29The horizontal transfer of large gene clusters by mobile elements is a key driver of prokaryotic 30 adaptation in response to environmental stresses. Eukaryotic microbes face similar 31 environmental stresses yet a parallel role for mobile elements has not yet been established. A 32 stress faced by all microorganisms is the prevalence of toxic metals in their environment. In 33 fungi, identified mechanisms for protection against metals generally rely on genes that are 34 dispersed within an organism's genome. Here we have discovered a large (~85 kb) region that 35 confers resistance to several metals in the genomes of some, but not all, strains of a fungus,
36Paecilomyces variotii. We name this region HEPHAESTUS (Hϕ) and present evidence that this 37 region is mobile within the P. variotii genome with features highly characteristic of a 38 transposable element. While large gene clusters including those for the synthesis of secondary 39 metabolites have been widely reported in fungi, these are not mobile within fungal genomes. 40 HEPHAESTUS contains the greatest complement of host-beneficial genes carried by a 41 transposable element in eukaryotes. This suggests that eukaryotic transposable elements might 42 play a role analogous to their bacterial counterparts in the horizontal transfer of large regions of 43 host-beneficial DNA. Genes within HEPHAESTUS responsible for individual metal resistances 44 include those encoding a P-type ATPase transporter, PcaA, required for cadmium and lead 45 resistance, a transporter, ZrcA, providing resistance to zinc, and a multicopper oxidase, McoA, 46 conferring resistance to copper. Additionally, a subregion of Hϕ conferring resistance to 47 arsenate was identified. The presence of a strikingly similar cluster in the genome of another 48 fungus, Penicillium fuscoglaucum, suggests that HEPHAESTUS arrived in P. variotii via horizontal 49 gene transfer.50 51 KEYWORDS 52 53 Eurotiales, gene cluster, metal homeostasis, horizontal gene transfer, transposon 54 3 INTRODUCTION 55 56Metals are used within diverse fields such as agriculture, construction, electronics and 57 pharmaceutical science. A number of these, such as cadmium, lead and arsenic, are toxic even 58 at low levels of exposure and thus environmental contamination resulting from their use is an 59 increasing concern 1 . The prevalence of toxic metals in the environment is of particular 60 relevance to fungal biology. Some fungi are able to accumulate metals to high concentrations, 61 making such fungi dangerous for human consumption 2,3 , allowing bioremediation of 62 contaminated sites 4 or concentrating valuable metals for extraction 5 . Zinc and copper play 63 essential roles in many cellular functions and are often important in fungal virulence 6 . However, 64 at high concentrations these metals can also be toxic; for example, copper-based molecules are 65 frequently used as fungicides in agriculture. 66 67 Compared to prokaryotes, our understanding of the evolutionary mechanisms through which 68 eukaryotes have ada...
