Abstract
BackgroundThe cyclic peptide toxins of Amanita mushrooms, such as α-amanitin and phalloidin, are encoded by the “MSDIN” gene family and ribosomally biosynthesized. Based on partial genome sequence and PCR analysis, some members of the MSDIN family were previously identified in Amanita bisporigera, and several other members are known from other species of Amanita. However, the complete complement in any one species, and hence the genetic capacity for these fungi to make cyclic peptides, remains unknown.ResultsDraft genome sequences of two cyclic peptide-producing mushrooms, the “Death Cap” A. phalloides and the “Destroying Angel” A. bisporigera, were obtained. Each species has ~30 MSDIN genes, most of which are predicted to encode unknown cyclic peptides. Some MSDIN genes were duplicated in one or the other species, but only three were common to both species. A gene encoding cycloamanide B, a previously described nontoxic cyclic heptapeptide, was also present in A. phalloides, but genes for antamanide and cycloamanides A, C, and D were not. In A. bisporigera, RNA expression was observed for 20 of the MSDIN family members. Based on their predicted sequences, novel cyclic peptides were searched for by LC/MS/MS in extracts of A. phalloides. The presence of two cyclic peptides, named cycloamanides E and F with structures cyclo(SFFFPVP) and cyclo(IVGILGLP), was thereby demonstrated. Of the MSDIN genes reported earlier from another specimen of A. bisporigera, 9 of 14 were not found in the current genome assembly. Differences between previous and current results for the complement of MSDIN genes and cyclic peptides in the two fungi probably represents natural variation among geographically dispersed isolates of A. phalloides and among the members of the poorly defined A. bisporigera species complex. Both A. phalloides and A. bisporigera contain two prolyl oligopeptidase genes, one of which (POPB) is probably dedicated to cyclic peptide biosynthesis as it is in Galerina marginata.ConclusionThe MSDIN gene family has expanded and diverged rapidly in Amanita section Phalloideae. Together, A. bisporigera and A. phalloides are predicted to have the capacity to make more than 50 cyclic hexa-, hepta-, octa-, nona- and decapeptides.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3378-7) contains supplementary material, which is available to authorized users.
Highlights
The cyclic peptide toxins of Amanita mushrooms, such as α-amanitin and phalloidin, are encoded by the “MSDIN” gene family and ribosomally biosynthesized
From a partial genomic sequence obtained by 454 pyrosequencing, it was shown that the genes for α-amanitin and phallacidin belong to a family of at least 15 genes in Amanita bisporigera (Ab) called the “MSDIN” family for the first five conserved amino acids in the precursor peptides [1]
We show that each species contains ~30 members of the MSDIN family, only three of which are in common between the two fungi
Summary
The cyclic peptide toxins of Amanita mushrooms, such as α-amanitin and phalloidin, are encoded by the “MSDIN” gene family and ribosomally biosynthesized. The characteristic toxins of poisonous agarics (mushrooms; Agaricales) in the genus Amanita include the amatoxins such as α-amanitin and the phallotoxins such as phalloidin. Both families of toxins are bicyclic peptides biosynthesized on ribosomes as precursor peptides [1]. From a partial genomic sequence obtained by 454 pyrosequencing, it was shown that the genes for α-amanitin and phallacidin belong to a family of at least 15 genes in Amanita bisporigera (Ab) called the “MSDIN” family for the first five conserved amino acids in the precursor peptides [1]. The MSDIN precursor peptides are 33–37 amino acids in length and comprise two conserved regions, a 10-amino acid “leader” and a 17-amino acid “follower”, flanking a highly variable “core” region of 6– 10 amino acids that contains the amino acids present in the mature toxins
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