Fig. 5. Myocardin acts in combination with other cardiac transcription factors to activate endogenous MLC2 expression in animal cap explants. (A) Expression of myocardin alone (lane labeled myocardin) activates SM22 and MHC expression, but is not sufficient to activate expression of the MLC2 gene. Expression of Nkx2-5, Gata4, or Tbx5 alone, or the combination of these three factors (lane labeled N+G+T) is not sufficient to activate expression of MLC2 or SM22 or MHCa. However, when myocardin is coexpressed with Nkx2-5, Gata4 and Tbx5, MLC2 gene expression is activated (lane labeled M+N+G+T). M, myocardin; N, Nkx2-5; G, Gata4; T, Tbx5. (B) Co-expression of combinations of transcription factors in animal cap explants shows that any combination of myocardin and Gata4 is sufficient to activate MLC2 expression.