Effects of the injection of exogenous DNAs on gene expression in early embryos and coenocytic egg cells of Xenopus laevis

In Xenopus laevis embryogenesis, the synthesis of heterogeneous mRNA-like RNA starts at the cleavage stage, whereas that of low-molecular-mass RNAs and rRNA occurs at the early blastula and late blastula stages, respectively. In coenocytic fertilized Xenopus egg cells, which fail to cleave, an excess of exogenously injected DNA (pBR322) induces 'premature' expression of previously injected exogenous genes (yeast tRNA genes). We have carried out experiments to discover whether the injection of excess exogenous DNAs of various origins modifies the expression of endogenous genes and previously injected exogenous genes in Xenopus embryogenesis. We found that injection of a relatively large amount of exogenous DNA (Xenopus rDNA clone) induces the premature expression, or enhanced expression, of previously injected bacterial chloramphenicol acetyltransferase genes in coenocytic cells. In embryos, however, the injection of exogenous DNAs of various origins did not appreciably modify the expression of either endogenous or previously injected CAT genes. The DNAs injected into fertilized eggs were not degraded and were partitioned into the nuclei of most (at least 80%) of the descendant blastomeres at least during early stages of development. Therefore, we concluded that the program of gene expression in normally developing embryos cannot easily be altered by the introduction of excess exogenous DNAs.