Transformed Xenopus embryos as a transient expression system to analyze gene expression at the midblastula transition.

The onset of transcriptional activity during embryogenesis in Xenopus laevis is at the 4000- to 8000-cell stage (stage 8-8.5) and is referred to as the midblastula transition (MBT). Most exogenous circular DNA that is microinjected into the fertilized egg also is expressed at the MBT. The transformed Xenopus embryo at these early stages was used as a transient expression system in order to determine the effects of (1) promoter strength, (2) physical conformation, (3) degree of replication, and (4) a regulatory molecule on the expression of an injected gene coding for chloramphenicol acetyl transferase. This gene linked to a relatively strong promoter (SV40 early promoter -pSV2CAT), a weak promoter (adenovirus early promoter -pE3CAT), or in circular or linear form is expressed at stage 8-8.5 following injection into fertilized eggs. pE3CAT coinjected with the E1a protein (enhances the transcription of the E3 promoter) is also expressed at stage 8.5, but expression is enhanced 2-7.6 fold. These data suggest that the inhibition of transcription prior to the MBT could not be perturbed by either the presence of different promoters or a positive regulatory molecule such as the E1a protein.