# Caroenoid Biosynthesis in _Fusarium_
## 4 Regulation by Light
The genes are grouped according to their functions: _ggs1, carRA, carB, carT,_ and _carD_ involved in NX biosynthesis and _carX_ and _carO_ involved in retinal and CarO rhodopsin production. Orange colors correspond to the genes organized as a cluster, and blue color to genes unlinked in the genome. Darker colors indicate cultures incubated in the dark and paler colors indicate cultures illuminated for one hour. (**A**) Above: wild-type strain IMI58289 of _F. fujikuroi_ and its _carS_ mutant SG39. Below: wild type strain 4287 of _F. oxysporum_ (f. sp. _lycerosici_) and its _carS_ mutant SX2; (**B**) Effect of light on the mRNA levels of the _carS_ mutant SG39. RNA-seq data and culture conditions correspond to analyses already described [71]. In brief, all strains were cultured for three days in dark in minimal medium (DG medium for _F. fujikuroi_ and DG with 3 g asparagine instead of NaNO\({}_{3}\) for _F. oxysporum_). In the cases indicated, the cultures were exposed to one hour of illumination. RNA samples were sequenced with Illumina technology and the resulting data were analyzed with software tools for read mapping (Bowtie and TopHat), transcriptome assembly (Cuffmerge and Cufflinks), and differential gene expression analysis (Cuffdiff and CummeRbund). The study of the transcriptional photoinduction of the structural genes for carotenogenesis in the mutants of the _wcoA_ and _cryD_ genes is consistent with different mechanisms of action for the encoded photoreceptors [65]. After illumination, the _wcoA_ mutants exhibit photoinduction of carotenogenesis, but they accumulate carotenoids more slowly and their total levels do not reach those of the wild type. However, both the carotenoid content and the mRNA levels for the structural _car_ genes are much lower in the dark in the _wcoA_ mutants than in the wild type. Even more, the transcriptional photoinduction of the _car_ genes is basically absent in the _wcoA_ mutants, with mRNA levels after illumination lower than those of the wild type in the dark. Therefore, the induction of carotenogenesis by the alternative photoreceptor, presumably CryD, must be done through a transcription-independent mechanism. The molecular basis for such a mechanism, that could involve mRNA stability or translation and enzymatic activities or stabilities, remains to be elucidated. The decay of the mRNA levels after photoinduction has been investigated in detail in _N. crassa_. In this fungus, the small flavin photoreceptor VIVID (VVD), its name due to the higher carotenoid levels of the _vvd_ mutants under light, plays a key role in deactivating the WC photoreceptor system after illumination [70]. In contrast, the mutation of the VIVID ortholog in _F. fujikuroi_, called _vvdA_, results in a lower carotenoid accumulation in this fungus [72]. The kinetics of carotenogenesis in the _vvdA_ mutants show a faster carotenoid accumulation immediately after light exposure, consistent with an attenuating function of VvdA on the photoactivated WcoA, but a slower accumulation after more prolonged growth under light, suggesting an up-regulating role during the second stage of carotenoid photoinduction [65].