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Updated: Apr 15, 2026

Purification of Active Photosystem I-Light Harvesting Complex I from Plant Tissues
Published on: February 3, 2023
Multiple copies of the PsbQ protein in a cyanobacterial photosystem II assembly intermediate complex
Haijun Liu1, Daniel A Weisz1,2, Himadri B Pakrasi3
1Department of Biology, CB1137, Washington University, 1 Brookings Drive, St. Louis, MO, 63130, USA.
Abstract:
Photosystem II (PSII) undergoes frequent damage owing to the demanding electron transfer chemistry it performs. To sustain photosynthetic activity, damaged PSII undergoes a complex repair cycle consisting of many transient intermediate complexes. By purifying PSII from the cyanobacterium Synechocystis sp. PCC 6803 using a histidine-tag on the PsbQ protein, a lumenal extrinsic subunit, a novel PSII assembly intermediate was isolated in addition to the mature PSII complex. This new complex, which we refer to as PSII-Q4, contained four copies of the PsbQ protein per PSII monomer, instead of the expected one copy. In addition, PSII-Q4 lacked two other lumenal extrinsic proteins, PsbU and PsbV, which are present in the mature PSII complex. We suggest that PSII-Q4 is a late PSII assembly intermediate that is formed just before the binding of PsbU and PsbV, and we incorporate these results into an updated model of PSII assembly.
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