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As a consequence of iron starvation, Synechocystis PCC 6803
expresses its isiA and isiB genes. Concomitant with this gene expression
is a drop in the level of PSI (33) and phycobiliproteins (5). We
have found that in addition to these well recognized responses to
iron limitation, Synechocystis forms a supercomplex composed of
a ring of 18 copies of the CP43' protein surrounding a PSI trimer.
The CP43'-PSI supercomplex was isolated by sucrose density centrifugation,
and size exclusion chromatography estimated its molecular mass to
be approximately 1900 kDa. This mass is consistent with that predicted
by the calculation for a PSI trimer (1068 kDa) plus 18 copies of
the CP43 protein (846 kDa).
Assuming that each CP43' subunit binds at least 12 chlorophylls
as does CP43 (29), the CP43' antenna ring of the PSI supercomplex
would contain 216 or more chlorophylls. It is for this reason that
the optical absorption spectrum of this supercomplex is significantly
different from that of the PSI trimer alone. The chlorophyll a molecules
bound within the CP43' protein have a long wavelength absorption
maximum at approximately 670 nm. Therefore, the long wavelength
absorption peak shifts from 680 nm for PSI to 673 nm for the CP43'-
PSI supercomplex. Some free CP43' in the supercomplex preparation
could also contribute to this blue shift, but fluorescence measurements
suggest that this contamination is notsignificant. When isolated,
the CP43' protein has a relatively high fluorescence yield at 77
K peaking at 685 nm. Although some emission at this wavelength was
detected from the CP43'- PSI supercomplex, the PSI low temperature
fluorescence peaking at 720 nm was the dominating emission. Only
after the addition of 0.1% Triton X-100 to dissociate the CP43'
protein from the PSI trimer was a large fluorescence emission seen
at 685 nm from the supercomplex. Therefore, we conclude that the
chlorophylls within the CP43' ring are excitonically coupled to
those within the PSI trimer core. Given that the PSI trimer binds
almost 300 chlorophyll a molecules (32), we can conclude that the
additional 216 chlorophylls in the CP43' ring increases the light-harvesting
capacity of the PSI reaction centers within the supercomplex by
at least 70%. It has previously been suggested that CP43' could
act as an additional antenna of PSI (16).
The results presented here and elsewhere (17) clearly show that
in response to iron deprivation, Synechocystis induces an
additional antenna system for PSI. The processing of top views of
the CP43'-PSI supercomplex indicate that the 18 CP43' subunits do
not form a perfect ring because of the fact that the PSI trimer
is not circular. The three-dimensional model presented in Fig. 6b
was constructed using a number of top, intermediate, and side views
showing that the supercomplex has a diameter of approximately 330
Å and a thickness of ~80 Å in negative stain. Because the hydrophobic
surfaces of the supercomplex must have a detergent layer, the true
diameter is likely to be slightly less. The two-dimensional class
averages (Fig. 6a) and three-dimensional reconstruction (Fig. 6b)
reveal rather flat stromal and luminal surfaces that are not expected
for the stromal surface since the PSI trimer normally binds extrinsic
PsaC, PsaD, and PsaE proteins. However, we are confident that the
three-dimensional model is composed of characteristic stromal and
luminal views because differences can be observed in the internal
density distribution of the PSI monomers within different two-dimensional
class averages, indicative of different orientations on the carbon
grid (see Fig. 6a).
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