RELIEF OF A REPRESSED GENE-EXPRESSION STATE IN THE MOUSE 1-CELL EMBRYO REQUIRES DNA-REPLICATION

In the mouse, transcriptional permissiveness is established in the fer tilized egg prior to the activation of zygotic genes at the 2-cell sta ge, Therefore, gene inactivity initiated at the end of gametogenesis r esults from a complex process, involving more than an inhibition of th e basal transcriptional apparatus, We have examined the ability of the first intron (I1) of the human hypoxanthine phosphoribosyl transferas e gene, which functions as an enhancer in embryonic stem cells, to act ivate a reporter gene when placed proximally to or at a distance from the HSV-tk promoter, or when integrated into the mouse genome as part of a stable transgene, In microinjected embryos, I1 functions as an en hancer sequence; however, its competence for long-range activation app ears only after the late 1-cell stage and depends on the first DNA rep lication. Moreover, activation of microinjected transgenes from proxim al enhancers occurs in the late 2-cell embryo and in the male pronucle us of 1-cell embryos blocked for DNA replication; whereas, for integra ted transgenes, proximal enhancer activity is subject to position effe cts in the 2-cell embryo and first occurs at the 2- or 4-cell stage, b ut only after completion of DNA replication, Therefore, the absence of long-range activation and a non-permissive genomic state (the relief of which both depend on DNA replication), together with an inactive tr anscriptional apparatus, appear to converge to prevent any gene activi ty in the 1-cell embryo, We propose that the embryo exploits the proce ss of DNA replication to relieve the transcriptionally repressive stat e that mas initially established to fulfil two purposes: (1) to arrest maternal gene expression in the maturing oocyte and (2) to protect th e unicellular egg and 1-cell embryo from premature differentiation. Re activation of gene expression by DNA replication would therefore serve to coordinate cell proliferation and differentiation in the preimplan tation embryo.