Figure 2. Amatoxin biosynthesis.

(A) Alignment of MSDIN proproteins. Red asterisks denote putatively nonfunctional copies in Galerina indica and G. sulciceps. The nonfunctional G. indica copy contains a single-nucleotide deletion in the core region that causes a frameshift; the affected codon position is represented by a dash in the translated alignment at the position corresponding to the conserved “N” core residue. (B) Schematic showing the biosynthetic pathway that produces α-amanitin or α-proamanitin. First, the core peptide sequence is trimmed from the MSDIN proprotein. The core peptide is then cyclized, creating the cyclic precursor. Finally, tailoring enzymes hydroxylate isoleucine and tryptophan to create α-proamanitin in G. indica. Additional tailoring enzymes hydroxylate the proline to create the canonical amatoxin α-amanitin in G. marginata and most other amatoxin-containing taxa. (C) Maximum-likelihood phylogeny of POPB protein sequences. The tree is rooted with paralogous housekeeping POP proteins that are not involved in amatoxin biosynthesis, using sequences from related Agaricales mushrooms in the genera Hypsizygus and Termitomyces. Branches overlaid with three sets of double slashes indicate that the displayed branch length has been compressed to one-eighth of its original length.