Identification, cloning and analysis of theAspergillus niger genepacC, a wide domain regulatory gene responsive to ambient pH

A wide domain regulatory gene implicated in modulating gene expression in response to ambient pH has been cloned and sequenced from the industrially useful filamentous fungusAspergillus niger. This gene,pacC, is able to restore apacC ⁺ phenotype toA. nidulans pacC ^c 11 andpacC ^c 14 mutants with respect to extent of conidiation, conidial pigment intensity and acid phosphatase regulation. ThepacC gene ofA. niger comprises three exons, encodes a three-zinc-finger protein of 677 amino acids, and shows pH-dependent regulation of expression: mRNA levels are elevated under alkaline conditions and considerably reduced under acidic conditions. The occurrence of PacC consensus binding targets within the sequences upstream ofpacC may indicate autoregulation.     

Regulation of acid phosphatases in anAspergillus niger pacC disruption strain

AnAspergillus niger strain has been constructed in which the pH-dependent regulatory gene,pacC, was disrupted. ThepacC gene ofA. niger, like that ofA. nidulans, is involved in the regulation of acid phosphatase expression. Disruptants were identified by a reduction in acid phosphatase staining of colonies. Southern analysis demonstrated integration of the disruption plasmid at thepacC locus and Northern analysis showed that the disruption strain produced a truncatedpacC mRNA of 2.2 kb (as compared to 2.8 kb in the wild type). The strain carrying thepacC disruption was used to assign thepacC gene to linkage group IV; this was confirmed by CHEF electrophoresis and Southern analysis. This strain further allowed us to determine which extracellular enzyme and transport systems are under the control ofpacC inA. niger. Expression of theA. niger pacC wild-type gene and the truncatedpacC gene showed that, in contrast to the auto-regulated wild-type expression, which was elevated only at alkaline pH, the truncatedpacC gene was deregulated, as high-level expression occurred regardless of the pH of the culture medium. Analysis of the phosphatase spectrum by isoelectric focussing and enzyme activity staining both in the wild-type and thepacC disruptant showed that at least three acid phosphatases are regulated by thepacC. For the single alkaline phosphatase no pH regulation was observed.     

Regulation of acid phosphatases in anAspergillus niger pacC disruption strain

AnAspergillus niger strain has been constructed in which the pH-dependent regulatory gene,pacC, was disrupted. ThepacC gene ofA. niger, like that ofA. nidulans, is involved in the regulation of acid phosphatase expression. Disruptants were identified by a reduction in acid phosphatase staining of colonies. Southern analysis demonstrated integration of the disruption plasmid at thepacC locus and Northern analysis showed that the disruption strain produced a truncatedpacC mRNA of 2.2 kb (as compared to 2.8 kb in the wild type). The strain carrying thepacC disruption was used to assign thepacC gene to linkage group IV; this was confirmed by CHEF electrophoresis and Southern analysis. This strain further allowed us to determine which extracellular enzyme and transport systems are under the control ofpacC inA. niger. Expression of theA. niger pacC wild-type gene and the truncatedpacC gene showed that, in contrast to the auto-regulated wild-type expression, which was elevated only at alkaline pH, the truncatedpacC gene was deregulated, as high-level expression occurred regardless of the pH of the culture medium. Analysis of the phosphatase spectrum by isoelectric focussing and enzyme activity staining both in the wild-type and thepacC disruptant showed that at least three acid phosphatases are regulated by thepacC. For the single alkaline phosphatase no pH regulation was observed.     

