Lost in Translation: Challenges with Heterologous Expression of Lichen Polyketide Synthases

The ability to functionally express proteins in hosts is a precondition to an advanced understanding of the biosynthetic pathways that are responsible for producing life's complex molecules. The study of secondary metabolites in lichen-forming fungi has long been hampered by slow growth. This study, reports on heterologous expression trials of four polyketide synthase (PKS) genes from C. uncialis in Aspergillus oryzae NSAR1. Isolation of mRNA and RT-PCR demonstrated that A. oryzae can transcribe all lichen genes and remove introns to produce translationally-coherent mRNA. Transformation of A. oryzae with a codon-optimized PKS did not result in metabolite production, nor did co-expression of a number of accessory genes restore function to any lichen PKS. Genes encoding an orsellinic acid synthase (OAS) from Fusarium sp. and a 6-methylsalicylic acid synthase (6MSAS) from Penicillum sp. were transformed into A. oryzae. Readily detectable amounts of de novo orsellinic acid and 6-methylsalicylic acid biosynthesis were observed in A. oryzae when transformed with these non-lichen PKS genes. However, transformation with functionally homologous PKS genes from C. uncialis produced no detectable product. This work demonstrates that lichen PKS genes are correctly transcribed by A. oryzae but that polyketide biosynthesis failed for a reason that is presently unknown but may be attributable to a fault of translation