Antenna System Of Photosystem Research Articles

Photosynthetic inner antenna CP47 plays important roles in ephemeral plants in adapting to high light stress

Throughout 3.5 billion years of evolution, photosynthesis of land plants has developed a complicated antenna system to cope with the ever-changing environments. The antenna system of photosystem (PS) II includes the outer antennae and inner antennae. The inner antennae CP43 and CP47, located in the closest peripheral of PSII reaction center (RC), play important roles in facilitating excitation energy transport from the outer antennae to the PSII RC. Although PSII RC is the last station of energy transport, it is the inner antenna CP47, not the RC, which possesses the lowest energy level in PSII. Berteroa incana (B. incana), which is a vascular plant grown in the Gobi region, can sustain very high photosynthesis capacity under very high light conditions. It has been discovered that the thylakoid membrane of B. incana possesses a unique low fluorescence emission spectrum because the fluorescence emission of CP47 (695 nm) is the main fluorescence emission peak of PSII. In this paper, the thylakoid membrane, isolated from B. incana grown under a light condition of 100 μM photons m−2 s-1 and subjected to high light treatment (1600 μM photons m-2 s-1 for 1.5 h or 3 h) was employed for the research. It has been found that the high fluorescence emission of CP47 decreased remarkably upon exposure to the high light treatment. Further observation revealed that the drastic changes in the CP47 fluorescence emission were accompanied by a slight reduction in the amount of CP47 and an enhancement of the CP29-LHCII-CP24 assembly. It is proposed that CP47 enables the functional switch between the excitation energy transfer towards PSII RC, and the overexcitation quenching in the PSII core. Together with CP43, CP47 plays important roles in regulating excitation energy distribution in PSII core complexes.

Polypeptide composition, assembly and phosphorylation patterns of the photosystem II antenna system of Chlamydomonas reinhardtii

In recent years major progress has been made in describing the gene families that encode the polypeptides of the light-harvesting antenna system of photosystem II (PSII). At the same time, advances in the biochemical characterization of these antennae have been hampered by the high degree of similarity between the apoproteins. To help interpret the molecular results, we have re-examined the composition, the assembly and the phosphorylation patterns of the light-harvesting antenna of PSII (LHCII) in the green alga Chlamydomonas reinhardtii Dang, using a non-Tris SDS-PAGE system capable of resolving polypeptides that differ by as little as 200 daltons. Research to date has suggested that in C. reinhardtii the LHCII comprises just four polypeptides (p11, p13, p16 and p17), and CP29 and CP26 just one polypeptide each (p9 and p10, respectively), i.e. a total of six polypeptides. We report here that these antenna systems contain at least 15 polypeptides, 10 associated with LHCII, 3 with CP29, and 2 with CP26. All of these polypeptides have been positively identified by means of appropriate antibodies. We also demonstrate substantial heterogeneity to the pattern of in-vitro phosphorylation, with major differences found among members of closely spaced and immunologically related polypeptides. Most intriguing is the fact that the polypeptides that cross-react with the anti-type 2 LHCII antibodies of higher plants (p16, and to a lesser extent p11) are not phosphorylated, whereas in higher plants these are the most highly phosphorylated polypeptides. Also, unlike in higher plants, CP29 is heavily phosphorylated. Phosphorylation does not appear to have any effect on the mobility of polypeptides on fully denaturing SDS-PAGE gels. To learn more about the accumulation and organization of the light-harvesting polypeptides, we have also investigated a chlorophyll b-less mutant, cbn1-48. The LHCII is almost completely lost in this mutant, along with at least some LHCI. But the accumulation of CP29 and CP26 and their binding to PSII core complexes, is relatively unaffected. As expected, the loss of antenna polypeptides is accompanied by a reduction of the size of large reaction-center complexes. Following in-vitro phosphorylation the number of phosphorylated proteins is greatly increased in the mutant thylakoids compared to wildtype thylakoids. We present a model of the PSII antenna system to account for the new polypeptide complexity we have demonstrated.

Trapping and annihilation in the antenna system of Photosystem I

The primary charge separation in Photosystem I of pea chloroplasts was measured as a photovoltage in the pico- and nanosecond time range by applying laser flashes at 532 nm of variable energy and different duration (12 ns and 30 ps, respectively). Contributions to the photovoltage from Photosystem II was eliminated by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The dependence of the photovoltage amplitude on the excitation energy could be described by an exponential saturation law when the excitation flash had a duration of 12 ns. Nearly the same dependence was found when the excitation source was the train of a mode-locked laser (approx. ten 30-ps flashes spaced by 7 ns; highest energy of a single flash, 80 μJ / cm −2). Even with single 30-ps flashes the photovoltage was only slightly smaller than the one elicited by 12-ns flashes of the same energy. These findings demonstrate that trapping of excitation energy by the reaction center of Photosystem I is much more effective than losses by annihilation and other loss processes. The photovoltage yield was nearly independent of the fraction of closed traps, thus demonstrating that the absorption cross section of Photosystem I is not altered by the closing of its reaction centers. By recording the rise time of the photovoltage with our highest time resolution we found that the trapping rate of the excitation energy in Photosystem I depended on the energy of the 30-ps flashes: at low excitation energies (less than 10 14 photons / cm 2 per pulse) trapping occurred within 90 ± 15 ps and at high excitation energy (10 15 photons / cm 2 per pulse) trapping and charge stabilization occurred within the time resolution of the apparatus, i.e., up to 50 ps. The trapping rate at low energies is in agreement with the one determined by fluorescence decay kinetics. Up to 50 ns there was no further detectable electrogenic phase (neither forward nor backward reactions). This demonstrates that all the electrogenicity, produced by the charge separation, takes place in less than 50 ps.