NUT1, a major nitrogen regulatory gene inMagnaporthe grisea,is dispensable for pathogenicity

NUT1, a gene homologous to the major nitrogen regulatory genesnit-2 ofNeurospora crassa andareA ofAspergillus nidulans, was isolated from the rice blast fungus,Magnaporthe grisea. NUT1 encodes a protein of 956 amino acid residues and, likenit-2 andareA, has a single putative zinc finger DNA-binding domain. Functional equivalence ofNUT1 toareA was demonstrated by introducing theNUT1 gene by DNA-mediated transformation into anareA loss-of-function mutant ofA. nidulans. The introducedNUT1 gene fully complemented theareA null mutation, restoring to the mutant the ability to utilize a variety of nitrogen sources. In addition, the sensitivity ofAspergillus NUT1 transformants to ammonium repression of extracellular protease activity was comparable to that of wild-typeA. nidulans. Thus,NUT1 andareA encode functionally equivalent gene products that activate expression of nitrogen-regulated genes. A one-step gene disruption strategy was used to generatenutl ^? mutants ofM. grisea by transforming a rice-infecting strain with a disruption vector in which a gene for hygromycin B phosphotransferase (Hyg) replaced the zinc-finger DNA-binding motif ofNUT1. Of 31 hygromycin B (hyg B)-resistant transformants shown by Southern hybridization to contain a disruptedNUT1 gene (nut1::Hyg), 26 resulted from single-copy replacement events at theNUT1 locus. Althoughnut1 ^? transformants ofM. grisea failed to grown on a variety of nitrogen sources, glutamate, proline and alanine could still be utilized. This contrasts withA. nidulans where disruption of the zinc-finger region ofareA prevents utilization of nitrogen sources other than ammonium and glutamine. The role ofNUT1 and regulation of nitrogen metabolism in the disease process was evaluated by pathogenicity assays. The infection efficiency ofnut1 ^? transformants on susceptible rice plants was similar to that of the parental strain, although lesions were reduced in size. These studies demonstrate that theM. grisea NUT1 gene activates expression of nitrogen-regulated genes but is dispensable for pathogenicity.     

Stable transformation and regulated expression of an inducible reporter construct inCandida albicans using restriction enzyme-mediated integration

To allow the regulated expression of cloned genes inCandida albicans, a plasmid was constructed using the inducible promoter of theC. albicans MAL2 gene. To demonstrate that theMAL2 promoter could regulate cloned genes placed under its control, a fusion construct was made with the coding sequence of theC. albicans URA3 gene. This plasmid was introduced into a Ura^? strain ofC. albicans using the process of restriction enzyme-mediated integration (REMI). This procedure involves the transformation of theBamHI-linearized plasmid in the presence ofBamHI enzyme. The majority of transformants generated contained insertions of the plasmid at chromosomalBamHI sites. All transformants examined were inducible forURA3 expression, which was determined by growth analysis and by measuring the level ofURA3 gene product activity. The Ura⁺ phenotype of the transformants was stable during growth under nonselective conditions. This system offers the advantages of stable transformation, easy recovery of integrated DNA, and inducible expression of genes inC. albicans.     

Oxalic acid production by citric acid-producing Aspergillus niger overexpressing the oxaloacetate hydrolase gene oahA

The filamentous fungus Aspergillus niger is used worldwide in the industrial production of citric acid. However, under specific cultivation conditions, citric acid-producing strains of A. niger accumulate oxalic acid as a by-product. Oxalic acid is used as a chelator, detergent, or tanning agent. Here, we sought to develop oxalic acid hyperproducers using A. niger as a host. To generate oxalic acid hyperproducers by metabolic engineering, transformants overexpressing the oahA gene, encoding oxaloacetate hydrolase (OAH; EC 3.7.1.1), were constructed in citric acid-producing A. niger WU-2223L as a host. The oxalic acid production capacity of this strain was examined by cultivation of EOAH-1 under conditions appropriate for oxalic acid production with 30 g/l glucose as a carbon source. Under all the cultivation conditions tested, the amount of oxalic acid produced by EOAH-1, a representative oahA-overexpressing transformant, exceeded that produced by A. niger WU-2223L. A. niger WU-2223L and EOAH-1 produced 15.6 and 28.9 g/l oxalic acid, respectively, during the 12-day cultivation period. The yield of oxalic acid for EOAH-1 was 64.2 % of the maximum theoretical yield. Our method for oxalic acid production gave the highest yield of any study reported to date. Therefore, we succeeded in generating oxalic acid hyperproducers by overexpressing a single gene, i.e., oahA, in citric acid-producing A. niger as a host.     

