Abstract:
The cyanobacterium Synechococcus sp. PCC 7942 (Anacystis
nidulans R2) adjusts its photosynthetic function by changing one
of the polypeptides of photosystem II. This polypeptide, called
Dl, is found in two forms in Synechococcus sp. PCC 7942.
Changing the growth light conditions by increasing the light
intensity to higher levels results in replacement of the original
form of D 1 polypeptide, D 1: 1, with another form, D 1 :2. We
investigated the role of these two polypeptides in two mutant
strains, R2S2C3 (only Dl:l present) and R2Kl (only Dl:2 present)
In cells with either high or low PSI/PSII.
R2S2C3 cells had a lower amplitude for 77 K fluorescence
emission at 695 nm than R2Kl cells. Picosecond fluorescence
decay kinetics showed that R2S2C3 cells had shorter lifetimes
than R2Kl cells.
The lower yields and shorter lifetimes observed in the D 1 and Dl:2 containing cells.
containing cells suggest that the presence of D 1: 1 results in more
photochemical or non-photochemical quenching of excitation
energy In PSII. One of the most likely mechanisms for the
increased quenching in R2S2C3 cells could be an increased
efficiency in the transfer of excitation energy from PSII to PSI.
However, photophysical studies including 77 K fluorescence
measurements and picosecond time resolved decay kinetics
comparing low and high PSI/PSII cells did not support the
hypothesis that D 1: 1 facilitates the dissipation of excess energy by
energy transfer from PSII to PSI.
In addition physiological studies of oxygen evolution
measurements after photoinhibition treatments showed that the two mutant cells had no difference in their susceptibility to
photoinhibition with either high PSI/PSII ratio or low PSI/PSII
ratio. Again suggesting that, the energy transfer efficiency from
PSII to PSI is likely not a factor in the differences between Dl:l
and Dl:2 containing cells.
