New elements in the regulation of carotenoid biosynthesis in Fusarium
Fusarium species produce a large variety of secondary metabolites ranging from toxins, antibiotics to pigments. Among the latter is the carotenoid neurosporaxanthin (NX), synthesized after illumination and presumably involved in protection against oxidative stress. In Fusarium fujikuroi and Fusarium oxysporum, light induces transcription of structural genes carRA, carB, carO, and carT, involved in NX biosynthesis.
Until now, no specific transcription factors of the car genes have been demonstrated to bind to their promoters. However, deep-pigmented strains accumulating large NX amounts have been characterized as well as the gene responsible of this phenotype: carS. An increase of carS mRNA, modulated by the Tet-on system, has shown to reduce the accumulation of NX, supporting the role of CarS as a repressor. The protein CarS does not bind to the carRA promoter, a key gene in the NX pathway, but it has two RING finger domains characteristic of ubiquitin ligases, suggesting a control by interaction with other regulatory proteins. In order to find putative specific transcription factors, pull-down experiments with biotinylated car promoters have been carried out and putative candidates genes have been identified.
RNA-seq data of the the upstream 5’ region of carS in F. fujikuroi and F. oxysporum led to identify a non-annotated transcript in their genomes, that we named carP. In F. fujikuroi, expression of carP is enormously affected by the carS mutation. Present data indicate that carP is a lncRNA transcribed in the same direction that carS. Deletion of carP provokes a down regulation of the structural car genes and an increase of carS mRNA, which correlate with a lack of NX accumulation. Altogether indicate a complex regulation of carotenogenesis in Fusarium in which participate not only regulatory proteins but also a lncRNA, currently under detailed study.