Photosystem II in a mutant of Chlamydomonas reinhardtii lacking the 23 kDa psbP protein shows increased sensitivity to photoinhibition in the absence of chloride

Abstract

The psbP gene product, the so called 23 kDa extrinsic protein, is involved in water oxidation carried out by Photosystem II. However, the protein is not absolutely required for water oxidation. Here we have studied Photosystem II mediated electron transfer in a mutant of Chlamydomonas reinhardtii, the FUD 39 mutant, that lacks the psbP protein. When grown in dim light the Photosystem II content in thylakoid membranes of FUD 39 is approximately similar to that in the wild-type. The oxygen evolution is dependent on the presence of chloride as a cofactor, which activates the water oxidation with a dissociation constant of about 4 mM. In the mutant, the oxygen evolution is very sensitive to photoinhibition when assayed at low chloride concentrations while chloride protects against photoinhibition with a dissociation constant of about 5 mM. The photoinhibition is irreversible as oxygen evolution cannot be restored by the addition of chloride to inhibited samples. In addition the inhibition seems to be targeted primarily to the Mn-cluster in Photosystem II as the electron transfer through the remaining part of Photosystem II is photoinhibited with slower kinetics. Thus, this mutant provides an experimental system in which effects of photoinhibition induced by lesions at the donor side of Photosystem II can be studied in vivo.

Lack of the Small Plastid-encoded PsbJ Polypeptide Results in a Defective Water-splitting Apparatus of Photosystem II, Reduced Photosystem I Levels, and Hypersensitivity to Light

“…This is in line with the earlier finding that a mutant of Chlamydomonas (FUD 39), which lacks the extrinsic 23-kDa protein of PSII (PsbP; Ref. 43), shows chloride ion deficiency in PSII and is highly susceptible to photodamage (37,45).…”

Destruction of a single chlorophyll is correlated with the photoinhibition of photosystem II with a transiently inactive donor side.

“…Both samples generated a similar amount of long-living cations, indicating that the primary charge separation from P680 to the primary quinone is-still intact after partial inhibition of the watersplitting activity. The loss ofwater-splitting activity is therefore due to an irreversible damage of the donor side in agreement with earlier results (6,7,(10)(11)(12)(13)(14).…”

“…In case of a transient malfunction of the water-splitting reaction, P680+ has an extended lifetime during which it can degrade (4) or damage other PSII components, including carotenoids, chlorophylls (Chl), possibly Z, the manganese binding sites, and other amino acids of the PSII proteins (5)(6)(7)(8)(9)(10). One or several of these damages result in an irreversible inhibition of the electron transfer from Z to P680+ and hence in an irreversible loss of the water-splitting activity (6,7,(10)(11)(12)(13)(14). Despite many investigations it is still not clear which of the various damages actually causes this inhibition.…”

PsbP Protein, But Not PsbQ Protein, Is Essential for the Regulation and Stabilization of Photosystem II in Higher Plants

“…In previous studies, a Chlamydomonas reinhardtii mutant lacking PsbP showed decreased oxygen-evolving activity (Mayfield et al, 1987), and an increased concentration of Cl 2 was required to restore oxygen evolution in isolated thylakoid membranes (Rova et al, 1994). However, the in vivo photosynthetic properties of the mutant cells were not well characterized in these studies.…”