Functional heterologous expression of the reversible Cu-decarboxylase from the lichen, Cladonia uncialis

Abstract

Despite the isolation of over 1000 known bioactive lichen mycobiont-derived secondary metabolites (SMs), understanding the genetic basis of their biosynthesis remains elusive. Biosynthetic gene clusters (BGCs) have been tentatively linked to chemical structures, with core genes such as polyketide synthases (PKSs) surrounded by accessory genes like decarboxylases. In this study, we focused on a decarboxylase gene from the genome of the lichen cladonia uncialis (named as Cu-decarboxylase) to elucidate its role in SM biosynthesis. A 963 bp gene was cloned from C. uncialis and expressed in Escherichia coli (BL21(DE3) cells using the pQE80L expression vector. The resulting 35 kDa protein was purified by applying a Ni⁺-NTA column using an FPLC system. Functional activity assays revealed the decarboxylation and reversible carboxylation of resorcinol to 2,4-dihydroxybenzoic acid and orcinol to orsellinic acid. This suggests a potential role for this Cu-decarboxylase in SM biosynthesis.

Furthermore, the lack of activity on substrates like anthranilic acid and aniline highlighted the importance of the phenolic OH group in facilitating these reactions. The 3D protein structure was predicted with AlphaFold3, based on sequence similarity with a known decarboxylases and revealed the importance of a zinc cofactor for the catalytic activity of the enzyme. The optimization of the reaction conditions, particularly for orsellinic acid production from orcinol, may enhance conversion rates and offer a viable route for industrial-scale production of bioactive compounds. This study marks the first known instance of functional heterologous expression of a non-codon-optimized gene isolated from lichen in E. coli.