The isolation and characterization of cnx genes from the filamentous fungus "Aspergillus nidulans"

Abstract

The filamentous fungus Aspergillus nidulans relies on a good source of nitrogen for healthy growth. One of the most important sources of this nitrogen is from nitrate. A. nidulans can assimilate the nitrogen from nitrate via the action of its nitrate reductase enzyme. This enzyme requires a molybdenum-pterin containing cofactor for its activity. In A. nidulans, there are six loci known to be involved in synthesis of the Mo-cofactor - the so-called cnx genes. This work reports on the isolation and characterization of the cnxABC locus. cnxABC was found to consist of one gene containing an intron which separates two domains which encode one bifunctional protein. The two regions, namely cnxA and cnxC (each of which are able to phenotypically repair cnxA and cnxC fungal mutants respectively) show a high degree of homology with the two protein products of moaA and nioaC. of Escherichia coli, as well as with two proteins encoded by genes isolated from Arabidopsis thaliana - cnx2 and cnx3. Nitrate is also shown to act as an inducer of cnx ABC mRNA synthesis. If cnx ABC plays a similar role to its homologues, it has its function at the early stages of Mo-pterin biosynthesis, ie. conversion of guanosine into precursor Z, which is then converted to the molybdopterin which is incorporated into the Mo-cofactor. Mo-co is a universal molecule which is found in various enzymes throughout the species. Research into the synthesis of this cofactor will provide results which will prove to have important consequences in the understanding of the action of various important enzymes within many species.