Solving the Polyketide Pigmentation Puzzle in Fusarium solani: Linking biosynthetic genes to compounds

Fusarium pigmentation is dictated by a set of two polyketide synthase (PKS) gene clusters where one is expressed during mycelial growth and the other during perithecial development. In the vast majority of Fusarium species, perithecial pigmentation relies on the PKS3 gene cluster responsible for biosynthesis of fusarubins and bostrycoidins. In these species, mycelial pigmentation is mediated by bikaverin or aurofusarin. However, the situation is different for F. solani where mycelial pigmentation is controlled by the PKS3 gene cluster, while the less common PKS35 is responsible for the perithecial pigmentation, although no actual compound(s) has ever been associated with the latter.
We set out to characterize the polyketide pigments of F. solani. The PKS3 gene cluster of F. solani shares seven genes with the previously characterized clusters in F. graminearum and F. fujikuroi. However, it differs from the previously described clusters by containing additional genes, some with predicted enzymatic function related to secondary metabolism. When we overexpressed the cluster specific transcription factor in F. solani, we observed a massive increase in production of javanicin, bostrycoidin, fusarubin and dihydrofusarubin.
In order to investigate the perithecial pigment chemistry we performed heterologous expression of the PKS35 in Saccharomyces cerevisiae, yielding prephenalenone and dehydroxyprephenalenone as the initial polyketide intermediates. Secondly, we overexpressed the local transcription factor gene in F. solani, resulting in a dark green and orange/red phenotype. We identified three new candidate compounds by mass spectrometry. To our surprise, the mutant also produced elevated levels of javanicin and bostrycoidin obscuring the observation and isolation of PKS35-related compounds. Interestingly, this co-expression indicates the regulation of both PKS3 and PKS35 gene clusters is somehow connected.