Temperature compensation of the circadian period length--a special case among general homeostatic mechanisms of gene expression?
In Neurospora crassa, as well as in other organisms, the expression of housekeeping genes is transiently suppressed after exposure to higher temperatures (30-45 degrees C); expression is then reactivated and adapts after a few-hours to values closer to the initial rates. Adaptive mechanisms apparently exist in the processes of transcription, RNA processing, and translation and render protein synthesis rates temperature compensated. Heat shock proteins (HSPs) play an important role within these mechanisms ("acquired thermotolerance of protein synthesis"), but their function is as yet not exactly known. Adaptive mechanisms seem also to involve intracellular ion changes after exposure to moderate temperature elevation. The expression of heat shock genes is transiently enhanced after exposure to higher temperatures and also adapts after a few hours. The adaptation mechanism includes inactivation of the heat shock transcription factor (HSF) by means of phosphorylation changes and possibly by binding of a gene product (HSP70)-a mechanism representing a negative feedback control. These examples demonstrate the existence of general adaptive mechanisms at different levels of gene expression that may also be at work in the temperature compensation of clock gene expression. Apart from such adaptation processes, antagonistic reactions within the processes of gene expression and protein modification might be equally enhanced or suppressed by temperature changes, leaving the equilibrium unaffected or balanced (antagonistic balance, see Ruoff et al., this issue of Chronobiology International). This principle is shown to apply to the effect of temperature elevation on total protein synthesis and degradation. It may also apply to other antagonistic processes such as phosphorylation-dephosphorylation or monomer-dimer formation. The circadian clock mechanism is assumed to consist of several processes that can either adapt or produce a balance. Single amino acid changes in a clock protein are assumed to partially upset this adaptation or balance.
Rensing
L
L
Institute of Cell Biology, Biochemistry and Biotechnology, University of Bremen, Germany.
Mohsenzadeh
S
S
Ruoff
P
P
Meyer
U
U
D002940:Circadian Rhythm; D015966:Gene Expression Regulation, Fungal; D005800:Genes, Fungal; D005801:Genes, Homeobox; D006360:Heat-Shock Proteins / Q000096:biosynthesis; D006706:Homeostasis; D008954:Models, Biological; D009492:Neurospora crassa / Q000502:physiology*; D013696:Temperature
D006360:Heat-Shock Proteins