Nuclear translocation of a maternal CCAAT factor at the start of gastrulation activates Xenopus GATA-2 transcription.

The transcription factor GATA-2 is present in blood cell precursors and plays a pivotal role in the control of erythroid differentiation. In Xenopus embryos, low levels of GATA-2 mRNA are maternally derived, while the onset of zygotic GATA-2 expression coincides with commitment to haematopoietic lineages. However, its initial transcriptional activation is not restricted to the presumptive blood islands, but occurs throughout ventral and lateral regions, in all three germ layers. In order to determine how this expression pattern is controlled, we have isolated and characterized the Xenopus GATA-2 gene. We show that 1.65 kb of 5' flanking sequences are sufficient to direct both correct transcriptional initiation in oocytes and appropriate temporal and spatial gene expression in early embryos. The transgene is activated during gastrulation and by neurula stages in predominantly expressed in the ventral hemisphere. We demonstrate that a CCAAT element is necessary for gene activity in both systems and that extracts prepared from oocytes and embryos contain a factor which specifically recognizes this element. We also show that cytoplasmic localization inhibits the function of this CCAAT factor until the beginning of gastrulation, when the zygotic GATA-2 gene is activated. These observations extend our understanding of the mechanisms by which maternal factors control the temporal activation of transcription in early vertebrate embryos.