Abstract:
In the light-harvesting antenna of the Synechocystis
PCC 6803 phycobilisome (PB), the core consists
of three cylinders, each composed of four disks, whereas
each of the six rods consists of up to three hexamers (Arteni
et al., Biochim Biophys Acta 1787(4):272–279, 2009).
The rods and core contain phycocyanin and allophycocyanin
pigments, respectively. Together these pigments
absorb light between 400 and 650 nm. Time-resolved difference
absorption spectra from wild-type PB and rod
mutants have been measured in different quenching and
annihilation conditions. Based upon a global analysis of
these data and of published time-resolved emission spectra,
a functional compartmental model of the phycobilisome
is proposed. The model describes all experiments
with a common set of parameters. Three annihilation time
constants are estimated, 3, 25, and 147 ps, which represent,
respectively, intradisk, interdisk/intracylinder, and intercylinder
annihilation. The species-associated difference
absorption and emission spectra of two phycocyanin and two allophycocyanin pigments are consistently estimated,
as well as all the excitation energy transfer rates. Thus, the
wild-type PB containing 396 pigments can be described
by a functional compartmental model of 22 compartments.
When the interhexamer equilibration within a rod is not
taken into account, this can be further simplified to ten
compartments, which is the minimal model. In this model,
the slowest excitation energy transfer rates are between the
core cylinders (time constants 115–145 ps), and between
the rods and the core (time constants 68–115 ps).
