Lichen ketosynthase domains are not responsible for inoperative polyketide synthases in Ascomycota hosts

Abstract

Efforts by lichenologists to characterize lichen polyketide synthases (PKS) through heterologous expression experiments have so far proved unfruitful. A determination of systematic causes of failure is therefore required. Three hypotheses involving the ketosynthase (KS) domain of lichen polyketide synthases (PKS) from Cladonia uncialis are tested: (1) Horizontal versus vertical gene transfer; (2) Typical versus atypical active site residues; (3) Typical versus atypical tertiary protein structure and active site architecture. Phylogenetics, amino acid sequence alignment, and protein modelling indicate that C. uncialis PKS evolved through vertical transfer from Ascomycota fungi, possess Cys-His-His catalytic triads typical of KS from most organisms, and possess protein and catalytic site architecture identical to well-characterized KS from non-lichen organisms. Though the reason for lack of functional activity in heterologous hosts remains unknown, complications involving the KS are ruled out as a likely explanation. Heterologous translation of lichen PKS (or parts thereof) have not been reported. We demonstrate heterologous translation of two lichen KS domains in E. coli.

Keywords: Ketosynthase; Lichen; Mass spectrometry; Polyketides; Protein expression; Usnic acid.