Effect of light intensity and various organic acids on the growth of Rhodobacter sphaeroides LHII-deficient mutant in a turbidostat culture.

The composition of photosynthetic apparatus of Rhodobacter sphaeroides wild strain 2.4.1 and its LHII-deficient mutant DBCΩ was compared. The absence of LHII in the mutant was confirmed by comparison of chromatophores spectra and by the absence of electrophoretic band corresponding to LHII complex. Continuous turbidostat cultures of wild strain and its LHII-deficient mutant were compared in response to different light intensities. Cultures were grown using lactate, mixture of lactate and acetate or succinate as carbon source. For comparative analysis, an approximation of experimental data by Monod and Gompertz equations were used. Cultures of DBCΩ had lower growth rates than wild strain when grown on lactate as electron donor and carbon source. Cultures of both strains grown on lactate and acetate or on succinate had similar growth rates. The cultures showed maximum growth rates when grown with succinate. Bacteriochlorophyll a content increased in both strains with decrease of incident light intensity. However, the variation of Bchl a content in wild strain was much more significant. Under light-limiting conditions, bacteriochlorophyll a content in DBCΩ was 4-5 times lower than in the wild strain. Under light-saturating conditions, it was only 1.5-2.5 times lower. Growing with lactate or with lactate and acetate, the mutant switched from light limitation under low light intensities to limitation by organic acids under higher light, whereas the parental strain had similar switch of limiting factor only when growing with lactate and acetate mixture. DBCΩ mutant has higher minimal light intensity enabling growth on any organic acid as a substrate. When growing with lactate or with lactate and acetate, the mutant reached maximum growth rate at lower light intensities than the wild strain. This phenomenon was observed for the first time. Taking into account the concentration of BChl a under light-limiting conditions, the thickness of the suspension capable of effective light absorption could be increased by 4-5 times, which is favorable for intensive cultivation.

Comparison of the protective effectiveness of NPQ in Arabidopsis plants deficient in PsbS protein and zeaxanthin.

The efficiency of protective energy dissipation by non-photochemical quenching (NPQ) in photosystem II (PSII) has been recently quantified by a new non-invasive photochemical quenching parameter, qPd. PSII yield (ФPSII) was expressed in terms of NPQ, and the extent of damage to the reaction centres (RCIIs) was calculated via qPd as: ФPSII=qPd×(F v/F m)/{1+[1-(F v/F m)]×NPQ}. Here this approach was used to determine the amount of NPQ required to protect all PSII reaction centres (pNPQ) under a gradually increasing light intensity, in the zeaxanthin-deficient (npq1) Arabidopsis mutant, compared with PsbS protein-deficient (npq4) and wild-type plants. The relationship between maximum pNPQ and tolerated light intensity for all plant genotypes followed similar trends. These results suggest that under a gradually increasing light intensity, where pNPQ is allowed to develop, it is only the amplitude of pNPQ which is the determining factor for protection. However, the use of a sudden constant high light exposure routine revealed that the presence of PsbS, not zeaxanthin, offered better protection for PSII. This was attributed to a slower development of pNPQ in plants lacking PsbS in comparison with plants that lacked zeaxanthin. This research adds further support to the value of pNPQ and qPd as effective parameters for assessing NPQ effectiveness in different types of plants.

PsbS protein modulates non-photochemical chlorophyll fluorescence quenching in membranes depleted of photosystems.

Plants with varying levels of PsbS protein were grown on lincomycin. Enhanced levels of non-photochemical fluorescence quenching (NPQ) in over-expressers of the protein have been observed. This was accompanied by increased amplitude of the irreversible NPQ component, qI, previously considered to reflect mainly photoinhibition of PSII reaction centres (RCII). However, since RCIIs were largely absent the observed qI is likely to originate from the LHCII antenna. In chloroplasts of over-expressers of PsbS grown on lincomycin an abnormally large NPQ (∼7) was characterised by a 0.34 ns average chlorophyll fluorescence lifetime. Yet the lifetime in the Fm state was similar to that of wild-type plants. 77K fluorescence emission spectra revealed a specific 700 nm peak typical of LHCII aggregates as well as quenching of the PSI fluorescence at 730 nm. The aggregated state manifested itself as a clear change in the distance between LHCII complexes detected by freeze-fracture electron microscopy. Grana thylakoids in the quenched state revealed 3 times more aggregated LHCII particles compared to the dark-adapted state. Overall, the results directly demonstrate the importance of LHCII aggregation in the NPQ mechanism and show that the PSII supercomplex structure plays no role in formation of the observed quenching.

Psb28 protein is involved in the biogenesis of the photosystem II inner antenna CP47 (PsbB) in the cyanobacterium Synechocystis sp. PCC 6803.

The role of the Psb28 protein in the structure and function of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. The protein was localized in the membrane fraction and, whereas most of the protein was detected as an unassembled protein, a small portion was found in the PSII core complex lacking the CP43 antenna (RC47). The association of Psb28 with RC47 was further confirmed by preferential isolation of RC47 from the strain containing a histidine-tagged derivative of Psb28 using nickel-affinity chromatography. However, the affinity-purified fraction also contained a small amount of the unassembled PSII inner antenna CP47 bound to Psb28-histidine, indicating a structural relationship between Psb28 and CP47. A psb28 deletion mutant exhibited slower autotrophic growth than wild type, although the absence of Psb28 did not affect the functional properties of PSII. The mutant showed accelerated turnover of the D1 protein, faster PSII repair, and a decrease in the cellular content of PSI. Radioactive labeling revealed a limitation in the synthesis of both CP47 and the PSI subunits PsaA/PsaB in the absence of Psb28. The mutant cells contained a high level of magnesium protoporphyrin IX methylester, a decreased level of protochlorophyllide, and released large quantities of protoporphyrin IX into the medium, indicating inhibition of chlorophyll (Chl) biosynthesis at the cyclization step yielding the isocyclic ring E. Overall, our results show the importance of Psb28 for synthesis of Chls and/or apoproteins of Chl-binding proteins CP47 and PsaA/PsaB.

Lack of the light-harvesting complex CP24 affects the structure and function of the grana membranes of higher plant chloroplasts.

The photosystem II (PSII) light-harvesting antenna in higher plants contains a number of highly conserved gene products whose function is unknown. Arabidopsis thaliana plants depleted of one of these, the CP24 light-harvesting complex, have been analyzed. CP24-deficient plants showed a decrease in light-limited photosynthetic rate and growth, but the pigment and protein content of the thylakoid membranes were otherwise almost unchanged. However, there was a major change in the macroorganization of PSII within these membranes; electron microscopy and image analysis revealed the complete absence of the C(2)S(2)M(2) light-harvesting complex II (LHCII)/PSII supercomplex predominant in wild-type plants. Instead, only C(2)S(2) supercomplexes, which are deficient in the LHCIIb M-trimers, were found. Spectroscopic analysis confirmed the disruption of the wild-type macroorganization of PSII. It was found that the functions of the PSII antenna were disturbed: connectivity between PSII centers was reduced, and maximum photochemical yield was lowered; rapidly reversible nonphotochemical quenching was inhibited; and the state transitions were altered kinetically. CP24 is therefore an important factor in determining the structure and function of the PSII light-harvesting antenna, providing the linker for association of the M-trimer into the PSII complex, allowing a specific macroorganization that is necessary both for maximum quantum efficiency and for photoprotective dissipation of excess excitation energy.