A DNA fragment of the rat embryonic myosin heavy-chain promoter (MHCemb) has been found to specifically bind a nuclear factor (NFe) present in extracts prepared from mouse C2 myoblasts, myotubes, and HeLa cells. The nucleotide sequence of the binding site (BSe) has been identified as 5'-GTGTCAGTCA-3' and was located between -93 and -84. Transient expression studies on MHCemb promoter deletion constructs in C2 myoblasts and C2 myotubes suggested that NFe is a transcriptional factor. Deletion of the NFe-binding site resulted in four- to sixfold and twofold reduction of promoter activity in C2 myotubes and C2 myoblasts, respectively. Furthermore, point mutations at the BSe not only abolished the NFe-binding activity of the MHCemb promoter but also resulted in reduction of the promoter activity to levels similar to those of the deletion constructs in C2 myotubes, myoblasts, and Hela cells (four- to sixfold). Although BSe and the binding site of the recently identified transcriptional factors AP-1 and ATF share significant homology, the results from competition binding assays indicated that NFe is different from both AP-1 and ATF.