Biomedical Engineering Reference
In-Depth Information
by inhibiting the pathway when suffi cient β-carotene is available. The
molecular functions of
carD
and
carF
genes are not known for the moment.
Mutants containing less β-carotene than the wild type have been
isolated. The
carI
mutant was isolated because a reduce response to retinol
(Roncero and Cerdá-Olmedo 1982) and the
carC
mutants (Revuelta and
Eslava 1983) are disturbed in the mechanism of negative control by the end
product mediated by the CarS product of gene
carS
. Carotene overproducing
mutants have been isolated in
Mucor
(Navarro et al. 1995, Iturriaga et al.
2000) and
Blakeslea
(Mehta and Cerdá-Olmedo 1995), but their β-carotene
content is not more than 5-10 times that of the wild type, suggesting that
the mechanism of overproduction of β-carotene may be different to that
described in
Phycomyces
. A major regulator of β-carotene biosynthesis in
Mucor
is the product of the
crgA
gene, a protein with similarities to ubiquitin
ligases that repress carotenogenesis by inhibiting the accumulation of
carB
and
carRP
messenger mRNAs (Navarro et al. 2001, Lorca-Pascual et
al. 2004, Corrochano and Garre 2010).
crgA
mutants exhibit a β-carotene
overproducing phenotype.
Sexual and Chemical Activation
Activation of carotenogenesis when strains of opposite sex grow near each
other is an old observation regarding the life cycle of Mucorales (Blakeslee
1904). The sexual activation of carotenogenesis is not a general rule in
Mucorales; depending on the species or the strains it may occur or not,
or may take place under different conditions (reviewed Cerdá-Olmedo
1987). Sexual stimulation in Mucorales is carried out by retoid-derivate
pheromone, the trisporic acids, whose biosynthetic pathway is currently
the subject of very active research (Schimek et al. 2005, Burmester et al.
2007, Schimek and Wöstemeyer 2009, Polaino et al. 2010). The stimulation
of β-carotene biosynthesis is exploited for biotechnological production
with the industrial fungus
B. trispora
, particularly suitable for carotene
synthesis when the two strains of opposite sex are growing under
submerged conditions. Sexual activation requires a functional β-carotene
pathway. Thus
carB
or
carR
mutants are sterile and their carotene contents
are not changed by the presence of opposite sex strains.
There is no general mechanism of chemical activation of carotenoid
biosynthesis either in the Mucorales or in other fungi. Even organisms
as closely related as
Phycomyces
and
Blakeslea
are activated by different
compounds. In addition to sexual hormones, two classes of chemical
compound stimulate carotene biosynthesis in
Phycomyces
; retinoids and
phenols (Bejarano et al. 1988). In the retinoids group the presence of a
β-ionone-ring is the common feature of all of them. They are thought to
compete for the action site of β-ring-apocarotenoids, which inhibit the
