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of Chl a biosynthesis than Mp(e) and Pchlide a , it indicates that in cucumber, the
Chl a -protein biosynthesis subcenter is a highly folded entity, where linear
distances between intermediates and end products bear little meaning (see discus-
sion). On the other hands, in barley, a DMV-LDMV plant species (Abd-El-Mageed
et al. 1997 ) distances separating Proto from various Chl a acceptors were generally
longer than those separating Mp(e) and MV Pchlide a from the Chl a acceptors
(Table 15.2 ). This in turn suggests that the tetrapyrrole-protein complex folding in
cucumber (DV subcenters) is different than in barley (MV subcenters).
On the other hands the shorter distances separating anabolic tetrapyrroles
from Chl-protein complexes (Table 15.2 ) are compatible with the
SBP-multilocation and MBP-sublocation models. Since overwhelming experi-
mental evidence argues against the operation of a single-branched Chl biosyn-
thetic pathway in plants (Rebeiz et al. 2003b ) that leaves us with the
MBP-sublocation model alternative. In this model, the unified multibranched
Chl a/b biosynthetic pathway, is visualized as the template of a Chl-protein
biosynthesis center where the assembly of PSI, PSII and LHC takes place
(Rebeiz et al. 1999 ). The multiple Chl biosynthetic routes are visualized,
individually or in groups of one or several adjacent routes, as Chl-apoprotein
biosynthesis subcenters earmarked for the coordinated assembly of individual
Chl-apoprotein complexes. Apoproteins destined to some of the subcenters may
possess specific polypeptide signals for specific Chl biosynthetic enzymes peculiar
to that subcenter, such as 4-vinyl reductases, formyl synthetases or Chl a and Chl b
synthetases. Once an apoprotein formed in the cytoplasm or in the plastid reaches
its subcenter destination and its signal is split off, it binds nascent Chl formed via
one or more biosynthetic routes, as well as carotenoids. During pigment binding,
the apoprotein folds properly and acts at that location, while folding or after
folding, as a template for the assembly of other pigment-proteins. This model is
certainly compatible with the lateral heterogeneity of the PSU and can account for
the observed resonance excitation energy transfer and the short distances separating
anabolic tetrapyrroles from Chl-protein complexes in the distinct PSI, PSII and
shuttling LHCII entities that compose the PSU.
In all cases, it was observed that while distances separating metabolic
tetrapyrroles from Chl a E670F685 and Chl a E677F695 were in the same
range, those separating Chl a E704F735 from the anabolic tetrapyrroles were
much shorter (Table 15.2 ). As may be recalled, it is believed that the fluorescence
emitted at F685 nm arises from the Chl a of the light-harvesting Chl-protein
complexes (LHCII and LHCI-680), that emitted at F695 nm originates mainly
from the PSII antenna Chl a (CP47 and/or CP29), while that emitted at F735 nm
originates primarily from the PSI antenna Chl a (LHCI-730)(Bassietal. 1990 ).
This in turn suggests that in the Chl a -protein biosynthesis subcenters, protein
folding is such that the PSI antenna Chl a (LHCI-730) is much closer to the
terminal steps of anabolic tetrapyrrole biosynthesis than the LHCII and LHCI-
680 Chl-protein complexes or the CP47 and/or CP29 PSII antenna Chl a
complexes.
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