PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

The requirement of carotenoids for the stable assembly of the photosynthetic complexes. Kulsam Ali¹, Michael Evans¹, Saul Purton^*,1, ¹ Department of Biology, London, United Kingdom

ABSTRACT- Carotenoids are found within all three electron transfer complexes of oxygenic phototrophs and in the associated light-harvesting complexes (LHCs) of plants and green algae. The recent structural analysis of photosystem I (PSI), photosystem II (PSII) and the cytochrome b₆f complex has revealed that each contains a number of â-carotene molecules: namely 22 in PSI, approximately 9 in PSII and a single molecule in the b₆f complex. It is well established that carotenoids play an important role in light harvesting and photoprotection. What is less certain is whether carotenoids are required for complex assembly and stability, and whether they play a direct role in electron transfer. To address this we examined a mutant (FN68) of the green alga Chlamydomonas that is completely devoid of carotenoids, probably because of a lesion at the level of phytoene synthase. Green gel analysis reveals that the major LHCs are absent in the mutant but that the PSI core accumulates to near wild-type levels. This was confirmed by western analysis which showed that the major PSI subunits are assembled into a stable complex. Of all the subunits examined, the only one that fails to assemble is the peripheral subunit, PsaN, which is normally bound to the lumenal face of PSI by electrostatic interactions. Analysis of PSI from FN68 by EPR spectroscopy revealed that the electron transfer properties are not grossly affected by the absence of the carotenoids, and light-induced electron transfer from P700 to the terminal acceptors is similar to that observed in wild-type PSI. In contrast to the PSI situation, PSII appears to be completely destabilized in FN68 such that the D1 protein is undetectable and CP43 accumulates only to a low level. No EPR signal arising from the PSII tyrosine radical Y_D. is detected in the mutant cells. Surprisingly, the loss of the single â-carotene from the b₆f complex also appears to result in the complete destabilization of this complex. These findings suggest that carotenoids are required for the stable accumulation of the LHCs, PSII and the b₆f complex, but not PSI.

KEY WORDS: photosystem 1, carotenoids