Construction and characterization of a functional mutant of Synechocystis 6803 harbouring a eukaryotic PSII-H subunit

A Synechocystis 6803 mutant carrying a chimaeric photosystem II (PSII), in which the Zen mays PsbH subunit (7.7 kDa calculated molecular mass) replace s the cyanobacterial copy (7.0 kDa), was constructed. With the exception of the N-terminal 12 amino acid extension, which has a phosphorylatable threo nine, the eukaryotic polypeptide is 78% homologous to its bacterial counter part. Biochemical characterization of this mutant shows that it expresses t he engineered gene correctly and is competent for photoautotrophic growth. Fluorescence analysis and oxygen evolution measurements in the presence of exogenous accepters indicate that the observed phenotype results from a chi maeric PSII rather than from the absence of function associated with PsbH, suggesting that the heterologous protein is assembled into a functional PSI I. Inhibition of oxygen evolution by herbicides belonging to different clas ses shows that the sensitivity of the mutant PSII is changed only towards p henolic compounds. This result indicates slight conformational modification of the Q(B)/herbicide binding pocket of the D1 polypeptide caused by the b ulky PsbH protein in the mutant, and also suggests close structural interac tion of the D1 and PsbH subunits in the topological arrangement of PSII.