In fungal secondary metabolites, the decalin scaffold is typically synthesised by an iterative type I highly reducing polyketide synthase (HR-PKS) or by a hybrid PKS-nonribosomal peptide synthetase (PKS-NRPS). For example, fusarielin A is synthesised from short-chain carboxylic acids by an HR-PKS, while equisetin is produced by a PKS-NRPS that additionally incorporates L-serine, forming a tetramic acid appended to a decalin in the structure (Fig. ). The cyclisation from the linear polyketide substrate can occur via a Diels-Alder reaction catalysed within the PKS, or by a Diels-Alderase enzyme. Given that the genome of A. burnettii MST FP2249 has been sequenced, we attempted to identify the gene cluster encoding burnettiene A (1). Previous genome analysis revealed that A. burnettii harbours 25 putative BGCs containing an HR-PKS. Among them, the putative burnettiene gene cluster (bue) was identified by its similarity to the fusarielin cluster from Fusarium graminearum PH-1 (Fig. ). As fusarielin is a decalin polyketide resembling 1, the bue gene cluster was selected as a candidate for the biosynthesis of 1. The HR-PKS enzyme BueA shows 50% identity with FSL1 and shared domain architecture (Fig. , Table †). The bue cluster also contains the transacting enoyl reductase (ER) BueB, which displays 60% identity with the trans-ER FSL5. There is also homology between the gene products BueC/D/E and FSL2/3/4 (ranging from 39-48% identity), which are an α/β hydrolase, epimerase, and cytochrome P450, respectively (Fig. , Table †). Additionally, bueR encodes a transcription factor (TF) that presents homology to the cluster specific TF FSL7. BueR also contains a Zn2Cys6 DNA-binding domain, which is the most common type of cluster-specific TF. To define the bue gene cluster boundaries, we searched for homologues of the putative bue and neighbouring genes in NCBI GenBank with cblaster. We found a wide distribution of this BGC family across several fungi from the Eurotiomycetes and Sordariomycetes classes, as it was previously observed in phylogenetic analysis of the FSL cluster. Importantly, we found close similarity to uncharacterised BGCs from several Aspergilli, including A. niger, A. fumigatus and A. clavatus (Fig. ). For example, six gene products from the bue cluster share 74-88% identity to the homologous cluster from the opportunistic human pathogen A. fumigatus Af293 (Fig. ). This cluster, A. fumigatus cluster 4, has been extensively analysed across A. fumigatus strains at a genetic level by Lind and co-workers, and it was observed to undergo pseudogenisation in many strains. Lind et al. also attempted to find the evolutionary origin of this cluster family, but as there are many gain and loss events in both Sordariomycetes and Eurotiomycetes fungi, it remains uncertain. Interestingly, we observed that the putative methyltransferase BueF and a major facilitator superfamily (MFS) transporter BueG, which lacked homologues within the fusarielin cluster, seem to be highly conserved in other species (Fig. ). The flanking genes downstream of bueA were not conserved in other species, indicating a cluster boundary (Fig. ). As the bue cluster is located at the end of an A. burnettii genome scaffold, we decided to delimit the putative bue cluster to the genes bueA/B/C/D/E/F/G/R (ETB97_004590-004597), that are encoded within a ∼22 kb region of A. burnettii genome.