Modulation of Activity of a Carotenoid Pathway Through the Use of the TeT-on Regulatory System: Application in the Fungus Fusarium fujikuroi

Carotenoids are widespread pigments in photosynthetic species, but they are also found in nonphotosynthetic microorganisms, such as bacteria and fungi. The amenability of fungi to genetic studies have made some fungal species advantageous models in the study of the genetics and biochemistry of carotenoid biosynthesis, while others have been used for biotechnological carotenoid production. The availability of molecular techniques that allow modulating the expression of target genes is a powerful tool in the manipulation of carotenoid synthesis. An example of an adjustable gene expression is based on the tetracycline-controlled transcriptional activation system, known as Tet-on. We describe here the material and protocols for the construction of a Tet-on regulated gene, its introduction in the filamentous fungus F. fujikuroi, and its use to modulate the expression of a negative regulator of carotenoid biosynthesis.

HPLC Analysis of Carotenoids in Neurosporaxanthin-Producing Fungi

The ascomycetous fungi Fusarium fujikuroi and Neurospora crassa are widely used as research models in the study of secondary metabolism and photobiology, respectively. Both fungi exhibit a similar carotenoid pathway, for which all the genes and enzymes have been identified. Under standard laboratory conditions, either F. fujikuroi or N. crassa accumulate a mixture of neurosporaxanthin, a carboxylic apocarotenoid acid, and several of its carotene precursors. We formerly described methods for the identification and quantification of neurosporaxanthin. However, the differences in polarity between this acidic xanthophyll and neutral carotenes make their global analysis cumbersome. Here we propose a simple HPLC methodology for the efficient separation of neurosporaxanthin and earlier pathway intermediates in a single HPLC run. This method should be useful to check the abundance of neurosporaxanthin under different experimental conditions and to evaluate the relative proportions of their different carotene precursors. To assess the validity of the method, we have compared the carotenoid profiles in samples of mycelia of F. fujikuroi and conidia of N. crassa, in both cases obtained from surface cultures of a wild strain of each species.