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when the rate of Chl
a
accumulation is at its peak, only traces of the PORA protein are
detected (Santel and Apel
1981
). The disappearance of PORA from etiolated tissues
during greening was confirmed by Kay and Griffiths (
1983
). These observation and
further experimentation have led to the proposal that in etiolated tissues, although
PORA functions only for a short period of time after the onset illumination, it is
required for normal greening (Runge et al.
1996
).
9.3.2 Protochlorophyllide a Oxidoreductase B (PORB)
It has been proposed that PORB provides the means to sustain light-dependent Chl
biosynthesis in fully greened mature plants, in the absence of PORA and
t-LW-Pchlide a H (E450 F655) (Runge et al.
1996
). In other words, it was suggested
that in some t-SW-Pchlide
a
H species, the apoprotein consists of PORB.
The photoreduction of Pchlide
a
by purified PORB overexpressed heterolo-
gously in
E. coli
has recently been described (Lebedev and Timko
1999
). The
PORB reaction is described as consisting of two steps. In a first photochemical step,
a single quantum mechanism leads to the formation of an unstable tetrapyrrole
intermediate with a putative free electron. In a second step, the free radical
intermediate is spontaneously converted to Chlide
a
. Both steps appear to proceed
at subzero temperatures. At room temperature, the rate of the reaction depends
linearly on enzyme and substrate concentrations, and the reduction kinetics are
consistent with one mole of substrate bound per active PORB monomer.
9.3.3 Protochlorophyllide a Photooxidoreductase C (PORC)
Like PORA and PORB, PORC is light-and-NADPH-dependent. In contrast to the
PORA
and
PORB
mRNAs, the
PORC
mRNA accumulates only after the beginning
of illumination (Oster et al.
2000
). In light-adapted mature plants only
PORB
and
PORC
mRNAs were detectable, and the amounts of both mRNAs exhibited pro-
nounced diurnal rhythmic fluctuations (Su et al.
2001
). However, differences were
observed between
PORB
and
PORC
. The differences can be summarized as
follows: (a) While the oscillations of
PORB
mRNA are under the control of the
circadian clock, that of
PORC
is not, (b) Upon transferring to darkness seedlings
grown under continuous white light, the concentration of
PORC
mRNA rapidly
declined and became undetectable, while
PORB
mRNA did not, (c) When seedlings
were exposed to different light intensities, the amounts of PORB mRNA remained
the same, while the mRNAs of
PORA
and
PORC
were modulated in an inverse way
by light intensity changes.
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