Complete genome of the toxic mold Aspergillus pseudotamarii isolate NRRL 25517 reveals genomic instability of the aflatoxin biosynthesis cluster

Authors: LEGAN, ANDREW - Oak Ridge Institute For Science And Education (ORISE); Mack, Brian; Mehl, Hillary; WISSOTSKI, MARINA - University Of Arizona; CHING'ANDA, CONNEL - University Of Arizona; MAXWELL, LOURENA - University Of Arizona; Callicott, Kenneth

Submitted to: G3, Genes/Genomes/Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2023
Publication Date: 7/4/2023
Citation: Legan, A.W., Mack, B.M., Mehl, H.L., Wissotski, M., Ching'anda, C., Maxwell, L.A., Callicott, K.A. 2023. Complete genome of the toxic mold Aspergillus pseudotamarii isolate NRRL 25517 reveals genomic instability of the aflatoxin biosynthesis cluster. G3, Genes/Genomes/Genetics. 13(9). Article jkad150. https://doi.org/10.1093/g3journal/jkad150.
DOI: https://doi.org/10.1093/g3journal/jkad150

Interpretive Summary: Molds from the genus Aspergillus often create toxic chemicals, and when these molds grow on crops, the toxins can enter the food supply. The most important toxic chemical made by Aspergillus is aflatoxin, which can cause cancer or death when consumed. The genes for producing aflatoxins were identified for Aspergillus flavus and A. parasiticus, which are two very common sources of crop contamination. In these species, the genes needed for producing aflatoxin are clustered together in one region of the genome. This paper presents a high quality genome of A. pseudotamarii, a mold which produces aflatoxins as well as more than 70 other chemicals, some of which may also be toxic. Comparing this genome to that of A. flavus reveals that A. pseudotamarii has the same genes in the same order within this cluster of aflatoxin genes, but the cluster itself is in a totally different part of the genome. Studying toxin producing genes and their location in different genomes can reveal how these genes have evolved and may help to improve our ability to keep aflatoxins out of the food supply.

Technical Abstract: Fungi are capable of synthesizing a deluge of secondary metabolite chemicals. Genes underpinning biosynthesis of a particular fungal secondary metabolite are typically arranged in a tightly linked cluster in the genome. For example, ~25 genes responsible for the biosynthesis of carcinogenic aflatoxins by Aspergillus section Flavi species are grouped in a ~70 Kb cluster. Genome assembly fragmentation prevents analyses of the role of structural genomic variation in secondary metabolite evolution in this clade. More comprehensive analyses of secondary metabolite evolution will be possible by analyzing more complete and accurate genomes of taxonomically diverse Aspergillus species. Here, we combined short and long read DNA sequencing to generate a highly contiguous genome of the aflatoxigenic fungus, Aspergillus pseudotamarii (isolate NRRL25517 = CBS766.97; scaffold N50 = 5.5 Mb). The nuclear genome is 39.4 Mb and encodes 12,639 putative protein-coding genes and between 74 and 97 candidate secondary metabolite biosynthesis gene clusters. The circular mitogenome is 29.7 Kb and contains 14 protein-coding genes that are highly conserved across the genus. This highly contiguous A. pseudotamarii genome assembly enables comparisons of genomic rearrangements between Aspergillus section Flavi series Kitamyces and series Flavi. Although the aflatoxin biosynthesis gene cluster of A. pseudotamarii is conserved with Aspergillus flavus in gene content, genomic rearrangement positioned the A. pseudotamarii aflatoxin gene cluster in inverted orientation relative to the telomere on a different chromosome compared to A. flavus. As an aflatoxin producer morphologically similar to closely-related non-aflatoxigenic species, the high quality genome of A. pseudotamarii will be useful for developing genetic screens to identify this toxic mold in crops and stored food products.