Mutagenesis of histidine-469 in the photosystem II chlorophyll a-binding protein CP47 in Synechocystis sp. PCC 6803

Abstract

Histidine residues in membrane-spanning domains of CP47 represent candidates as axial ligands for chlorophyll binding. However, a recent model suggests that, in helix VI, His-455, His-466 and His-469 are located at 6.97, 6.47 and 7.45 Å, respectively, from their adjacent chlorophylls and are therefore unlikely to act as ligands (Barber, J., Morris, C. and Büchel, C. (2000) Biochim. Biophys. Acta 1459:239-247). Examples are known where Glu has successfully substituted for His residues as an axial ligand and the strains H455Q, H466Q and H469Q have been created. Whereas the phenotypes of H465Q and H466Q were similar to wild type the mutant H469Q showed a five-fold reduction in the number of PSII centers assembled and a reduction in the corresponding rate of photoautotrophic growth (Eaton-Rye, J.J. and Vermaas, W.F.J. (1992) Research in Photosynthesis, Vol. I:239-242). A spontaneous revertant was obtained that restored photoautotrophic growth and PSII centers in H469Q to near wild-type levels. Sequencing confirmed that the change responsible was not in psbB, the gene encoding CP47, and therefore the revertant contained an intergenic suppressor mutation. A psbB deletion mutant strain was created in the suppressor mutation background and the strains H469K, H469P and H469Y were created in this, and the wild-type background. In the wild-type background H469K, H469P and H469Y were not photoautotrophic, however, PSII assembly and photoautotrophic growth were restored in the background of the suppressor mutation. Since Lys and Pro are unlikely to serve as axial ligands this result supports the interpretation that His-469 is not a chlorophyll ligand.