Plant Physiol, November 2002, Vol. 130, pp. 1201-1212
Regulation of the Distribution of Chlorophyll and
Phycobilin-Absorbed Excitation Energy in Cyanobacteria. A
Structure-Based Model for the Light State
Transition1
Michael D.
McConnell,
Randy
Koop,
Sergej
Vasil'ev, and
Doug
Bruce*
Department of Biological Sciences, Brock University, St.
Catharines, Ontario, Canada L2S 3A1
The light state transition regulates the distribution of
absorbed excitation energy between the two photosystems (PSs) of photosynthesis under varying environmental conditions and/or metabolic demands. In cyanobacteria, there is evidence for the redistribution of
energy absorbed by both chlorophyll (Chl) and by phycobilin pigments,
and proposed mechanisms differ in the relative involvement of the two
pigment types. We assayed changes in the distribution of excitation
energy with 77K fluorescence emission spectroscopy determined
for excitation of Chl and phycobilin pigments, in both wild-type and
state transition-impaired mutant strains of
Synechococcus sp. PCC 7002 and
Synechocystis sp. PCC 6803. Action spectra for the
redistribution of both Chl and phycobilin pigments were very similar in
both wild-type cyanobacteria. Both state transition-impaired mutants
showed no redistribution of phycobilin-absorbed excitation energy, but
retained changes in Chl-absorbed excitation. Action spectra for the
Chl-absorbed changes in excitation in the two mutants were similar to
each other and to those observed in the two wild types. Our data show
that the redistribution of excitation energy absorbed by Chl is
independent of the redistribution of excitation energy absorbed by
phycobilin pigments and that both changes are triggered by the same
environmental light conditions. We present a model for the state
transition in cyanobacteria based on the x-ray structures of PSII, PSI,
and allophycocyanin consistent with these results.
1
This work was supported by the Natural and
Scientific Engineering Research Council of Canada.
*
Corresponding author; e-mail dbruce{at}spartan.ac.brocku.ca; fax
905-688-1855.
© 2002 American Society of Plant Biologists
