Amino Acid Biosynthesis and the COP9 Signalosome in Aspergillus nidulans

Abstract

The filamentous fungus Aspergillus nidulans represents an eukaryotic model system versatile in metabolism and developmental properties. This work focuses on the impact of modified enzyme activities whithin two regulatory networks on the fungal organism. The cross-pathway control ensures proper biosynthesis of amino acids upon corresponding starvation conditions in fungi. The COP9 signalosome is part of a regulatory network which is essential for the development of higher eukaryotes. Components of both systems have been isolated from A. nidulans and their contribution to the molecular control of physiology and morphology was examined.Two amino acid biosynthesis genes, hisB and lysA, have been identified as target genes of the cross-pathway control. HISB, the imidazole glycerol-phosphate dehydratase (E.C. 4.2.1.19), is essential for histidine biosynthesis. Supplemen-tation of a hisB deletion strain with traces of histidine enables growth of this auxotrophic strain. In contrast to the wild-type grown under the same conditions, this strain exhibits an induced cross-pathway control and arrests sexual development at the level of micro-cleistothecia. When high histidine supply represses the cross-pathway control, the sexual cycle can be complete. This indicates a link between the regulatory network of amino acid biosynthesis and sexual development in A. nidulans.The lysA gene, encoding saccharopine dehydratase (E.C. 1.5.1.7), catalyses the ultimate step of lysine formation in the branched lysine/penicillin biosynthesis pathway. Transcription of lysA is increased upon amino acid starvation due to an activated cross-pathway control, whereas the lysF gene of the pathway"s common stem as well as penicillin production were negatively affected. Thus, the regulatory network of amino acid biosynthesis is involved in a cross-talk between regulation of primary and secondary metabolism in A. nidulans.The COP9 signalosome is a multiprotein complex with at least two assumed associated enzyme activities, a protein kinase and a deneddylase, which contribute to the regulation of targeted protein degradation. For the first time, two genes encoding subunits four and five of the COP9 signalosome of filamentous fungi, csnD and csnE of A. nidulans, were identified. Deletion of either csn subunit resulted in multiple mutant phenotypes. It can be concluded that the COP9 signalosome of A. nidulans is involved in repression of pigment production and maintenance of cell polarity in vegetative hyphae as well as in light-dependence of developmental induction and completion of the sexual cycle.These results indicate that changes in single enzyme activities whithin genetic networks ultimately affect the metabolic and developmental potential of the entire fungal organism and suggest cross-connections between the different regulatory circuits.