HSF1 is the transcription regulator of the heat shock responsein Chlamydomonas (Schulz-Raffelt et al., 2007). Previous studieshave indicated that HSF1 is weakly expressed under non-stress conditions and rapidly stimulated by heat treatment (Schulz-Raffeltet al., 2007; Fig. 6A, insert). Therefore, we examined whether cold-shock treatment resulted in the accumulation of HSF1 mRNA. Thetranscript levels were increased after 2 h and reached a maximumwithin 6–24 h of cold treatment (Fig. 6A). After 24 h, the tran-scription level of HSF1 only slightly decreased and then was quite constant. Thus, HSF1 is a cold-responsive gene. We found that a shiftin temperature from 22 to 7◦C led to an increased accumulation ofHSF1 protein (Fig. 6B).