PsaK2 Subunit in Photosystem I Is Involved in State Transition under High Light Condition in the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

To avoid the photodamage, cyanobacteria regulate the distribution of light energy absorbed by phycobilisome antenna either to photosystem II or to photosystem I (PSI) upon high light acclimation by the process so-called state transition. We found that an alternative PSI subunit, PsaK2 (sll0629 gene product), is involved in this process in the cyanobacterium Synechocystis sp. PCC 6803. An examination of the subunit composition of the purified PSI reaction center complexes revealed that PsaK2 subunit was absent in the PSI complexes under low light condition, but was incorporated into the complexes during acclimation to high light. The growth of the psaK2 mutant on solid medium was inhibited under high light condition. We determined the photosynthetic characteristics of the wild type strain and the two mutants, the psaK1 (ssr0390) mutant and the psaK2 mutant, using pulse amplitude modulation fluorometer. Non-photochemical quenching, which reflects the energy transfer from phycobilisome to PSI in cyanobacteria, was higher in high light grown cells than in low light grown cells, both in the wild type and the psaK1 mutant. However, this change of non-photochemical quenching during acclimation to high light was not observed in the psaK2 mutant. Thus, PsaK2 subunit is involved in the energy transfer from phycobilisome to PSI under high light condition. The role of PsaK2 in state transition under high light condition was also confirmed by chlorophyll fluorescence emission spectra determined at 77 K. The results suggest that PsaK2-dependent state transition is essential for the growth of this cyanobacterium under high light condition.The effective absorption of light energy is the first step in photosynthesis. All oxygenic photosynthetic organisms share common core antenna pigments of ϳ40 chlorophyll a in PSII 1 and ϳ100 chlorophyll a in PSI (1). Cyanobacteria have an additional light-harvesting system, phycobilisome, which is primarily associated with PSII (2, 3). Thus, cyanobacteria use two kinds of antenna pigments with totally different absorption wavelengths.Because neither light quality nor light quantity is constant in natural environments, cyanobacteria have to distribute the light energy absorbed by antenna pigments to two photosystems in order to optimize the photosynthetic performance in response to changing light environments. When cells are exposed to illumination favoring either PSII or PSI, the distribution of light energy between two photosystems would be adjusted (4). When cells have been pre-illuminated with light mainly absorbed by phycobiliproteins (PSII light), the energy transfer to PSI increases, whereas the energy transfer to PSII decreases (state 2). Pre-illumination of cells with light mainly absorbed by chlorophyll a (PSI light) causes a reverse effect leading to the increase of energy transfer to PSII and the decrease of energy transfer to PSI (state 1) (5). Distribution of light energy to two photosystems is regulated in response to the light regime not only in cyanobacteria but also in green ...

The Small Regulatory RNA SyR1/PsrR1 Controls Photosynthetic Functions in Cyanobacteria

“…Interestingly, both psaK1 and psaK2 (together with its adjacent gene sll0630) were among the most strongly affected transcripts in psrR1 + (Supplemental Table 1). These genes encode two alternative PSI subunits that are involved in state transitions and high-light adaptation of the PSI complex (Fujimori et al, 2005;Dühring et al, 2007). PsaK is incorporated as the final subunit during PSI assembly (Dühring et al, 2007), supporting the idea that PsrR1 causes a retardation in trimer accumulation upon changes in light intensity, by regulating the final steps of complex assembly.…”

The Membrane-Associated CpcG2-Phycobilisome in Synechocystis: A New Photosystem I Antenna

“…PsaK2, an alternative of PSI peripheral subunit, was proposed on the photosystem side in the state transition in Synechocystis. It was expressed and incorporated into the PSI complex instead of PsaK1 during acclimation to high light (Hihara et al, 2001b;Fujimori et al, 2005). Furthermore, the state transition induced under high-light conditions was found to be impaired in the psaK2 disruptant.…”

Effects of low temperature on tropical and temperate isolates of marine Synechococcus

“…To avoid over-reduction, photosynthetic organisms increase the transfer of energy to PSI or dissipate energy as heat (Huner et al, 1998). In freshwater cyanobacterium, Synechocystis, PsaK is involved in increased transfer of energy from PBS to PSI under high light conditions, which aid in avoiding photodamage (Fujimori et al, 2005). The upregulation of PsaK in response to low temperature in WH8102 may have a similar role in protecting PSII from overreduction.…”