Genetic analysis of amdS transformants of Aspergillus niger and their use in chromosome mapping

Summary TheAspergillus nidulans gene coding for acetamidase (amdS) was introduced intoA. niger by transformation. Twelve Amd⁺ transformants were analysed genetically. TheamdS inserts were located in seven different linkage groups. In each transformant the plasmid was integrated in only a single chromosome. Our (non-transformed)A. niger strains do not grow on acetamide and are more resistant to fluoroacetamide than the transformants. Diploids hemizygous for theamdS insert have the Amd⁺ phenotype. We exploited the opportunity for two-way selection inA. niger: transformants can be isolated based on the Amd⁺ phenotype, whereas counter-selection can be performed using resistance to fluoroacetamide. On this basis we studied the phenotypic stability of the heterologousamdS gene inA. niger transformants as well as in diploids. Furthermore, we mapped the plasmid insert of transformant AT1 to the right arm of chromosome VI betweenpabA1 andcnxA1, providing evidence for a single transformational insert. The results also show that theamdS transformants ofA. niger can be used to localize non-selectable recessive markers and that the method meets the prerequisites for efficient mitotic mapping. We suggest the use ofamdS transformants for mitotic gene mapping in other fungi.     

Expression of Lactate Dehydrogenase in Aspergillus niger for L-Lactic Acid Production

Different engineered organisms have been used to produce L-lactate. Poor yields of lactate at low pH and expensive downstream processing remain as bottlenecks. Aspergillus niger is a prolific citrate producer and a remarkably acid tolerant fungus. Neither a functional lactate dehydrogenase (LDH) from nor lactate production by A. niger is reported. Its genome was also investigated for the presence of a functional ldh. The endogenous A. niger citrate synthase promoter relevant to A. niger acidogenic metabolism was employed to drive constitutive expression of mouse lactate dehydrogenase (mldhA). An appraisal of different branches of the A. niger pyruvate node guided the choice of mldhA for heterologous expression. A high copy number transformant C12 strain, displaying highest LDH specific activity, was analyzed under different growth conditions. The C12 strain produced 7.7 g/l of extracellular L-lactate from 60 g/l of glucose, in non-neutralizing minimal media. Significantly, lactate and citrate accumulated under two different growth conditions. Already an established acidogenic platform, A. niger now promises to be a valuable host for lactate production.     

Regulation of purine biosynthetic genes expression inSalmonella typhimurium IV Oc mutation site ofpurG and its function analysis

Salmonella typhimurium 5 phosphoribosylformylglycinamide (FGAR) amidotransferase encoded bypurG gene catalyzes the conversion of FGAR to formylglycinamide ribonucleotide (FGAM) in the presence of glu- tamine and ATP for thede novo purine nucleotide biosynthesis.purG gene is negatively regulated by a repressor-operator system. The O⁺ purG and O^c purG were cloned respectivelyin vivo. Restriction enzymes analysis of preliminary clones pLBG-1 (O⁺) and pLBG-2 (O^c) were carried out. The hybrid plasmids pLB1933 (O⁺) and pLB1927 (O^c) containing 5′ control region ofpurG were constructed and the DNA sequences were determined respectively, DNA sequences data showed that O^c mutation ofpurG occurred at the 3rd position of 16 bp PUR box in the 5′ control region (G→A). Gel retardation experiment indicated that the repressor bound well with O⁺ PUR box, but not with O^c PUR box. The result strongly supported the idea that PUR box is the binding region of repressor protein and the 3rd position base G of PUR box is essential for the binding function with repressor protein.