DIFFERENT POTENTIAL OF CELLULAR AND VIRAL ACTIVATORS OF TRANSCRIPTIONREVEALED 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 tra nscription factors using GAL4 fusion proteins. Here we have tested the ir transcriptional activity in oocytes and developing embryos of the c lawed toad Xenopus laevis. We find that the ''acidic'' C-terminal doma in of the herpesvirus VP16 (= Vmw65) activator, which is active from y east to man, is also very active in the two Xenopus systems. The const itutive nature of this viral domain may have evolved to be refractory to cellular defense mechanisms. By contrast, activation domains from c ellular eukaryotic transcription factors (TFE3, ITF2, MTF-1) are diffe rentially active in oocytes and early embryos. This indicates that the ir activity can be regulated by protein modification and/or availabili ty of specific coactivators. We have also compared VP16 induced enhanc ement of transcription from remote and promoter-proximal positions. In both oocytes and late blastula embryos, activation from a promoter-pr oximal position was more than 50 fold, while only a moderate stimulati on (3-8 fold) was observed from remote positions. This may mean that f rog oocyte and early embryos are not yet fully geared for gene control by remote enhancers, i.e. respond predominantly to close-by regulator y 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.