A functional compartmental model of the Synechocystis PCC 6803 phycobilisome

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).Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-017-0424-5) contains supplementary material, which is available to authorized users.

The phycobilisome terminal emitter transfers its energy with a rate of (20 ps)^-1 to photosystem II

“…These observations can further be tested with the help of a target analysis. Assuming that PB is the same in vivo and in vitro (Tian et al 2011(Tian et al , 2012, we adopt the functional compartmental model for the PB from van Stokkum et al (2017). It is based upon the schematic structure depicted in Fig.…”

“…where the populations are determined by an unknown compartmental model, that depends upon the unknown kinetic parameters θ. In the target analysis constraints on the SAS are needed to estimate all parameters θ and SASl(λ) (van Stokkum et al 2004, Snellenburg et al 2013.…”

“…Together these pigments absorb light between 400 and 650 nm, and excitations of the antenna pigments are efficiently transferred to the chlorophyllcontaining PSI and PSII (Gillbro et al 1985, Sandström et al 1988, Tian et al 2011, 2013bScott et al 2006, Liu et al 2013. These photosystems convert the excitations to chemical energy via initial charge separation (van Grondelle et al 1994), and the combined action of PBs and photosystems (the light reactions of photosyn-thesis) provides the energy input to the cell (Govindjee et al 2017). Until now, the rate at which the phycobilisome transfers its energy to PSII is not known.…”

“…PCC 6803 (Shen et al 1993). With the help of target analysis (Holzwarth 1996, van Stokkum et al 2004 we estimated the unknown rate of energy transfer from PB to PSII. Fig.…”

“…2 depicts the different pigments present in a PB-PSII complex. Recently, a functional compartmental model of the PB has been developed (van Stokkum et al 2017). A compartment contains pigments with the same light-harvesting function that are considered equivalent.…”