The Genetics and Molecular Biology of Carotenoid Biosynthesis in Mucorales - Biotechnology of Fungal Genes - page 33

In Phycomyces the failure to fi nd mutants that accumulate signifi cant
amounts of other intermediates of the route different from phytoene and
lycopene, did suspect that these two structural genes, carB and carRA,
were solely responsible for the transformation of phytoene to β-carotene.
Later, quantitative complementation and interallelic complementation
studies (De la Guardia et al. 1971, Aragon et al. 1976, Sanz et al. 2002)
demonstrated that this assumption was correct and that the conversion of
phytoene to β-carotene is carried out by an enzyme aggregate composed
of four copies of the phytoene dehydrogenase enzyme encoded by the
carB gene and two copies of the lycopene cyclase enzyme encoded by
the carRA gene. The four copies of the phytoene dehydrogenase enzyme
act sequentially on the phytoene molecule, turning it into phytofl uene,
ζ-carotene neurosporene and lycopene. The two copies of the lycopene
cyclase enzyme act successively on the ends of the lycopene molecule
fi rst making γ-carotene and then β-carotene. In Mucor a similar enzyme
organization has been described (Velayos et al. 2000b) (Figure 3).
Figure 3. Proposed model for carotenogenic enzymes aggregate in M. circinelloides
(Velayos, et al. 2000). The structural genes carB and carRP are controlled by the same
promoter. The enzymes encoded by these genes catalyze the dehydrogenation of
phytoene ( carB ) and both the synthesis of phytoene and the cyclization of lycopene
( carRP ). Two and four carB carRP products interact to form an aggregate. Each phytoene
dehydrogenase would make a dehydrogenation and each lycopene cyclase a cyclization
in each molecule that passes through them. In C it is shown the likely real location of the
aggregate, anchored in the membrane according to the deducted hydropathic profi les
of CarB and CarRP proteins.
THE REGULATION OF THE CAROTENOGENIC PATHWAY
The biosynthesis of carotene is much more conserved than its regulation,
as is often the case in the evolution of metabolic pathways. This is true
also in Mucorales in such a way that overproducing strains may have
different regulatory mechanisms in different species. In Mucorales, P.
blakesleeanus is the most studied and has been used in carotenoid studies
since the middle of the last century (reviewed in Cerdá-Olmedo 1987).
In P. blakesleeanus β-carotene plays a key role in the control of its own