Xenopus oocytes reactivate muscle gene transcription in transplanted somatic nuclei independently of myogenic factors.

Transplantation into eggs or oocytes is an effective means of achieving the reprogramming of somatic cell nuclei. We ask here whether the provision of gene-specific transcription factors forms part of the mechanism by which a gene that is repressed in somatic cells is transcribed in oocytes. We find that M1 oocytes have an extremely strong transcription-inducing activity. They cause muscle genes of nuclei from non-muscle somatic cells, after injection into oocytes, to be transcribed to nearly the same extent as muscle genes in muscle cells. We show, surprisingly, that the myogenic factor MyoD and other known myogenic factors are not required to induce the transcription of muscle genes in a range of non-muscle somatic cell nuclei after transplantation to Xenopus oocytes. The overexpression of Id, a dominant-negative repressor of MyoD, prevents maternal MyoD from binding to its consensus sequences; nevertheless, muscle genes are activated in somatic nuclei to the same extent as without Id. We conclude that M1 oocytes can reprogram somatic nuclei in a different way to other experimental procedures: oocytes do not suppress the transcription of inappropriate genes and they activate a gene without the help of its known transcription factors. We suggest that these characteristics might be a special property of amphibian oocytes, and possibly of oocytes in general.