Biology Reference
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recently, Yuichi and Bauer reported demonstration of dark-Pchlide
a
reductase
activity in reconstituted systems from
R. capsulatus
, a purple nonsulfur photosyn-
thetic bacterium (Yuichi and Bauer
2000
). Two of the putative three subunits, BchL
and BchN were expressed in
R. capsulatus
as S tag fusion proteins. The third subunit,
BchB, copurified with the
BchN
protein, thus indicating that the BchN and BchB
proteins form a tight complex. Dark Pchlide
a
reductase activity was shown to be
dependent on the presence of all three subunits, on ATP, and on the reductant
dithionite. In angiosperms, the corresponding gene products ChlL, ChlN, and ChlB,
also appear to be evolutionarily related to the subunits of the eubacterial nitrogenase
enzyme complex (Armstrong
1998
).
9.2 Kinetics of the Photoconversion of Pchlide
a
-H (E650 F657) to Chlide
a
9.2.1 Action Spectrum of the Photoconversion
Pchl(ide)
a
H (E650 F657) is the photoreceptor for its own photoconversion to
Chlide
a
(Koski et al.
1951
). In an albino corn mutant lacking carotenoids, the
action spectrum exhibited two prominent peaks, one at 650 nm and one at 445 nm
that corresponded to the absorption spectrum of LW t-Pchlide
a
H of the mutant.
9.2.2 Effect of Temperature on the Photoconversion
The phototransformation of LW t-Pchlide
a
HtoChlide
a
was completely inhibited
at
70
C.
At temperatures beyond 50
C, photoconversion was progressively inhibited due to
apoprotein denaturation. Dependency of the photoconversion upon temperature
indicates that the phototransformation is not a purely photochemical reaction but
also involves a thermochemical component.
195
C (Smith and Benitez
1954
). Partial photoconversion took place at
9.2.3 Quantum Yield of the Photoconversion
The average quantum yield of the photoconversion at 642 nm amounts to about 0.6
(Smith and Benitez
1954
). Therefore it is not clear from this work whether one or
two quanta of light are required for the photoconversion. On the other hands,
Thorne (
1971
) proposed a two quantum process for the photoconversion.
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