Photosynthetic electron transport in genetically altered photosystem II reaction centers of chloroplasts.

Abstract

Using a cotransformation system to identify chloroplast transformants in Chlamydomonas reinhardt&, we converted histidine-195 of the photosystem H reaction center D1 protein to a tyrosine residue. The mutants were characterized by a reduced quantum efficiency for photosynthetic oxygen evolution, which varied in a pH-dependent manner, a reduced capacity to oxidize artificial donors to photosystem H, and P680) reduction kinetics (microsecond) that were essentially similar to wild type. In addition, a dark-stable radical was detected by ESR in mutant photosystem II particles but not in wild-type particles. This radical was similar in g value and lineshape to chlorophyll or carotenoid cations but could have arisen from a tyrosine-195 cation. The ability of the photosystem II trap (P680) to oxidize tyrosine residues suggests that the mutant tyrosine residue could be used as a redox-sensitive probe to investigate the environment around the photosystem H trap.It is generally accepted that the photosystem II (PSII) reaction center of chloroplasts is structurally and functionally similar to the bacterial photosynthetic reaction center (1-5). Both reaction center types have two core polypeptides that coordinate the primary electron donor, presumably a chlorophyll (Chl) special pair, and the primary electron acceptors, pheophytin and the plastoquinone Qa. These polypeptides, the L and M subunit of the bacterial reaction center and the D1 and D2 subunits of PSII, have significant levels of sequence identity (25%) (6). In addition, the D1 and D2 polypeptides have the same number and orientations of transmembrane domains as the L and M subunits (3). These and other similarities suggest that the protein domains involved in the coordination of the PSII trap and binding of electron acceptors are structurally similar to those of the L and M polypeptides of the bacterial reaction center (1-3). The bacterial and PSII reaction centers, however, differ with respect to the organization of secondary electron donors and the redox potential of the reaction center traps. The Chl special pair of the bacterial reaction center is reduced by a cytochrome, whereas the PSII trap, P680', is reduced by a peptidyl tyrosine residue that in turn is reduced by electrons derived from the oxidation ofwater. Since the redox potential of a Chl dimer in solution is not sufficiently positive to drive the oxidation of water, it has been generally assumed that the redox potential of the PSII trap is determined by interactions between the chromophores and proteins. Modified Chl molecules containing point charges or the presence of charged residues near Chl may alter spectral properties or the redox potential of Chl (7-9). A survey of the amino acid sequences of the PSII core polypeptides D1 and D2 was conducted to identify conserved and potentially charged residues that could influence the electronic properties of the PSII trap. We identified a potentially ionizable residue, histidine-195 (H195) of the D1 protein, which is projected to be adjacent to ...

The Deletion of petG in Chlamydomonas reinhardtii Disrupts the Cytochrome bf Complex

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