Muscle specificity encoded by specific serum response factor-binding sites

Serum response factor (SRF) is a MADS box transcription factor that regulat es muscle-specific and growth factor-inducible genes by binding the consens us sequence CC(A/T)(6)GG, known as a CArG box. Because SRF expression is no t restricted solely to muscle, its expression alone cannot account for the muscle specificity of some of its target genes. To understand further the r ole of SRF in muscle-specific transcription, we created transgenic mice har boring lacZ transgenes linked to tandem copies of different CArG boxes with flanking sequences. CArG boxes from the SM22 and skeletal cy-actin promote rs directed highly restricted expression in developing smooth, cardiac, and skeletal muscle cells during early embryogenesis, In contrast, the CArG bo ar and flanking sequences from the c-fos promoter directed expression throu ghout the embryo, with no preference for muscle cells. Systematic swapping of the core and flanking sequences of the SM22 and c-fos CArG boxes reveale d that cell type specificity was dictated in large part by sequences immedi ately flanking the CArG box core. Sequences that directed widespread embryo nic expression bound SRF more strongly than those that directed muscle-rest ricted expression. We conclude that sequence variations among CArG boxes in fluence cell type specificity of expression and account, at least in part, for the ability of SRF to distinguish between growth factor-inducible and m uscle-specific genes in vivo.