Eukaryot Cell 3(2): 527-535

LaeA, a Regulator of Secondary Metabolism in <em>Aspergillus</em> spp.

Department of Plant Pathology, University of Wisconsin—Madison, Madison, Wisconsin 53706

^{Corresponding author. Mailing address: Department of Plant Pathology, University of Wisconsin, 1630 Linden Dr., Madison, WI 53706. Phone: (608) 262-9795. Fax: (608) 263-2626. E-mail:}ude.csiw.htaptnalp@kpn.

Received 2003 Nov 5; Accepted 2003 Dec 16.

Abstract

Secondary metabolites, or biochemical indicators of fungal development, are of intense interest to humankind due to their pharmaceutical and/or toxic properties. We present here a novel Aspergillus nuclear protein, LaeA, as a global regulator of secondary metabolism in this genus. Deletion of laeA (ΔlaeA) blocks the expression of metabolic gene clusters, including the sterigmatocystin (carcinogen), penicillin (antibiotic), and lovastatin (antihypercholesterolemic agent) gene clusters. Conversely, overexpression of laeA triggers increased penicillin and lovastatin gene transcription and subsequent product formation. laeA expression is negatively regulated by AflR, a sterigmatocystin Zn₂Cys₆ transcription factor, in a unique feedback loop, as well as by two signal transduction elements, protein kinase A and RasA. Although these last two proteins also negatively regulate sporulation, ΔlaeA strains show little difference in spore production compared to the wild type, indicating that the primary role of LaeA is to regulate metabolic gene clusters.

Abstract

A complex and fascinating aspect of fungal development is the production of secondary metabolites. These compounds, frequently associated with sporulation processes, are considered part of the chemical arsenal required for niche specialization and have garnered intense interest by virtue of their biotechnological and pharmaceutical applications (9, 11). Many of them display a broad range of useful antibiotic, antiviral, antitumor, antihypercholesterolemic, and immunosuppressant activities as well as less desirable phyto- and mycotoxic activities. Hawksworth's studies of fungal biodiversity led to the conclusion that nearly 1.5 million fungal species exist on Earth, with only 5% identified thus far (16). Thus, the potential for fungal secondary metabolite discovery is vast. Furthermore, the discovery of global regulators for fungal secondary metabolite production is critical, as it would allow for universal manipulation of secondary metabolite production.

A large number of known fungal secondary metabolites have been ascribed to the Ascomycete genus Aspergillus. Studies of Aspergillus nidulans have demonstrated the power of using a model system to elucidate the biochemistry and molecular genetics of fungal secondary metabolism, principally penicillin (PN, an antibiotic) and sterigmatocystin (ST, a carcinogen biochemically related to the agricultural contaminant aflatoxin) biosynthesis (6, 17). These studies have established several characteristics of fungal secondary metabolism, including the clustering of biosynthetic and regulatory genes and a genetic connection linking secondary metabolite biosynthesis with sporulation through a shared signal transduction pathway (9).

The discovery of the G protein-cyclic AMP (cAMP)-protein kinase A regulation of ST, aflatoxin, and other fungal secondary metabolites (4, 17, 29, 34) has been helpful for establishing a model of global regulation of secondary metabolism. However, all of the signal transduction mutants described in the literature have pleiotropic effects on fungi, the most notable effect being the gross impact on spore production and vegetative hyphal growth (1, 9, 18, 29). Similarly, mutations in other major regulators of ST biosynthesis, such as RcoA, a WD protein (19), and SpdA, or spermidine synthase (20), also have gross effects on fungal morphology. Thus, currently available Aspergillus mutants are so impaired in fungal development that further elucidation of genes that are specific for the regulation of secondary metabolism is difficult.

In a previous mutagenesis hunt, 23 A. nidulans mutants that displayed a phenotype of loss of ST production but had normal sporulation were isolated (8). Here we describe the identification of a gene called laeA that complements one of these mutants. laeA encodes a nuclear protein that is required for the expression of secondary metabolite genes. We propose that LaeA is a regulator of secondary metabolism in Aspergillus, as it is required not only for ST biosynthesis but also for PN biosynthesis and the biosynthesis of mycelial pigments in A. nidulans and gliotoxin and mycelial pigments in Aspergillus fumigatus. Furthermore, this protein is required for expression of the heterologous lovastatin (LOV) gene cluster in A. nidulans as well as for native LOV expression in Aspergillus terreus. Interestingly, the protein appears to be conserved in filamentous fungi, but it is not present in Saccharomyces cerevisiae, a fungus devoid of secondary metabolites. Unlike other genes that regulate secondary metabolism, the loss of laeA has a negligible impact on morphological developmental processes.

Acknowledgments

This research was funded by NSF grant MCB-9874646 to N.P.K.

Acknowledgments

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