Biology Reference
In-Depth Information
2. Evolutionary history
All P450s of the CYP73A family characterized biochemically to date do
encode C4H, and thus it appears that highly specific 4-hydroxylation of
cinnamate is the sole biological function of this family. However, phyloge-
netic reconstructions indicate that the CYP73A family can be subdivided into
two major groups, called class I and class II, that diverged via gene duplica-
tion early in vascular plant evolution (
Ehlting et al., 2006
;
Fig. 2
). This
duplication clearly occurred prior to monocot-eudicot divergence, and pos-
sibly even prior to the angiosperm-gymnosperm split (
Fig. 2
). Monocot
sequences had previously all been annotated as class II subfamilies (
Ehlting
et al., 2006
), but with the addition of more complete monocot gene families,
it is now more likely that the previous class IIA monocot group actually
constitutes a sister clade to eudicot class I proteins and is thus referred to as
monocot class I instead (
Fig. 2
). Class I and class II sequences share amino
acid similarities in the range of 60% to each other, thus clearly belong to a
single CYP subfamily (these are defined by sequence similarity exceeding
55%). Based on 22 nearly completed genome sequences, it appears that all
plants maintained a single or recently duplicated, paralogous pair of class I
CYP73A sequences. Class I members are highly conserved, share protein
sequence similarities exceeding 80%, and are all predicted to be localized in
the endoplasmic reticulum (ER). For some class I C4Hs, targeting to the ER
has been experimentally proven (
Achnine et al., 2004, Ro et al., 2001
). In
contrast, class II genes have been maintained in most but not all species,
notably no class II sequences are present in the Brassicacea including the
model plant Arabidopsis thaliana. Class II sequences are more divergent to
each other with sequence similarity down to 70% on the protein level and
many do not contain a typical ER-targeting signal that suggests a non-ER
subcellular localization (
Ehlting et al., 2006
).
Taken together, this evolutionary history would suggest that class I
CYP73As maintained an essential function that does not allow these genes
to be lost or even changed much, and it is appealing to assume that this
essential function is developmental lignification. In contrast, class II genes
appear to have taken over more specialized functions, which are not abso-
lutely required in all lineages or species and that have undergone more
evolutionary change presumably due to relaxed or distinct selection pressure
acting on the different lineages. This evolutionary history would thus be
consistent with class II proteins having functions in specialized or secondary
metabolism. Consistent with this distinction of physiological functions are
reports on the differential expression of class I and class II genes. For
example, the paralogous poplar class I CYP73As are predominantly
expressed in xylem, while the class II CYP73A is expressed to very low levels
