Biomedical Engineering Reference
In-Depth Information
synthesis. Total carotene synthesis is greatly increased when the amount of
β-carotene is reduced by enzymatic block of the pathway produced either
by mutation in the structural genes,
carB
or
caR
(affected in the R domain
of the
carRA
gene) or by chemical block with specifi c dehydrogenase or
cyclase inhibitors. In the absence of other activation, the phytoene and
lycopene content of
carB
and
carR
mutants, respectively, is about fi fty
times that of the β-carotene content of the wild type. The increase is less
pronounced in the corresponding mutants of
M. circinelloides
(Navarro
et al. 1995, Velayos et al. 1997) and
B. trispora
(Mehta and Cerdá-Olmedo
1995). Apart from the infl uence of environmental conditions, what can
be deduced from those results is that β-carotene synthesis in
Phycomyces
is feed-back regulated by the end product (Eslava et al. 1974, Bejaro et
al. 1988) while in
Mucor
and
Blakeslea
the regulation of the pathway by
the end product seems to be absent or is different from that described in
Phycomyces
.
Several external factors have been shown to exert a positive or negative
effect on carotenoid production in Mucorales. The effects of media and
culture conditions were described in
Phycomyces
by W.G. Lilly, T.W. Godwin
and their coworkers and in
Blakeslea
by A. Ciegler and collaborators
(reviewed in Avalos and Cerdá-Olmedo 2004). The β-carotene content of
Phycomyces
is infl uenced by agents such as blue light, sexual interactions,
chemicals and mutations in several genes (Cerdá-Olmedo 2001). The extent
of the action in other Mucorales and even their existence depends on the
species and on the strain used.
Regulatory Mutants
The existence of overproducing carotenoid mutants suggests that there
must be regulatory mechanisms. Mutants have been found in
Phycomyces
,
Mucor
and
Blakeslea
. In
Phycomyces
it is easy to isolate super-yellow
mutants that contain a large amount of β-carotene. The β-carotene content
is increased about 100 times in mutants of genes
carS
,
carD
and
carF
(Murillo and Cerdá-Olmedo 1976, Salgado et al. 1989, Mehta et al. 1997).
The CarS protein plays a central role in the regulation of the pathway
and it has been proposed that the end product regulation of the pathway
is mediated by an interaction of β-carotene with the
carS
and
carA
gene
products (Murillo and Cerdá-Olmedo 1976, Bejarano et al. 1988). However
the molecular nature of the end-product regulation is not known. Recently
(Tagua et al. unpublised results) it has been shown that
carS
encodes
a β-carotene-cleaving oxygenase that catalyzes the biosynthesis of a
β-ring-containing apocarotenoid that inhibits the activity of the phytoene
synthase, the
carA
gene product, which is the fi rst enzyme specifi c for
carotenogenesis. The β-ring-apocarotenoid thus exerts a feed-back control
