Expression of exogenously introduced bacterial chloramphenicol acetyltransferase genes in Xenopus laevis embryos before the midblastula transition.

Previous papers have reported that DNAs exogenously injected into Xenopus laevis fertilized eggs are expressed only at and after the midblastula transition (MBT). We have injected fertilized eggs of Xenopus laevis with circular plasmids that contained bacterial chloramphenicol acetyltransferase (CAT) genes connected to the promoter of viral genes (pSV2CAT and pAd12.E1aCAT) or the Xenopus cardiac actin gene (actin-CAT fusion gene), and examined whether these DNAs are expressed during the stage before the MBT. We found that expression of CAT enzyme can be detected before the MBT when CAT genes connected to viral promoters were injected. The activity was low during the cleavage stage on a per-embryo basis; however, the time course of the accumulation of the CAT enzyme activity roughly paralleled the increase in cell number. Therefore, CAT enzyme activity per cell was constant during cleavage and did not change dramatically before and after the MBT stage. CAT mRNA level, detected by CAT antisense RNA, was roughly proportional to the levels of CAT enzyme. Therefore, we assume that the observed enzyme activity reflects the transcriptional activity of injected CAT genes before and after the MBT. When the actin-CAT fusion gene was injected, however, no enzyme activity was detected until embryos had reached the neurula stage, a stage when endogenous actin genes are first activated. On the basis of these results, we conclude that the concept of an initial transcriptional activation of exogenous genes at amphibian MBT has to be changed. We propose that the expression of polymerase-II-transcribed genes is regulated by the nature of the promoters connected to the genes rather than by changes associated with MBT.