Different potential of cellular and viral activators of transcription revealed in oocytes and early embryos of Xenopus laevis.

Many protein domains for transcriptional activation also function when fused to a heterologous DNA binding domain. In mammalian/HeLa cells, we have previously characterized the activation domains of several transcription factors using GAL4 fusion proteins. Here we have tested their transcriptional activity in oocytes and developing embryos of the clawed toad Xenopus laevis. We find that the "acidic" C-terminal domain of the herpesvirus VP16 (= Vmw65) activator, which is active from yeast to man, is also very active in the two Xenopus systems. The constitutive nature of this viral domain may have evolved to be refractory to cellular defense mechanisms. By contrast, activation domains from cellular eukaryotic transcription factors (TFE3, ITF2, MTF-1) are differentially active in oocytes and early embryos. This indicates that their activity can be regulated by protein modification and/or availability of specific coactivators. We have also compared VP16 induced enhancement of transcription from remote and promoter-proximal positions. In both oocytes and late blastula embryos, activation from a promoter-proximal position was more than 50 fold, while only a moderate stimulation (3-8 fold) was observed from remote positions. This may mean that frog oocyte and early embryos are not yet fully geared for gene control by remote enhancers, i.e. respond predominantly to close-by regulatory sequences. The fact that cellular enhancers are naturally located at various distances from the responsive promoters may thus be exploited by multicellular organisms for differential gene control at early and late stages of development.