Cosuppression of photosystem I subunit PSI-H in Arabidopsis thaliana. Efficient electron transfer and stability of photosystem I is dependent upon the PSI-H subunit.
PSI-H is an intrinsic membrane protein of 10 kDa that is a subunit of photosystem I (PSI). PSI-H is one of the three PSI subunits found only in eukaryotes. The function of PSI-H was characterized in Arabidopsis plants transformed with a psaH cDNA in sense orientation. Cosuppressed plants containing less than 3% PSI-H are smaller than wild type when grown on sterile media but are similar to wild type under optimal conditions. PSI complexes lacking PSI-H contain 50% PSI-L, whereas other PSI subunits accumulate in wild type amounts. PSI devoid of PSI-H has only 61% NADP+ photoreduction activity compared with wild type and is highly unstable in the presence of urea as determined from flash-induced absorbance changes at 834 nm. Our data show that PSI-H is required for stable accumulation of PSI and efficient electron transfer in the complex. The plants lacking PSI-H compensate for the less efficient PSI with a 15% increase in the P700/chlorophyll ratio, and this compensation is sufficient to prevent overreduction of the plastoquinone pool as evidenced by normal photochemical quenching of fluorescence. Nonphotochemical quenching is approximately 60% of the wild type value, suggesting that the proton gradient across the thylakoid membrane is decreased in the absence of PSI-H.
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Plant Biochemistry Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, 40 Thorvaldsensvej, DK-1871 Frederiksberg C, Copenhagen, Denmark.
Haldrup
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Scheller
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D017360:Arabidopsis / Q000254:growth & development / Q000378:metabolism*; D018076:DNA, Complementary; D018744:DNA, Plant; D004579:Electron Transport; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D045331:Photosystem I Protein Complex; D030821:Plants, Genetically Modified / Q000254:growth & development / Q000378:metabolism; D011485:Protein Binding
D018076:DNA, Complementary; D018744:DNA, Plant; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex