Illumina-based de novo transcriptome sequencing and analysis of Amanita exitialis basidiocarps

Amanita exitialis is a lethal mushroom that was first discovered in Guangdong Province, China. The high content of amanitin in its basidiocarps makes it lethal to humans. To comprehensively characterize the A. exitialis transcriptome and analyze the Amanita toxins as well as their related gene family, transcriptome sequencing of A. exitialis was performed using Illumina HiSeq 2000 technology. A total of 25,563,688 clean reads were collected and assembled into 62,137 cDNA contigs with an average length of 481 bp and N50 length of 788 bp. A total of 27,826 proteins and 39,661 unigenes were identified among the assembled contigs. All of the unigenes were classified into 166 functional categories for understanding the gene functions and regulation pathways. The genes contributing to toxic peptide biosynthesis were analyzed. From this set, eleven gene sequences encoding the toxins or related cyclic peptides were discovered in the transcriptome. Three of these sequences matched the peptide toxins α-amanitin, β-amanitin, and phallacidin, while others matched amanexitide and seven matched unknown peptides. All of the genes encoding peptide toxins were confirmed by polymerase chain reaction (PCR) in A. exitialis, and the phylogenetic relationships among these proprotein sequences were discussed. The gene polymorphism and degeneracy of the toxin encoding sequences were found and analyzed. This study provides the first primary transcriptome of A. exitialis, which provided comprehensive gene expression information on the lethal amanitas at the transcriptional level, and could lay a strong foundation for functional genomics studies in those fungi.