Background: Aspergillus oryzae, a useful industrial filamentous fungus, produces limited varieties of secondary metabolites, such as kojic acid. Thus, for the production of valuable secondary metabolites by genetic engineering, the species is considered a clean host, enabling easy purification from cultured cells. A. oryzae has been evaluated for secondary metabolite production utilizing strong constitutive promoters of genes responsible for primary metabolism. However, secondary metabolites are typically produced by residual nutrition after microbial cells grow to the stationary phase and primary metabolism slows. We focused on a promoter of the secondary metabolism gene kojA, a component of the kojic acid biosynthetic gene cluster, for the production of other secondary metabolites by A. oryzae.
Results: A kojA disruptant that does not produce kojic acid was utilized as a host strain for production. Using this host strain, a mutant that expressed a polyketide synthase gene involved in polyketide secondary metabolite production under the kojA gene promoter was constructed. Then, polyketide production and polyketide synthase gene expression were observed every 24 h in liquid culture. From days 0 to 10 of culture, the polyketide was continuously produced, and the synthase gene expression was maintained. Therefore, the kojA promoter was activated, and it enabled the continuous production of polyketide for 10 days.
Conclusions: The combined use of the kojA gene promoter and a kojA disruptant proved useful for the continuous production of a polyketide secondary metabolite in A. oryzae. These findings suggest that this combination can be applied to other secondary metabolites for long-term production.
Keywords: Aspergillus oryzae; Kojic acid; Polyketide; Secondary metabolism; Transcriptional regulation.