PARENT SESSION
Posters P1B Photo-oxidative stress, photoinhibition. Abstracts (394-443)

Analysis of electron transfer and PSII repair in cytochrome b₅₅₉ mutants of Chlamydomonas reinhardtii. E. Franco^*,1, M. Hamilton¹, Z. Deak², I. Vass², P. Nixon¹, ¹ Department of Biological Sciences, Imperial College London, ., London, UK²

ABSTRACT- Cytochrome b-559 (Cyt b-559) is an essential part of photosystem II (PSII), the light driven water-plastoquinone oxidoreductase found within the thylakoid membrane of oxygenic photosynthetic organisms. Cyt b-559 is a heterodimer composed of the a and b subunits, of 9 kDa and 6 kDa respectively. The haem group is ligated by single His residues in each subunit. The function of Cyt b-559 is still not clear although a number of possible functions have been proposed for Cyt b-559 including a role in the photoprotection of PSII and as a plastoquinol oxidase. To probe the function of Cyt b-559 we have generated a H23C mutant of the a subunit of Cyt b-559 in a His-tagged PsbH strain of C. reinhardtii. The H23C mutant assembles PSII at 15-20% of wild type levels, evolves oxygen but is incapable of photoautotrophic growth. PSII supercomplexes isolated from this strain show WT levels of oxygen evolution but contain no detectable haem, similar to other mutants constructed at this residue (Morais et al., 2001). Thus the presence of the haem is not critical for photosynthetic oxygen evolution or for stabilisation of the supercomplex. Chlorophyll fluorescence and thermoluminescence techniques have been used to analyse electron transfer in the H23C mutant in vivo. The data suggest that the redox properties of plastoquinone QB, but not QA, are altered in the H23C mutant, consistent with a role for cytochrome b-559 in stabilising QB-. PSII activity in the H23C mutant is more vulnerable to light damage than the WT. In contrast to the WT, the recovery of PSII activity from the mutant strain is strongly impaired after light damage. Immunoblotting and pulse-labelling experiments suggest that the major cause of photoinhibition is an impaired PSII repair cycle at the level of assembly of the PSII complex.

KEY WORDS: C. reinhardtii H23C mutant , Assembly of the PSII complex., Cytochrome b-559 , PSII activity