The effect of high temperature stress on the expression of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase was examined in wheat (Triticum aestivum L.) leaves, which normally possess 46- and 42-kDa activase forms. Heat stress at 38 degrees C significantly reduced total activase mRNA levels compared to controls, and recovery of activase transcription was only marginal 24 h after alleviating heat stress. In contrast to transcript abundance, immunoblot analysis indicated that heat stress increased the accumulation of the 42-kDa activase and induced a putative 41-kDa form. Heat stress did not affect the amounts of the 46- and 42-kDa activase forms (present as 51- and 45-kDa preproteins) recovered after their immunoprecipitation from in vitro translation products. De novo protein synthesis in vivo in the presence of [35S]Met/Cys showed an increase in the amount of newly synthesized 42-kDa subunit after 4 h of heat stress, and synthesis of the putative 41-kDa activase was apparent. In contrast to activase, heat stress led to a rapid and large reduction in the de novo synthesis of the large and small subunits of Rubisco. Long-term (48-h) heat stress further increased the amounts of de novo synthesized 42- and 41-kDa activase forms. After 24 h of recovery from heat stress, de novo synthesis of the 42-kDa activase returned to control levels, while a small amount of 41-kDa protein was still expressed. Southern analysis suggested the presence of a single activase gene. These results indicate that heat stress alters activase expression, most likely posttranscriptionally, and suggest that the heat-induced expression of the 42- and 41-kDa subunits of wheat leaf Rubisco activase may be related to the maintenance and acclimation of photosynthetic CO2 fixation during high temperature stress in wheat.