Expression of microinjected hsp 70/CAT and hsp 30/CAT chimeric genes in developing Xenopus laevis embryos.

The expression of microinjected chimeric genes containing Drosophila hsp 70 and Xenopus hsp 70 and hsp 30 promoters linked to the reporter gene coding for bacterial chloramphenicol acetyltransferase (CAT) was examined during early development of Xenopus laevis. Heat-inducible expression of fusion genes containing either the Drosophila hsp 70 promoter (1100 bp) or the Xenopus hsp 70 promoter (750 bp) was first detectable after the midblastula stage of development. This coincides with the embryonic stage at which the endogenous hsp 70 gene is first heat-inducible. A Xenopus hsp 30/CAT fusion gene containing 350 bp of promoter sequences was also heat-inducible after the midblastula stage unlike the endogenous hsp 30 genes which were not heat-inducible until the early tailbud stage (stage 23-24). Sequences that are present within either the coding or 3' region of the hsp 30 clone do not cause the microinjected hsp 30 gene to be developmentally regulated in a normal manner. Additionally, microinjected hsp 30 gene sequences have no effect on the developmental regulation of endogenous hsp 30 genes which continue to be activated at the tailbud stage of development. Our data suggest, that an inhibitory system, which may control the expression of the endogenous hsp 30 gene during development, does not regulate the expression of the injected hsp 30 gene.