CONSTITUTIVE EXPRESSION OF A SOMATIC HEAT-INDUCIBLE HSP70 GENE DURINGAMPHIBIAN OOGENESIS

We isolated and characterized a sequence coding for heat-shock protein 70 (HSP70) of the amphibian Pleurodeles valtl. Results from S1 nuclea se protection assays led us to conclude that an hsp70 gene, strictly i nducible in somatic cells during heat shock, is constitutively active during oogenesis. By quantitative northern and western blot analysis, we showed that both hsp70 mRNA and HSP70-related protein levels increa sed in oocytes from stage II to stage VI under physiological condition s. Furthermore, by in situ hybridization to the nascent transcripts of lampbrush chromosome loops, we provided evidence for a clear-cut rela tionship between this increase in hsp70 mRNA and transcriptional activ ity during the lampbrush stage of oogenesis. These results strongly su ggest that hsp70 genes are actively transcribed throughout oogenesis. HSP70-related proteins localized in the cytoplasm of young oocytes are progressively transferred to the nucleus in the course of oogenesis a nd preferentially accumulated in the nuclei of some stage VI oocytes.