The ATP-dependent Clp protease is essential for acclimation to UV-B and low temperature in the cyanobacterium Synechococcus.

ClpP is the proteolytic subunit of the ATP-dependent Clp protease in eubacteria, mammals and plant chloroplasts. Cyanobacterial ClpP protein is encoded by a multigene family, producing up to four distinct isozymes. We have examined the importance of the first ClpP protein (ClpP1) isolated from the cyanobacterium Synechococcus sp. PCC 7942 for acclimation to ecologically relevant UV-B and low-temperature regimens. When the growth light of 50 mumol photons m-2 s-1 was supplemented with 0.5 W m-2 UV-B for 8 h, the constitutive level of ClpP1 rose eightfold after an initial lag of 1 h. Wild-type cells readily acclimated to this UV-B level, recovering after the initial stress to almost the same growth rate as that before UV-B exposure. Growth of a clpP1 null mutant (delta clpP1), however, was severely inhibited by UV-B, being eight times slower than the wild type after 8 h. In comparison, ClpP1 content increased 15-fold in wild-type cultures shifted from 37 degree C to 25 degree C for 24 h. Wild-type cultures readily acclimated to 25 degree C after 24 h, whereas the delta clpP1 strain did not and eventually lost viability with prolonged cold treatment. During acclimation to either UV-B or cold, photosynthesis in the wild type was initially inhibited upon the shift but then recovered. Photosynthesis in delta clpP1 cultures, however, was more severely inhibited by the stress treatment and failed to recover. Acclimation was also monitored by examining the exchange of photosystem II reaction centre D1 proteins that occurs in wild-type Synechococcus during conditions of excitation stress. During both cold and UV-B shifts, wild-type cultures replaced the acclimative form of D1 (D1:1) with the alternative D1 form 2 (D1:2) within the first hours. Once acclimated to either 25 degree C or 0.5 W m-2 UV-B, D1:2 was exchanged back for D1:1. In delta clpP1 cultures, this second exchange between D1 forms did not occur, with D1:2 remaining the predominant D1 form. Our results demonstrate that the ATP-dependent Clp protease is an essential component of the cold and UV-B acclimation processes of Synechococcus.