Changes in permissiveness for the expression of microinjected DNA during the first cleavages of mouse embryos

LacZ DNA and LacZ RNA were microinjected during the first cleavages of embryos. LacZ DNA was not expressed before 18–19 h post insemination (hpi) but LacZ RNA was translated. Before 22 hpi LacZ DNA was expressed in the pronuclei of the one-cell embryos and the polypeptides of the minor, but not the major activation period of the genome were synthesized. This suggests a negative control of transcription before 18–19 hpi and demonstrates that its resumption is independent of the first cleavage and of the major activation of the genome. At the time of the minor activation the eggs contain the trans -acting elements to express a variety of genes that they do not express. It may indicate that, the minor and the major activation of the genome are differently controlled.