Muscle-specific (CArG) and serum-responsive (SRE) promoter elements are functionally interchangeable in Xenopus embryos and mouse fibroblasts.

The Xenopus cardiac actin gene contains four copies of a promoter element, the CArG box, which is conserved amongst striated muscle actin genes and is essential for tissue-specific expression in the developing Xenopus embryo. Our aim is to identify embryo and muscle proteins that interact with the CArG box as a step towards understanding the molecular basis of this developmentally regulated gene expression. The CArG box shares some sequence similarity with the Serum Response Element (SRE), which mediates the transcriptional activation by serum of genes such as c-fos and cytoskeletal actin. We show here that the most proximal cardiac actin CArG box is recognized by the same binding activity as the cytoskeletal actin SRE in nuclear extracts from both Xenopus embryos and mammalian muscle cells. This activity is indistinguishable from the previously characterized HeLa cell SRE-binding activity, Serum Response Factor (SRF). Importantly, we extend these in vitro studies to demonstrate that the CArG box and SRE are functionally interchangeable, both in Xenopus embryos and mouse fibroblasts. This implies that the CArG box and SRE can bind the same protein in vivo, as well as in vitro. Our results identify an SRF-like protein as a CArG box-binding factor and we discuss the implication that a common mechanism may be utilized in both muscle-specific gene expression and serum-responsive transcription.