Expression, activity, and subcellular localization of the Yin Yang 1 transcription factor in Xenopus oocytes and embryos

Yin Yang 1 (YY1) is a multifunctional transcription factor that acts as an activator, repressor, or initiator of transcription of numerous cellular an d viral genes. Previous studies in tissue culture model systems suggest YY1 plays a role in development and differentiation in multiple cell types, bu t the biological role of YY1 in vertebrate oocytes and embryos is not well understood, Here we analyzed expression, activity, and subcellular localiza tion profiles of YY1 during Xenopus laevis development. Abundant levels of YY1 mRNA and protein were detected in early stage oocytes and in all subseq uent stages of oocyte and embryonic development through to swimming larval stages. The DNA binding activity of YY1 was detected only in early oocytes (stages I and II) and in embryos after the midblastula transition (MBT), wh ich suggested that its potential to modulate gene expression may be specifi cally repressed in the intervening period of development. Experiments to de termine transcriptional activity showed that addition of YY1 recognition si tes upstream of the thymidine kinase promoter had no stimulatory or repress ive effect on basal transcription in oocytes and post-MET embryos. Although the apparent transcriptional inactivity of YY1 in oocytes could be explain ed by the absence of DNA binding activity at this stage of development, the lack of transcriptional activity in post-MET embryos was not expected give n the ability of WI to bind its recognition elements. Subsequent Western bl ot and immuno cytochemical analyses showed that YY1 is localized in the cyt oplasm in oocytes and in cells of developing embryos well past the MET. The se findings suggest a novel mode of YY1 regulation during early development in which the potential transcriptional function of the maternally expresse d factor is repressed by cytoplasmic localization.