Abstract:
Cyanobacteria are able to regulate the distribution of absorbed light energy between
photo systems 1 and 2 in response to light conditions. The mechanism of this regulation (the state
transition) was investigated in the marine cyanobacterium Synechococcus sp. strain PCC 7002.
Three cell types were used: the wild type, psaL mutant (deletion of a photo system 1 subunit
thought to be involved in photo system 1 trimerization) and the apcD mutant (a deletion of a
phycobilisome subunit thought to be responsible for energy transfer to photo system 1).
Evidence from 77K fluorescence emission spectroscopy, room temperature fluorescence
and absorption cross-section measurements were used to determine a model of energy
distribution from the phycobilisome and chlorophyll antennas in state 1 and state 2. The data
confirm that in state 1 the phycobilisome is primarily attached to PS2. In state 2, a portion of the
phycobilisome absorbed light energy is redistributed to photo system 1. This energy is directly
transferred to photo system 1 by one of the phycobilisome terminal emitters, the product of the
apcD gene, rather than via the photo system 2 chlorophyll antenna by spillover (energy transfer
between the photo system 2 and photo system 1 chlorophyll antenna). The data also show that
energy absorbed by the photo system 2 chlorophyll antenna is redistributed to photo system 1 in
state 2. This could occur in one of two ways; by spillover or in a way analogous to higher plants
where a segment of the chlorophyll antenna is dissociated from photo system 2 and becomes part
of the photo system 1 antenna.
The presence of energy transfer between neighbouring photo system 2 antennae was
determined at both the phycobilisome and chlorophyll level, in states 1 and 2. Increases in
antenna absorption cross-section with increasing reaction center closure showed that there is
energy transfer (connectivity) between photosystem 2 antennas. No significant difference was
shown in the amount of connectivity under these four conditions.
