Acclimation of the photosynthetic machinery to high temperature in Chlamydomonas reinhardtii requires synthesis de novo of proteins encoded by the nuclear and chloroplast genomes

The mechanism responsible for the enhancement of the thermal stability of t he oxygen-evolving machinery of photosystem II during acclimation of Chlamy domonas reinhardtii to high temperatures such as 35 degrees C remains unkno wn. When cells that had been grown at 20 degrees C were transferred to 35 d egrees 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 deg rees C. Such enhancement of thermal stability was prevented by cycloheximid e and by lincomycin, suggesting that the synthesis de novo of proteins enco ded by both the nuclear and the chloroplast genome was required for this pr ocess. No increase in thermal stability was observed when cells that had be en grown at 35 degrees C were exposed to heat shock at 41 degrees C, optimu m conditions for the induction of the synthesis of homologs of three heat s hock proteins (Hsps), namely, Hsp60, Hsp70, and Hsp22. Moreover, no synthes is of these homologs of Hsps was induced at 35 degrees C. Thus it appears l ikely that Hsps are not involved in the enhancement of the thermal stabilit y of the oxygen-evolving machinery.