Eukaryotic transposable elements as "cargo carriers": the forging of metal resistance in the fungus Paecilomyces variotii

The horizontal transfer of large gene clusters by mobile elements is a key driver of prokaryotic adaptation in response to environmental stresses. Eukaryotic microbes face similar environmental stresses yet a parallel role for mobile elements has not yet been established. A stress faced by all microorganisms is the prevalence of toxic metals in their environment. In fungi, identified mechanisms for protection against metals generally rely on genes that are dispersed within an organisms genome. Here we have discovered a large ([~]85 kb) region that confers resistance to several metals in the genomes of some, but not all, strains of a fungus, Paecilomyces variotii. We name this region HEPHAESTUS (H{phi}) and present evidence that this region is mobile within the P. variotii genome with features highly characteristic of a transposable element. While large gene clusters including those for the synthesis of secondary metabolites have been widely reported in fungi, these are not mobile within fungal genomes. HEPHAESTUS contains the greatest complement of host-beneficial genes carried by a transposable element in eukaryotes. This suggests that eukaryotic transposable elements might play a role analogous to their bacterial counterparts in the horizontal transfer of large regions of host-beneficial DNA. Genes within HEPHAESTUS responsible for individual metal resistances include those encoding a P-type ATPase transporter, PcaA, required for cadmium and lead resistance, a transporter, ZrcA, providing resistance to zinc, and a multicopper oxidase, McoA, conferring resistance to copper. Additionally, a subregion of H{phi} conferring resistance to arsenate was identified. The presence of a strikingly similar cluster in the genome of another fungus, Penicillium fuscoglaucum, suggests that HEPHAESTUS arrived in P. variotii via horizontal gene transfer.