Comparison of Expression of Secondary Metabolite Biosynthesis Cluster Genes in Aspergillus flavus, A. parasiticus, and A. oryzae

Abstract

Fifty six secondary metabolite biosynthesis gene clusters are predicted to be in the Aspergillus flavus genome. In spite of this, the biosyntheses of only seven metabolites, including the aflatoxins, kojic acid, cyclopiazonic acid and aflatrem, have been assigned to a particular gene cluster. We used RNA-seq to compare expression of secondary metabolite genes in gene clusters for the closely related fungi A. parasiticus, A. oryzae, and A. flavus S and L sclerotial morphotypes. The data help to refine the identification of probable functional gene clusters within these species. Our results suggest that A. flavus, a prevalent contaminant of maize, cottonseed, peanuts and tree nuts, is capable of producing metabolites which, besides aflatoxin, could be an underappreciated contributor to its toxicity.

Sclerotia Formation and Toxin Production in Large Sclerotial <i>Aspergillus flavus</i> Isolates from Kenya

“…The small sclerotia Aspergillus produce relatively more sclerotia and toxins compared with the large sclerotia formers. Erlich and Mark [4] reported that the timing of expression for some of the gene clusters for secondary metabolism in Aspergillus flavus was coordinated with sclerotial production and that the associated metabolites accumulated preferentially in sclerotia. This study explored the possibility of using the ability to form sclerotia as indication of toxigenicity among the large sclerotia formers isolated from maize in Nandi County, Kenya.…”

<i>Aspergillus flavus</i> Secondary Metabolites: More than Just Aflatoxins

“…Another clade was characterized by high expression at most conditions and included clusters 23 (leporins), 55 (CPA), 26 (unknown) and 36 (unknown). Cluster 26 was also identified by Ehrlich and Mack 18) and Wang et al 51) as differentially expressed on solid media and during pathogen-host interaction in peanut, respectively. The study by Georgianna et al 8) was limited in that it could not account for the effect of accessory enzymes, but provided one of the first comprehensive profiling of secondary metabolic genes in A. flavus.…”

“…Ehrlich and Mack 18) was among the first to utilize RNA-sequencing to characterize secondary metabolic gene clusters in A. flavus, primarily for annotation purposes in identifying absolute expression activity and defining cluster boundaries. The authors identified eight clusters demonstrating high expression levels under their conditions and eleven clusters correlating with sclerotial development.…”

“…These types of gene clusters are termed cryptic or orphan and identification of their metabolite(s) cannot be achieved using classical knockout of a cluster gene followed by comparative metabolic analysis of the mutant and the wild-type by liquid chromatography-mass spectrometry (LC-MS). Analysis of ribonucleic acid sequencing (RNA-Seq) data from a number of A. flavus strains growing on artificial media indicates that approximately half of the gene clusters are silent or expressed at very low levels 18) . A number of methods have been used to overcome silent gene clusters thereby enabling identification of some previously unknown cluster metabolites [19][20][21] .…”

Tremorgenic and neurotoxic paspaline-derived indole-diterpenes: biosynthetic diversity, threats and applications

“…Similarly, the integration of IDT biosynthesis with other metabolic and morphogenetic processes also remains opaque. In A flavus , abundant aflatrem production is associated with sclerotia formation (Ehrlich and Mack 2014) while reduced aflatrem production results from the deletion of the ndsC gene encoding a global regulator of secondary metabolism and asexual development (Gilbert et al 2016). …”