Fig. 3. Deletional analysis of the Mef2c anterior heart field enhancer identifies an endogenous promoter and a minimal evolutionarily conserved enhancer that is necessary and sufficient for expression in vivo. (A) Genomic organization and deletion constructs of the Mef2c anterior heart field enhancer. The organization of the enhancer is denoted at the top. The black box denotes the third 5' untranslated exon; the red boxes indicate regions of mouse to human homology (>75%). Construct fragment number and nucleotides are indicated on the left. Anterior heart field expression of lacZ at 9.5 and 11.5 days post coitum (dpc) is denoted in the column to the right. The column on the far right indicates the number of independent transgenic lines or F₀ embryos that expressed lacZ in the anterior heart field as a fraction of the total number of transgene positive F₀ embryos or lines examined. The green box denotes the presence of the heterologous HSP68 promoter. (B-G) Representative embryos collected at 9.5 dpc for selected Mef2c anterior heart field enhancer deletion constructs. Fragment 2 (B), Fragment 3 (C), Fragment 6 (D) and Fragment 10 (G) directed strong expression to the pharyngeal mesoderm/anterior heart field (arrowheads), outflow tract and right ventricle (RV). Fragment 10 (G) represents a minimal sufficiency enhancer construct required for lacZ expression in the anterior heart field. Fragment 3 (C) utilizes an endogenous promoter from Mef2c.