Myosin heavy chain IIb (MyHC-IIb) promoter sequence alignment and activity in mouse and human muscle cells. (A) For the proximal 0.2 kb of the mouse and human MyHC-IIb promoters, the sequence identity was 94%. Both sequences contain a conserved E-Box and AT rich region (AT1), along with a consensus TATA box. However, the AT2 and CArG box regions contain sequence differences predicted to affect myocyte enhancer factor 2 (MEF2) and serum response factor (SRF) binding to the human sequence. (B) Activities of the 1.0 kb and 0.2 kb human constructs were significantly reduced compared to the corresponding mouse sequences in both the mouse and human cells. A chimeric construct containing the proximal human 0.2 kb region linked to the distal mouse 0.8 kb region (Chimera 2) showed reduced activity compared to the opposing construct (Chimera 1). Values (mean ± standard error of mean) represent the result of at least three duplicate experiments and are expressed as fold-activity relative to the mouse 1.0 kb MyHC-IIb reporter plasmid. *P < 0.01 Human 1.0 kb versus Mouse 1.0 kb; **P < 0.05 Human 0.2 kb versus Mouse 0.2 kb; ***P < 0.01 Chimera 2 versus Chimera 1.