Biomedical Engineering Reference
In-Depth Information
mutant affected in photomorphogenesis (Flores et al 1998). Such strains
may reveal the occurrence of auxiliary regulatory proteins. They could be
involved in phosphorylation and dephosphorylation steps, as suggested
by the effect of inhibitors for protein kinases and phosphatases on carotene
production (Tsolakis et al. 1999).
In summary, carotenogenesis in
Phycomyces
is regulated by a feed-
back mechanism that should be coordinated with photo-induction, but the
molecular basis is yet to be clarifi ed.
Molecular Analysis
Cloning and sequencing of the structural genes in
Phycomyces
,
Mucor
and
Blakeslea
permitted the analysis of the effect of light and other facts
on their mRNA levels. The analysis of the fi rst gene available,
carB
from
Phycomyces
, showed a fast photo-induction of its mRNA amounts, as
fast as the one found in
N. crassa
, but with a lower light intensity level
(Ruiz-Hidalgo et al. 1997, Blasco et al. 2001). Analogous experiments in
M. circinelloides
showed a similar response, but with a stronger induction
(Velayos et al. 2000a). An equivalent result was obtained with the
carRA
orthologous
carRP
(Velayos et al. 2000b).
The complex regulation of carotenogenesis in
Phycomyces
has recently
been investigated at the mRNA level for the genes of the pathway (Almeida
and Cerdá-Olmedo 2008). The results showed a similar light regulation
of
carRA
and
carB
mRNAs and no effect on those for early genes of the
terpenoid pathway such as
hmgR
and
hmgS
. Light accumulation of
carRA
and
carB
mRNAs was observed for different strains in structural
carA
,
carR
and
carB
mutants, although a tendency for a lower induction was manifested
in the
carRA
mutant strains. A clear light induction was also observed in the
regulatory mutants
carS
,
carF
and
carD
. However these mutations did not
affect the
carB
and
carRA
mRNA amounts in the dark. This suggests that
they have no relation with the light-activation mechanism. A clear induction
of
carRA
and
carB
mRNAs was also found upon several stimulations, but
these inductions were in addition to the one produced by light (Almeida
and Cerdá-Olmedo 2008).
All the enzymatic activities needed to synthesize β-carotene from
geranylgeranyl pyrophosphate in
Phycomyces, Mucor
and
Blakesleea
are
encoded by two closely linked genes divergently orientated in a cluster
like organization that seems to be characteristic of Zygomycetes but not of
other groups of fungi (Velayos et al. 2000b, Arrach et al. 2001, Sanz 2004,
Rodríguez-Sáiz et al. 2004). In
Phycomyces
a light pulse followed by periods
of darkness produces similar biphasic responses in the expression of the
carB
and
carRA
genes, indicating their coordinated regulation by light (Sanz
2004, Sanz et al. 2010). The same is true in
Mucor circinelloides
(Velayos et al.
