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would be observed. These excitation maxima would correspond to absorbance
maxima of the various tetrapyrrole donors, and would correspond to the peaks of
the resonance excitation energy transfer bands.
The SBP-Single Location Model Is Not Compatible with Resonance
Excitation Energy Transfer Between Anabolic Tetrapyrrole
Donors and chl
a
-Proteins Acceptors in Chloroplasts
The compatibility of the SBP-single location model with the formation of Chl
a
-thylakoid proteins was tested by monitoring resonance excitation energy transfer
between anabolic tetrapyrrole intermediates of the Chl biosynthetic pathway and
various thylakoid Chl
a
-protein complexes. Pronounced resonance excitation
energy transfer bands from Proto, Mp(e), and Pchl(ide) a to Chl
a
~F685, ~F695,
Assignment of
in situ
resonance excitation energy transfer maxima to various
metabolic tetrapyrroles was unambiguous except for a few cases at the short wave-
length and long wavelength extremes of excitation bands. It was surprising to observe
a significant diversity in the various intra-membrane environments of Proto, Mp(e),
and Pchl(ide)
a
(Kolossov and Rebeiz
2003
). A differential donation of resonance
excitation energy transfer frommultiple Proto, Mp(e) and Pchl(ide)
a
sites to different
Chl
a
-apoprotein complexes, expressed this diversity which was strongly compatible
with the biosynthetic heterogeneity of the Chl biosynthetic pathway.
Since resonance excitation energy transfer is insignificant at distances larger
than 100
˚
(Calvert and Pitts
1967
), the detection of pronounced resonance
excitation energy transfer from Proto, Mp(e), and Pchl(ide)
a
to Chl
a
~ F685,
were within distances of 100
˚
or less of the Chl
a
acceptors. This was incompatible
with the functionality of the SBP-single location Chl-thylakoid biogenesis model as
detailed below.
As mentioned in Chap.
15
, the early concept of a PSU consisting of about
500 antenna Chl per reaction center has evolved into two pigment systems each
with its own reaction center and antenna Chl (Allen and Forsberg
2001
; Anderson
2002
; Staehelin
2003
). The early visualization of the two photosystems consisted of
various pigment-protein complexes arrayed into a linear PSU (the continuous array
model), about 450
˚
in length and 130
˚
in width (Bassi et al.
1990
). In the PSU,
the LHCII was depicted as being shared between the two photosystems. More
recent models however, favor the concept of a laterally heterogeneous PSU
(Allen and Forsberg
2001
; Anderson
2002
; Staehelin
2003
). In this model LHCII
shuttles between PSI and PSII upon phosphorylation and dephosphorylation
(Allen and Forsberg
2001
). Furthermore while PSII is mainly (but not exclusively)
located in oppressed thylakoid domains, PSI is located in non-appressed stroma
thylakoids, grana margins, and end membranes (Anderson
2002
; Staehelin
2003
).
The SBP-single location model is incompatible with the linear continuous array
model, and the laterally heterogeneous PSU model. The SBP-single location model
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