 5.9 +/- 0.2. Only a single enzyme protein was found, although the enzyme appears to occur both in the mitochondrion and in the cytoplasm. Growth on organic acids as carbon sources or on NO3- as nitrogen source led to increased activities, whereas the presence of amino acids led to lower activities. The enzyme exhibits hyperbolic kinetics with respect to its substrates isocitrate and NADP+. Mn2+ and Mg2+ are obligatory for enzyme activity. The enzyme is inhibited by its products alpha-oxoglutarate and NADPH. Among various metabolites, ATP and citrate appear to inhibit the enzyme at a concentration considered to occur intracellularly. In both cases, however, the mechanism is a removal of the metal ion cofactor from the substrates. It is concluded that under physiological conditions, where the Mg2+ content is around 10 mM, the observed inhibition by ATP or citrate is of poor regulatory significance.

Regulation of aflatoxin biosynthesis: effect of glucose on activities of various glycolytic enzymes

Catabolism of carbohydrates has been implicated in the regulation of aflatoxin synthesis. To characterize this effect further, the activities of various enzymes associated with glucose catabolism were determined in Aspergillus parasiticus organisms that were initially cultured in peptone-mineral salts medium and then transferred to glucose-mineral salts and peptone-mineral salts media. After an initial increase in activity, the levels of glucose 6-phosphate dehydrogenase, mannitol dehydrogenase, and malate dehydrogenase were lowered in the presence of glucose. Phosphofructokinase activity was greater in the peptone-grown mycelium, but fructose diphosphatase was largely unaffected by carbon source. Likewise, carbon source had relatively little effect on the activities of pyruvate kinase, malic enzyme, isocitrate-NADP dehydrogenase, and isocitrate-NAD dehydrogenase. The results suggest that glucose may, in part, regulate aflatoxin synthesis via a carbon catabolite repression of NADPH-generating and tricarboxylic acid cycle enzymes.