Involvement of the HtrA family of proteases in the protection of the cyanobacterium Synechocystis PCC 6803 from light stress and in the repair of photosystem II

Photosystem II (PSII) is prone to irreversible light-induced damage, with the D1 polypeptide a major target. Repair processes operate in the cell to replace a damaged D1 subunit within the complex with a newly synthesized copy. As yet, the molecular details of PSII repair are relatively obscure despite the critical importance of this process for maintaining PSII activity and cell viability. We are using the cyanobacterium Synechocystis sp. PCC 6803 to identify the various proteases and chaperones involved in D1 turnover in vivo. Two families of proteases are being studied: the FtsH family (four members) of Zn²⁺-activated nucleotide-dependent proteases; and the HtrA (or DegP) family (three members) of serine-type proteases. In this paper, we report the results of our studies on a triple mutant in which all three copies of the htrA gene family have been inactivated. Growth of the mutant on agar plates was inhibited at high light intensities, especially in the presence of glucose. Oxygen evolution measurements indicated that, under conditions of high light, the rate of synthesis of functional PSII was less in the mutant than in the wild-type. Immunoblotting experiments conducted on cells blocked in protein synthesis further indicated that degradation of D1 was slowed in the mutant. Overall, our observations indicate that the HtrA family of proteases are involved in the resistance of Synechocystis 6803 to light stress and play a part, either directly or indirectly, in the repair of PSII in vivo.