| Research Abstract |
Exogenous DNAs were injected into Xenopus laevis oocytes, unfertilized eggs, and fertilized eggs, and replication and expression of the injected DNAs were studied. In oocyte nuclei; only circular DNAs were expressed, and the expression was not dependent on the promoter. Expression occurred similarly when DNAs were injected into the cytoplasm of unfertilized and fertilized eggs, but in the former case, the extent of the expression was extremely low. The expression from both unfertilized eggs and fertilized eggs depended on the strength of the promoters. Linearized DNAs were expressed, irrespective of the presence or absence of the promoter, and the expression accompanied always the formation of a large amount of large-sized concatemers. Using pSV2CAT, which was formerly reported to be expressed only after embryos reached the midblastula stage, we found here that this plasmid could be expressed already at the early cleavage stage. We also found that the circular gene with developmentally-regulated promoter such as CAT gene connected to the promoter of a-actin gene, could be expressed only when embryos reached the neurula stage, although such regulation was abolished when the DNA was linearized. In contrast to these data, which were all obtained with the wild-type embryos, the results obtained with the maternal-defect mutant were only at the preliminary ones. We found here that the developmental arrest which occurred at the gastrula stage was due to cessation of RNA transcription at the blastula stage by isotopic labeling experiment. The DNA synthesis, on the other hand, seemed to be normal even at the blastula stage. Thus, we expect that in the maternal-defect mutant embryos, the expression of the injected DNA would be very low.
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