EVIDENCE FOR THE INVOLVEMENT OF MOUSE HEAT-SHOCK FACTOR-1 IN THE ATYPICAL EXPRESSION OF THE HSP70.1 HEAT-SHOCK GENE DURING MOUSE ZYGOTIC GENOME ACTIVATION

The mouse HSP70.1 gene, which codes for a heat shock protein (hsp70), is highly transcribed at the onset of zygotic genome activation (ZGA). This expression, which occurs in the absence of stress, is then repre ssed. It has been claimed that this gene does not exhibit a stress res ponse until the blastocyst stage. The promoter of HSP70.1 contains fou r heat shock element (HSE) boxes which are the binding sites of heat s hock transcription factors (HSF). We have been studying the presence a nd localization of the mouse HSBs, mHSF1 and mHSF2, at different stage s of embryo development. We show that mHSF1 is already present at the one-cell stage and concentrated in the nucleus. Moreover, by mutageniz ing HSE sequences and performing competition experiments (in transgeni c embryos with the HSP70.1 promoter inserted before a reporter gene), we show that, in contrast with previous findings, HSE boxes are involv ed in this spontaneous activation, Therefore, we suggest that HSF1 and HSE are important in this transient expression at the two-cell stage and that the absence of typical inducibility at this early stage of de velopment results mainly from the high level of spontaneous transcript ion of this gene during the ZGA.