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Prochloron didemni, Prochlorothrix hollandica, and
Prochlorococcus marinus are oxygenic photosynthetic prokaryotes
known as prochlorophytes. They are closely related to cyanobacteria
but differ in that they contain chlorophyll (Chl) a/b light-harvesting
systems rather than phycobiliproteins (1). Their discovery has been
discussed in terms of the endosymbiotic theory, which suggests that
the Chl a/b containing chloroplasts of plants and green algae originate
from free living oxygenic photosynthetic prokaryotes (2). Prochloron
is symbiotic with didemnid ascidians of tropical waters (3, 4) and
as yet there is no report of its existence as a free-living organism
(5). In contrast, Prochlorothrix is a freshwater free-living
filamentous prochlorophytes (6), whereas Prochlorococcus
is a highly abundant, freeliving marine phytoplankton (7).
Based on small subunit ribosomal (16S) RNA, all of these organisms
fall within the clade of extant cyanobacteria (8); thus
there must have been greater diversity of light-harvesting pigments
within the cyanobacteria than previously conceived. This
is reinforced by the recent discovery of a group of oxygenic photosynthetic
bacteria containing Chl d as a major pigment (9), which also fall
within the cyanobacterial clade, based on 16S rRNA analysis (H.
Miyashita, personal communication).
In 1996 the sequence of the pcb genes encoding the Chl a/b-binding
proteins of prochlorophytes was reported (10).
Surprisingly, the sequence was found to be very different to that
of the cab genes that encode the Chl a/b-binding proteins of chloroplasts
(11). In fact, the pcb gene sequence was similar to that of the
gene that encodes the Chl a-binding protein CP43 of photosystem
II (PSII) and the IsiA protein induced in cyanobacteria under conditions
of limiting iron levels (12, 13).
Recent studies (14–16) have shown that the CP43-like IsiA protein
forms a ring of 18 subunits around the cyanobacterial trimeric photosystem
I (PSI) reaction center core complex. In so doing it increases the
light-harvesting capacity of PSI by ~80%. Following on from this,
it was discovered that a similar lightharvesting antenna ring is
present in a low light adapted strain SS120 of P. marinus (17).
In this case, the 18 subunits were Pcb proteins containing both
Chl a and Chl b, and the presence of this antenna ring did not depend
on iron deficiency.
In light of this finding, we set out to investigate whether the
closely related organism P. didemni contained a similar Pcb–PSI
supercomplex and to establish whether the 18 subunit antenna ring
of PSI was a common feature of Chl b containing prochlorophytes.
In so doing we have found that for Prochloron, isolated from
its native environment, Chl a/b binding Pcb protein subunits associate
with PSII rather than PSI. Here we report the structural analysis
of this association.
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