Gain and loss of a transcription factor that regulates late trichothecene biosynthetic pathway genes in Fusarium

Trichothecenes are among the mycotoxins of most concern to food and feed safety and are produced by species in two lineages of Fusarium: the F. incarnatum-equiseti (FIESC) and F. sambucinum (FSAMSC) species complexes. Previous functional analyses of the trichothecene biosynthetic gene (TRI) cluster in members of FSAMSC indicate that the transcription factor gene TRI6 activates expression of other TRI cluster genes. In addition, previous sequence analyses indicate that the FIESC TRI cluster includes TRI6 and another uncharacterized transcription factor gene (hereafter TRI21) that was not reported in FSAMSC. Here, gene deletion analysis indicated that in FIESC TRI6 functions in a manner similar to FSAMSC, whereas TRI21 activated expression of some genes that function late in the trichothecene biosynthetic pathway but not early-pathway genes. Consistent with this finding, TRI21 was required for formation of diacetoxyscripenol, a late-trichothecene-pathway product, but not for isotrichodermin, an early-pathway product. Although intact homologs of TRI21 were not detected in FSAMSC or other trichothecene-producing fungal genera, TRI21 fragments were detected in some FSAMSC species. This suggests that the gene was acquired by Fusarium after divergence from other trichothecene-producing fungi, was subsequently lost in FSAMSC, but was retained in FIESC. Together, our results indicate fundamental differences in regulation of trichothecene biosynthesis in FIESC and FSAMSC.