Developmental regulation of a serum response element binding activity in amphibian embryos

As part of our studies of transcriptional control during early development in vertebrates, we have examined embryos of the amphibian Xenopus laevis for the presence of sequence-specific DNA-binding proteins, using gel electrophoresis mobility-shift assays. Our analysis has focused on sequence elements in the cytoskeletal actin gene, whose embryonic transcription is initially activated at the gastrula stage, approximately 16 hours after fertilization. We detect activities capable of specific binding to two known transcriptional regulatory elements, the serum response element and the GC-box, located in the 5'-flanking region of the cytoskeletal actin gene. Binding activity specific for a region downstream of the transcriptional startsite is also detected, in a region which may be involved in controlling developmental activation of this gene. Serum response element-binding activity, as well as the downstream binding activity, is enriched in extracts from gastrula and neurula stage embryos, compared to egg extracts, suggesting that increased levels of one or both of these activities might play a role in developmentally timed transcriptional activation of the cytoskeletal actin gene in the embryo.