Plant Physiol, September 2000, Vol. 124, pp. 441-450

Acclimation of the Photosynthetic Machinery to High Temperature in Chlamydomonas reinhardtii Requires Synthesis de Novo of Proteins Encoded by the Nuclear and Chloroplast Genomes¹

Department of Regulation Biology, National Institute for Basic Biology, and Department of Molecular Biomechanics, The Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan

The mechanism responsible for the enhancement of the thermal stability of the oxygen-evolving machinery of photosystem II during acclimation of Chlamydomonas reinhardtii to high temperatures such as 35°C remains unknown. When cells that had been grown at 20°C were transferred to 35°C, the thermal stability of the oxygen-evolving machinery increased and within 8 h it was equivalent to that in cells grown initially at 35°C. Such enhancement of thermal stability was prevented by cycloheximide and by lincomycin, suggesting that the synthesis de novo of proteins encoded by both the nuclear and the chloroplast genome was required for this process. No increase in thermal stability was observed when cells that had been grown at 35°C were exposed to heat shock at 41°C, optimum conditions for the induction of the synthesis of homologs of three heat shock proteins (Hsps), namely, Hsp60, Hsp70, and Hsp22. Moreover, no synthesis of these homologs of Hsps was induced at 35°C. Thus it appears likely that Hsps are not involved in the enhancement of the thermal stability of the oxygen-evolving machinery.