HEAT-SHOCK PROTEINS AND CIRCADIAN-RHYTHMS

Significant circadian rhythms in heat shock gene expression were obser ved in a prokaryotic species (Synechocystis). In eukaryotes, in contra st, several heat shock genes (constitutive and inducible) were shown t o be constantly expressed. A few cases of circadian expression of heat shock proteins (HSPs), however, have been reported. Significant circa dian changes of thermotolerance were observed in yeast and several pla nt species. Higher thermotolerance can be attributed to a higher abund ance of HSPs, but also to other adaptive mechanisms. Zeitgeber effects of temperature changes can be explained on the basis of their direct effects on the state variables of the clock gene (per,frq) expression and its negative feedback loop. Effects of increased HSP concentration s, as observed after heat shock, but also after light and serotonin (5 HT), appear possible, in particular with respect to nuclear localizati on of the clock (PER) protein, but these effects have not been documen ted yet. Thus, the role of HSPs in the circadian clock system is littl e understood and, from our point of view, deserves more attention.