The Chlamydomonas reinhardtii alternative oxidase 1 is regulated by heat stress.

The alternative oxidase (AOX) is a non-energy conserving terminal oxidase that has emerged as an important mitochondrial component of the cell stress responses. Although the most studied abiotic condition in relation to Chlamydomonas reinhardtii is high temperature, changes in AOX capacity of the alga were studied only under oxidative stress and cold. To examine whether elevated temperatures affected AOX1 expression, we applied quantitative real-time PCR and pharmaceutical approaches. In this work, we demonstrated a sharp increase in AOX1 transcript and protein abundance under heat stress. Furthermore, C. reinhardtii cells displayed a large increase in alternative respiration in response to high temperature. Feeding with the protein kinase inhibitor staurosporine strongly retarded the AOX1 transcription. Finally, the addition of the calcium chelator EGTA prevented heat-induced AOX1 expression. Together, our results imply that heat-inducible Ca(2+) influx and protein kinase(s) may mediate AOX1 expression at elevated temperatures. Characterization of heat-induced AOX1 regulation in the green alga C. reinhardtii provides a framework for a more complete understanding of the function of this conserved protein.