Activation of Xenopus MyoD transcription by members of the MEF2 protein family.

Members of the MEF2 family of DNA binding proteins interact with a set of AT-rich sequences commonly found in the promoters and enhancers of muscle-specific genes. We have shown that a MEF2 binding site precisely overlaps the TFIID binding site (TATA box) in the Xenopus MyoDa (XMyoDa) promoter and appears to play an important role in muscle-specific activity of this promoter. To further investigate the potential role of MEF2 in the regulation of XMyoDa transcription, we have analyzed the appearance of factors that interact with the XMyoDa TATA/MEF2 site during early amphibian development. Proteins that bind specifically to this site were present at low levels during early development and increased in abundance during gastrulation and neurulation. Two related cDNAs were isolated that encode proteins that recognize the XMyoDa TATA motif. Both proteins are highly homologous to each other, belong to the MADS (MCM1 agamous deficiens SRF) protein family, and are most highly related to the mammalian MEF2A gene products. Xenopus MEF2A (XMEF2A) transcripts accumulated preferentially in forming somites after the appearance of XMyoD transcripts. Ectopic expression of XMEF2A and other members of the MEF2 gene family activated transcription of a reporter gene controlled by the XMyoDa promoter. Transcriptional activation of the XMyoDa promoter required only the conserved DNA binding domain of XMEF2A and was independent of a domain necessary for activity when this factor was bound to multiple upstream sites. These results suggest that the XMyoDa promoter can be activated by binding of MEF2 to the XMyoDa TATA motif and indicate that MEF2-dependent transcriptional activation occurs by different mechanisms depending on the location of the MEF2 binding site. We suggest that XMEF2 expression in myogenic cells contributes to the activation and stabilization of XMyoDa transcription during muscle cell differentiation.