DELETION OF THE GENE ENCODING THE PHOTOSYSTEM-II 33-KDA PROTEIN FROM SYNECHOCYSTIS SP PCC-6803 DOES NOT INACTIVATE WATER-SPLITTING BUT INCREASES VULNERABILITY TO PHOTOINHIBITION

Photosynthetic oxygen evolution is mediated by a Mn cluster at a site on the lumenal side of Photosystem II (PS II). The psbO gene product, the so-called extrinsic 33 kDa protein appears closely associated with this site, although its precise role remains unclear. We have used targeted mutagenesis to investigate whether the psbO protein is essential for PS II activity in the cyanobacterium Synechocystis sp. PCC 6803. The psbO gene was isolated and the locus was shown to be present as a single copy in the genome by low-stringency Southern hybridisations. Northern analysis detected a major psbO monocistronic transcript of 0.95-1.0 kb. Insertion and deletion psbO - mutant strains (named IC1 and IC2, respectively) were engineered and verified biochemically. The lesions in psbO have been confirmed by readdition of the psbO gene into both mutants using in situ complementation. This technique further indicated that the mutants may not be phenotypically identical. However, both mutants can grow photoautotrophically and exhibit appreciable rates of oxygen evolution. We therefore confirm independently the recently published results of Burnap and Sherman (Burnap, R.L. and Sherman, L.A. (1991) Biochemistry 30, 440-446). We also show that the mutated organism is highly susceptible to photoinhibition. This observation indicates that although the psbO protein is not absolutely required for water-splitting in Synechocystis sp. PCC 6803 it does play an important role in protecting against donor side photoinhibition.