Examination of the DNA sequence-specific binding properties of heat shock transcription factor in Xenopus laevis embryos.

The binding of heat shock transcription factor (HSF) to the heat shock element (HSE) is necessary for transcriptional activation of eukaryotic heat shock protein (HSP) genes. The properties of Xenopus embryo HSF were examined by DNA mobility shift analysis employing a synthetic oligonucleotide corresponding to the proximal HSE in the promoter of the Xenopus HSP70B gene. Heat shock induced activation of HSF binding in Xenopus neurulae was not affected by an inhibition of protein synthesis, indicating that the mode of activation may be posttranslational. Also, while HSF binding was activated in control Drosophila cell extracts by in vitro heat shock or other chemical treatments, HSF binding in Xenopus embryo or somatic cell extract was not. Thus, the activation of Xenopus HSE-HSF binding may occur via a different mechanism compared with Drosophila. Furthermore, we determined that the native size of heat-induced HSF in pre- and post-midblastula stage Xenopus embryos is approximately 530 kilodaltons (kDa), which corresponds to a hexamer made up of 88 kDa monomers. Finally, the slower accumulation of HSP70 mRNA to peak levels found at lower heat shock temperatures was not correlated with HSE-HSF binding activity.