High diversity of type I polyketide genes in Bacidia rubella as revealed by the comparative analysis of 23 lichen fungal genomes

Abstract

Fungi involved in lichen symbioses produce a large array of different secondary metabolites. The high diversity of those substances has been known for decades and are often considered characteristic for taxonomical delimitation in lichen-forming fungi. Polyketides, the most common secondary metabolites, are synthesized by the Type I Polyketide synthases (TI-PKS) comprised of different enzymatic domains. We present a phylogenetic overview of Type I PKS genes recovered from the de-novo sequenced genome of Bacidia rubella in the context of additional twenty-one fungal genomes from the largest radiation of lichen-forming Ascomycetes (Lecanoromycetes) as well as the lichen-forming Eurotiomycete, Endocarpon pusillum. Using de-novo gene prediction and functional annotation combined with a phylogenetic analysis, we provide insights into the biosynthetic potential and PKS gene diversity of lichen-forming fungi. We discuss genes predicted in the lichen-forming fungal genomes in relation to previously characterized PKS genes from other fungi and bacteria. Our results reveal a high number of biosynthetic gene clusters and their gene domain composition. PKS gene content outnumbers known secondary substances produced by the lichen-forming fungi. We were able to assign putative functions to several of those PKS genes in silico, based on similarity to already characterized genes. In particular, we identified a putative PKS23 gene of Bacidia rubella, producing the common lichen substance atranorin. However, we also found that several lichen-forming fungi still possess homologs of different biosynthetic genes without producing the corresponding substances in detectable amounts. Although many PKSs remain without functional assignments in our analysis, our findings highlight that genes from lichen-forming fungi represent an untapped source of novel polyketide compounds. However, additional experimental approaches are necessary to link biosynthetic genes and secondary metabolites with confidence.