Fig. 4. Transient expression analysis of the rat myosin light chain (mylc) promoter in zebrafish embryos and structure of the mylc-GFP (A) and mylc-MSTN^pro (B) transgenes. The mylc-GFP gene construct (A) contains a 1.3 kb rat myosin light chain promoter (mylc) at the 5', the 0.8 kb GFP coding region, a 0.8 kb SV40 poly(A) signal, and a mylc 1/3 enhancer from the rat mylc gene (Donoghue et al., 1991). The mylc-MSTN^pro gene construct (B) contains the 1.3 kb rat myosin light chain promoter (mylc) at the 5', the 0.8 kb prodomain coding region of zebrafish myostatin (MSTN^pro), the 0.8 kb SV40 polyA signal, and the mylc 1/3 enhancer from the rat mylc gene. The ATG start codon, RIRR proteolytic site and the TAA stop codon are indicated. (C,D) Muscle-specific expression of GFP reporter gene in 24 h.p.f. zebrafish embryos injected with the mylc-GFP: (C) fluorescence, (D) immunostaining. Scale bar, 100 µm. The mosaic pattern of expression is typical for transient expression studies in zebrafish embryos (Westerfield et al., 1992; Du and Dienhart, 2001).