Gene Expression from Endogenou and Exogenously-introduced DNAs in Early Embryogenesis of Xenopus laevis

In Xenopus laevis embryogenesis, gene expression from endogenous and exogenously-introduced DNAs is reported to start only after 12 rounds of cleavage, at the stage called midblastula transition (MBT). In isotopic labeling experiments, however, we found that the synthesis of heterogeneous mRNA-like RNA occurs from the early cleavage stage, and that gene expression from zygotic genomes occurs sequentially in three characteristically different phases: the pre-MBT phase, in which heterogeneous mRNA-like RNA and small amounts of small-molecular-weight RNAs are synthesized; the MBT phase, in which there is a large increase in 4S RNA synthesis, and the post-MBT phase, in which rRNA is also synthesized. Moreover in our studies on the expression of exogenously introduced DNA, we found that circular forms of bacterial chloramphenicol acetyltransferase (CAT) genes connected to viral promoters were expressed from the early cleavage stage, whereas circular forms of genes connected to Xenopus cardiac α-actin promoter were expressed only after the embryos reached the neurula stage, when the endogenous α-actin gene started to be expressed. We, therefore, conclude that in Xenopus embryogenesis, DNA-dependent RNA polymerases II, III and I are activated in this order, and that the promoter, not changes associated with the MBT, probably determine the timing of gene expression.