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H P X
2
G Y) (Hugueney et al. 1995). Although the cyclase activity of these
enzymes does not require the presence of cofactors, a third region has been
detected in them that seems to be a consensus region for FAD-binding
and other cofactors (D X
4
G X G X A X
4
A) (Cunningham et al. 1994). This
sequence could be the remainder of an ancestral protein (Cunningham
et al. 1996) or intervene in the stabilization of the enzyme (Schnurr et al.
1996). Furthermore, both types have the same hydropathic profi le type with
several potential transmembrane sequences suggesting a role in membrane
anchoring (Pecker et al. 1996). These similarities among different types of
phylogenetically unrelated enzymes could be the result of a convergent
evolution.
In
Myxococcus xanthus
two genes have been described that determine two
proteins similar to the lycopene cyclases (Botella et al. 1995). These enzymes
form a third group of lycopene cyclases with different characteristics from
the two previous families and with possibly independent origin.
A new type of lycopene cyclase enzyme has been described in
Xanthophyllomyces dendrorhous
(Verdoes et al. 1999b),
Mucor circinelloides
(Velayos et al. 2000b)
Phycomyces blakesleeanus
(Arrach et al. 2001, Sanz
2004),
N. crassa
(Arrach et al. 2002),
Gibberella fujikuroi
(Linnemannstons
et al. 2002) and
Blakesleea trispora
(Rodríguez-Sáiz et al. 2004) in which the
lycopene cyclase activity is located at the amino terminus of the phytoene
synthase enzyme, with both activities encoded by the same bifunctional
gene (
Figure 1).
In
P. blakesleeanus
the bifunctional nature of the
carRA
gene, encoding
the phytoene synthase and lycopene cyclase activities was proposed based
on the behaviour of different mutations in this gene (Torres-Martinez et
al. 1980). Thus, the lycopene cyclase domain, located in the N-terminal
hydrophobic region of the protein, was defi ned by different mutations
occurring in the 5' end of the gene that caused the accumulation of lycopene
in the mycelia. The phytoene synthase domain, located in the hydrophilic
C-terminal end was defi ned by mutations in the 3' region of the gene that
caused the absence of any carotenoid. Although the separation of these
activities into two distinct polypeptides was initially suggested due to
the presence of a possible cleavage site between the two domains, there is
evidence suggesting that this may not be the case at least in
Mucor
(Velayos et
al. 2000b). Thus, the existence of a bifunctional gene with phytoene synthase
and lycopene cyclase activities appears to be a common phenomenon
among different types of fi lamentous fungi (
Basidiomycetes
,
Zygomycetes
and
Ascomycetes
). In some bacteria, such as
Brevibacterium,
lycopene cyclase
activity is encoded by two small genes that form a heterodimer responsible
for cyclization reactions (Krubasik and Sandmann 2000). The analysis of
the
M. circinelloides carRP
gene sequence revealed that the lycopene cyclase
