Heat shock response in Chlorella protothecoides

Chlorella cells grown under different nutritional conditions were transferred from the normal growth temperature of 25°C to elevated temperatures (30°C and 35°C), and the effect of heat shock (HS) on their protein synthesis was studied. The HS treatment caused a decrease in the incorporation of amino acids into some of the algal proteins. Unlike the above group of proteins, synthesis of another set of proteins with molecular masses 96, 92, 80, 74, 70, 68, 66, 44, and 19kDa were induced by HS. Interestingly, both the induction of HS proteins and recovery of the proteins which were inhibited by HS were very fast under appropriate conditions. Results also show that the photosynthetic activity was unaffected at 35°C for a period of 1 h. Furthermore, the synthesis of RuBP carboxylase was only marginally affected during HS at 35°C. Experiments using a transcription inhibitor and inhibitors of chloroplast and cytoplasmic protein synthesis showed that chloroplast protein synthesis was not significantly sensitive to HS treatment. However, cytoplasmic protein synthesis was severely affected. Actinomycin-D did not affect the induction of HS proteins. Hence HS response in Chlorella seems to be regulated to a large extent at the level of translation, rather than at the level of transcription. Furthermore, Chlorella HS proteins are the conserved proteins since proteins of similar molecular mass are induced in other organisms also during HS treatment