Fig. 1. A Mef2c-lacZ bacterial artificial chromosome (BAC) transgene is sufficient to direct the complete pattern of Mef2c expression. (A) Schematic representation of the mouse Mef2c locus contained within BAC clone GS133. The BAC contains the first six known exons from Mef2c (vertical black lines), including the first two coding exons, which encode the MADS and MEF2 domains of Mef2c. The red box represents a region of high evolutionary conservation that functions as an anterior heart field specific transcriptional enhancer. (B) Schematic representation of Mef2c-F6-lacZ, which contains the Mef2c anterior heart field enhancer subcloned into the transgenic reporter plasmid HSP68-lacZ. Red box, Mef2c noncoding sequences representing the anterior heart field enhancer; black box, Mef2c untranslated exon 3; green box, HSP68 promoter (pr); blue box, lacZ gene; yellow box, SV40 splice and polyadenylation (polyA) sequence. (C) Expression of the Mef2c BAC transgene, Mef2c-L8-lacZ, at 7.5 days post coitum (dpc) shown in frontal view. The Mef2c-L8-lacZ BAC transgene directs expression exclusively to the anterior heart field at this stage, recapitulating the earliest expression of endogenous Mef2c. (D) Lateral view of a transgenic embryo collected at 8.0 dpc showing expression of Mef2c-L8-lacZ. Strong expression is evident in the anterior heart field at the arterial pole of the linear heart tube. (E) Ventral view of an 8.5 dpc Mef2c-L8-lacZ transgenic embryo. Strong expression remains evident in the anterior heart field at the arterial pole of the linear heart tube. Transgene expression is also evident in the neural crest (NC) at this stage and expression can begin to be seen in vascular endothelial cells within the dorsal aortae and in the yolk sac (not shown). (F) Lateral view of a 9.5 dpc Mef2c-L8-lacZ transgenic embryo. At this stage the expression directed by the BAC transgene completely recapitulated the pattern of endogenous Mef2c expression during embryonic development at this stage. Expression was present in vascular endothelium (V), neural crest, somites (S), branchial arches (BA), pharyngeal mesoderm, outflow tract and heart. RV, right ventricle. (C-F) Representative X-gal stained transgenic embryos at each stage. Arrowheads point to pharyngeal mesoderm/anterior heart field. Two F₀ transgenic embryos and embryos collected from an independent BAC transgenic line displayed identical patterns of expression at all stages.