4.1.1 Summary of Section 4

The integration of physiological and molecular responses links an environmental signal to a physiological response. Heat-shock proteins (Hsps) are chaperone proteins that maintain the structure and function of proteins in cells exposed to high temperatures. The initial response of cells to high temperatures is rapid transcription of heat-shock genes, e.g. Hsp70, which is possible because Hsp70 is normally partly transcribed by RNA polymerase II. Resumption of transcription requires only three steps: release, then trimerisation of HSTF (heat-shock transcription factor), followed by binding to HSEs (heat-shock regulatory elements) in the promoters.

At T _a 25°C, the desert reptiles Phrynocephalus interscapularis and Crossobamon eversmanni have high constitutive levels of Hsps in their cells, in contrast to a temperate species, Lacerta vivipara. Cells from P. interscapularis and Gymnodactylus caspius kept at 25°C contained high levels of active HSTF bound to HSE in contrast to cells from L. vivipara, suggesting that in desert species HSTF genes are expressed constitutively. Protein synthesis in liver cells of desert species continued normally following heat shock up to 45°C, whereas in L. vivipara protein synthesis in the liver plummeted after heat shock at 37°C. Cells of desert species are well prepared for heat shock.