Fig. 1. Identification of cis-acting cardiovascular enhancer regions in the myocardin locus. (A) Schematic representation of the genomic organization of the myocardin locus and the constructs used for the creation of transgenic mice (see Materials and methods for construct details). The enhancer element that drives expression in heart and smooth muscle (fragment 5) is located 20-30 kb upstream of myocardin's translational start in exon 1, and is shown in red. The number of genotype-positive E12.5 embryos obtained from a total of three injections per construct are indicated between brackets. Shown at the top are the first three exons of myocardin (in black) and the exon organization of an upstream gene (mRNA AK042647) in gray. (B) Expresssion of the myocardin enhancer (MyE) throughout embryonic development was determined using a stable transgenic line bearing fragment 5, linked to Hsp68lacZ. Embryos harvested from staged matings demonstrated that the enhancer recapitulates the expression pattern of myocardin. Note strong lacZ expression in the cardiac crescent (a,b). As the linear heart tube forms (c) and loops (d,f), lacZ staining is detected at high levels throughout the entire heart. In the vasculature we detected lacZ expression in the developing dorsal aorta as early as E9.0-9.5 (f,g), in branchial arch arteries at E10.5 (h,i) and carotid arteries at E12.5 (k,l). In adult tissues, lacZ expression was detected in the heart (e) and in the vasculature of the lung (j) and aorta (o).