| Summary: | Carotenoid biosynthesis is a frequent trait in fungi. In the ascomycete <i>Fusarium fujikuroi</i>, the synthesis of the carboxylic xanthophyll neurosporaxanthin (NX) is stimulated by light. However, the mutants of the <i>carS</i> gene, encoding a protein of the RING finger family, accumulate large NX amounts regardless of illumination, indicating the role of CarS as a negative regulator. To confirm CarS function, we used the Tet-on system to control <i>carS</i> expression in this fungus. The system was first set up with a reporter <i>mluc</i> gene, which showed a positive correlation between the inducer doxycycline and luminescence. Once the system was improved, the <i>carS</i> gene was expressed using Tet-on in the wild strain and in a <i>carS</i> mutant. In both cases, increased <i>carS</i> transcription provoked a downregulation of the structural genes of the pathway and albino phenotypes even under light. Similarly, when the <i>carS</i> gene was constitutively overexpressed under the control of a <i>gpdA</i> promoter, total downregulation of the NX pathway was observed. The results confirmed the role of CarS as a repressor of carotenogenesis in <i>F. fujikuroi</i> and revealed that its expression must be regulated in the wild strain to allow appropriate NX biosynthesis in response to illumination.
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