One possibility for the improvement of the metabolic productivity of an organism is genetic modification. This strategy can be successful when an increase of the flux through a pathway is achieved by, e. g., the overproduction of the rate-limiting enzyme, an increase of precursors, or the modification of the regulatory properties of enzymes. In the carotenogenic yeasts, mevalonate synthesis, which is an early step in terpenoid biosynthesis, is a key point of regulation of the carotenoid biosynthetic pathway. In fact, addition of mevalonate to a culture of X. dendrourhous stimulated both astaxanthin and total carotenoid biosynthesis four times (from 0.18 to 0.76 mg/ g and from 0.27 to 1.1 mg/ g dry cells, respectively). This indicates that the conversion of HMG-CoA to mevalonate by HMG-CoA reductase is a potential bottleneck on the road to modified strains with higher astaxanthin content (Verdoes et al., 2003).

Like carotenoids, ergosterol is an isoprenoid and it is biosynthetically related to them by common prenyl lipid precursor, FPP. Astaxanthin production by P. rhodozyma strain was enhanced (1.3-fold) when sgualene synthase phenoxypropylamine-type inhibitor for sterol biosynthesis was added to the medium. The isolation and characteristic of the carotenogenic genes of yeasts facilitates the study of the effect of their overexpression on carotenoid biosynthesis. Use of recombinant DNA technology for metabolic engineering of the astaxanthin biosynthetic pathway in X. dendrourhous was described too. In several transformants containing multiple copies of the phytoene synthase-lycopene cyclase­encoding gene (crtYB), the total carotenoid content was higher (with 82%) than in the control strain. This increase was mainly due to an increase of the beta-carotene and echinenone content (with 270%), whereas the total content of astaxanthin was unaffected or even lower.

Alternatively, in recent years, several food-grade non-pigmented yeasts (Saccharomyces cerevisiae, Candida utilis) have been engineered in order to obtain strains possessing the ability to produce selected carotenoids (Verwaal et al., 2007). Identification of genes of enzymes from the astaxanthin biosynthetic pathway and their expression in a non- carotenogenic heterologous host have led to the overproduction of beta-carotene. The possibility of the use of S. cerevisiaeas a host for efficient beta-carotene production by successive transformation with carotenogenic genes (crtYB which encodes a bifunctional phytoene synthase and lycopene cyclase; crtI, phytoene desaturase; crtE, heterologous GGPP synthase; tHMGI, HMG-CoA reductase) from X. dendrorhous was studied. Like X. dendrorhous, S. cerevisiae is able to produce FPP and converts it into GGPP, the basic building block of carotenoids. S. cerevisiae, the industrially important conventional yeast, cannot produce any carotenoid, while it synthesizes ergosterol from FPP by a sterol biosynthetic pathway. Conversion of FPP into GGPP is catalyzed by GGPP synthase encoded by BTS1 gene in S. cerevisiae. Construction of a strain, producing a high level of beta-carotene (5.9 mg/g dry cells) was succesful. Oleaginous yeasts are also suitable host strains for the production of lipophilic compounds due to their high lipid storage capacity. Recently, the carotenoid-producing Yarrowia lipolytica has been generated by metabolic engineering. Acording to these results entire biosynthetic pathways can be introduced into new host cells through recombinant DNA technology and carotenoids can be produced in organisms that do not normally produce carotenoids.