The BtpA protein stabilizes the reaction center proteins of photosystem I in the cyanobacterium Synechocystis sp PCC 6803 at low temperature

Specific inhibition of photosystem I (PSI) was observed under low-temperatu re conditions in the cyanobacterium Synechocystis sp. strain PCC 6803. Grow th at 20 degrees C caused inhibition of PSI activity and increased degradat ion of the PSI reaction center proteins PsaA and PsaB, while no significant changes were found in the level and activity of photosystem II (PSII). Btp A, a recently identified extrinsic thylakoid membrane protein, was found to be a necessary regulatory factor for stabilization of the PsaA and PsaB pr oteins under such low-temperature conditions. At normal growth temperature (30 degrees C), the BtpA protein was present in the cell, and its genetic d eletion caused an increase in the degradation of the PSI reaction center pr oteins. However, growth of Synechocystis cells at 20 degrees C or shifting of cultures grown at 30 degrees C to 20 degrees C led to a rapid accumulati on of the BtpA protein, presumably to stabilize the PSI complex, by lowerin g the rates of degradation of the PsaA and PsaB proteins. A btpA deletion m utant strain could not grow photoautotrophically at low temperature, and ex hibited rapid degradation of the PSI complex after transfer of the cells fr om normal to low temperature.