Regulation of Transketolase Like 1 Gene Expression in the Murine One-Cell Stage Embryos

In mice, transcription from the zygotic genome starts at the mid-one-cell stage after fertilization. Previous studies showed that an enhancer is not required for transcription at this stage, and that the enhancer-dependent mechanism of transcription is established during the two-cell stage. However, these results were obtained using reporter gene assays with promoters derived from viruses, rather than from endogenous genes. We conducted a reporter-gene assay using the promoter of Tktl1, which is transcribed after fertilization, to investigate the mechanism regulating gene expression at the one-cell stage. When a plasmid containing the 2467 bp upstream and 25 bp downstream of the Tktl1 transcription start site (TSS) was microinjected into the nuclei of growing oocytes, and one-cell stage and early and late two-cell-stage embryos, transcriptional activity was detected in the one-cell- and two-cell-stage embryos, but not in the oocytes. It was highest at the early two-cell stage and was reduced at the late two-cell stage. The decrease in activity at the late two-cell stage was prevented by inhibiting the second round of DNA replication, suggesting that the transcriptionally repressive state is established during the two-cell stage by a mechanism coupled to DNA replication. When the Tktl1 promoter was deleted to leave 56 bp upstream of the TSS which includes GC and TATA boxes, transcriptional activity was still detected in one-cell-stage embryos, but not early or late two-cell-stage embryos. The core promoter of Tktl1 alone seems to be able to induce basal transcription at the one-cell stage. These results suggest that repressive chromatin is established after fertilization in two steps, which occur during the transition from the one- to two-cell stage and during DNA replication at the two-cell stage.