The BtpA Protein Stabilizes the Reaction Center Proteins of Photosystem I in the Cyanobacterium Synechocystis sp. PCC 6803 at Low Temperature

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Abstract
Specific inhibition of photosystem I (PSI) was observed under low-temperature conditions in the cyanobacteriumSynechocystis sp. strain PCC 6803. Growth at 20°C caused inhibition of PSI activity and increased degradation of the PSI reaction center proteins PsaA and PsaB, while no significant changes were found in the level and activity of photosystem II (PSII). BtpA, a recently identified extrinsic thylakoid membrane protein, was found to be a necessary regulatory factor for stabilization of the PsaA and PsaB proteins under such low-temperature conditions. At normal growth temperature (30°C), the BtpA protein was present in the cell, and its genetic deletion caused an increase in the degradation of the PSI reaction center proteins. However, growth ofSynechocystis cells at 20°C or shifting of cultures grown at 30°C to 20°C led to a rapid accumulation of the BtpA protein, presumably to stabilize the PSI complex, by lowering the rates of degradation of the PsaA and PsaB proteins. A btpAdeletion mutant strain could not grow photoautotrophically at low temperature, and exhibited rapid degradation of the PSI complex after transfer of the cells from normal to low temperature.