Gliotoxin, and other related molecules, are encoded by multi-gene clusters and biosynthesized by fungi using non-ribosomal biosynthetic mechanisms. Almost universally described in terms of its toxicity towards mammalian cells, gliotoxin has come to be considered as a component of the virulence arsenal of Aspergillus fumigatus. Here we show that deletion of a single gene, gliT, in the gliotoxin biosynthetic cluster of two A. fumigatus strains, rendered the organism highly sensitive to exogenous gliotoxin and completely disrupted gliotoxin secretion. Addition of glutathione to both A. fumigatus ΔgliT strains relieved gliotoxin inhibition. Moreover, expression of gliT appears to be independently regulated compared to all other cluster components and is up-regulated by exogenous gliotoxin presence, at both the transcript and protein level. Upon gliotoxin exposure, gliT is also expressed in A. fumigatus ΔgliZ, which cannot express any other genes in the gliotoxin biosynthetic cluster, indicating that gliT is primarily responsible for protecting this strain against exogenous gliotoxin. GliT exhibits a gliotoxin reductase activity up to 9 µM gliotoxin and appears to prevent irreversible depletion of intracellular glutathione stores by reduction of the oxidized form of gliotoxin. Cross-species resistance to exogenous gliotoxin is acquired by A. nidulans and Saccharomyces cerevisiae, respectively, when transformed with gliT. We hypothesise that the primary role of gliotoxin may be as an antioxidant and that in addition to GliT functionality, gliotoxin secretion may be a component of an auto-protective mechanism, deployed by A. fumigatus to protect itself against this potent biomolecule.

Gliotoxin biosynthesis is encoded by the gli gene cluster in Aspergillus fumigatus. The biosynthesis of gliotoxin is influenced by a suite of transcriptionally-active regulatory proteins and a bis-thiomethyltransferase. A selfprotection system against gliotoxin is present in A. fumigatus. Several additional metabolites are also produced via the gliotoxin biosynthetic pathway. Moreover, the biosynthesis of unrelated natural products appears to be influenced either by gliotoxin or by the activity of specific reactions within the biosynthetic pathway. The activity of gliotoxin against animal cells and fungi, often mediated by interference with redox homeostasis or protein modification, is revealing new metabolic interactions within eukaryotic systems. Nature has provided a most useful natural product with which to reveal some of its many molecular secrets. Contextualizing and rethinking gliotoxinAspergillus fumigatus is an opportunistic fungal pathogen and primarily infects immunocompromised individuals where it can cause fatal invasive aspergillosis (IA) [1]. A. fumigatus exposure can also induce debilitating aspergillosis and allergy in immunocompetent individuals [2,3]. Selected secondary metabolites produced by A. fumigatus, in particular siderophores and the non-ribosomal peptide gliotoxin, are generally considered to be front-line virulence factors [4,5]. Gliotoxin is an epipolythiodioxopiperazine (ETP) of molecular mass 326 Da, and contains a disulfide bridge which can undergo repeating cleavage and reformation, thereby resulting in a potent intracellular redox activity (Figure 1) [6]. Indeed, the dithiol form of gliotoxin has also been posited to be responsible for the observed biological activities of gliotoxin [7]. Bisdethiobis(methylthio)gliotoxin (BmGT) and related gliotoxin metabolites (Figure 1) are also biosynthesized by A. fumigatus [8,9]. Incredibly, the gliotoxin biosynthetic pathway had remained elusive since the discovery of gliotoxin in 1936; however, recent studies have not only dissected this unusual molecular assembly system but have revealed the necessity for gliotoxin-producing fungi to possess an endogenous resistance system against gliotoxin [10,11]. Moreover, because gliotoxin can be considered as the prototype ETP, studies on gliotoxin can be instrumental in revealing the biosynthetic mechanisms of related ETPs which are biosynthesized by a range of fungi [12]. Amongst others, these include sirodesmin A, sporidesmin A, chaetocin, aranotin, and chetomin [13]. Studies on the biosynthetic mechanism of ETPs, particularly gliotoxin, are also serving to inspire new synthetic chemistry approaches for ETP synthesis and desulfurization, which are somewhat beyond the scope of the present review [13,14].In addition to studying how gliotoxin contributes to organismal virulence, it has also been deployed to explore and reveal novel biochemistry within both fungal and animal cells [15,16]. Thus, we contend that it is the ability of gliotoxin to interfere with so many cellular processes that makes it...

Gliotoxin is a redox-active nonribosomal peptide produced by Aspergillus fumigatus. Like many other disulfide-containing epipolythiodioxopiperazines, a bis-thiomethylated form is also produced. In the case of gliotoxin, bisdethiobis(methylthio)gliotoxin (BmGT) is formed for unknown reasons by a cryptic enzyme. Here, we identify the S-adenosylmethionine-dependent gliotoxin bis-thiomethyltransferase (GtmA), which converts dithiogliotoxin to BmGT. This activity, which is induced by exogenous gliotoxin, is only detectable in protein lysates of A. fumigatus deficient in the gliotoxin oxidoreductase, gliT. Thus, GtmA is capable of substrate bis-thiomethylation. Deletion of gtmA completely abrogates BmGT formation and we now propose that the purpose of BmGT formation is primarily to attenuate gliotoxin biosynthesis. Phylogenetic analysis reveals 124 GtmA homologs within the Ascomycota phylum. GtmA is encoded outside the gliotoxin biosynthetic cluster and primarily serves to negatively regulate gliotoxin biosynthesis. This mechanism of postbiosynthetic regulation of nonribosomal peptide synthesis appears to be quite unusual.

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