The molecular determinants that direct gene expression to the ventricles of the 
heart are for the most part unknown. Additionally, little data is available on 
how the anterior/posterior axis of the heart tube is determined and whether the 
left and right atrial and ventricular chambers are assigned as part of this 
process. Utilizing myosin light chain-2 ventricular promoter/beta-galactosidase 
reporter transgenes, we have determined the minimal cis-acting sequences 
required for ventricular-specific gene expression. In multiple independent 
transgenic mouse lines, we found that both a 250 base pair myosin light chain-2 
ventricular promoter fragment, as well as a dimerized 28 bp sub-element (HF-1) 
containing binding sites for HF1a and HF1b/MEF2 factors, directed 
ventricular-specific reporter expression from as early as the endogenous gene, 
at day 7.5-8.0 post coitum. While the endogenous gene is expressed uniformly 
throughout both ventricles, the transgenes were expressed in a right 
ventricular/conotruncal dominant fashion, suggesting that they contain only a 
subset of the elements which respond to positional information in the developing 
heart tube. Expression of the transgene was cell autonomous and its 
temporospatial characteristics not affected by mouse strain/methylation state of 
the genome. To determine whether ventricular-specific expression of the 
transgene was dependent upon regulatory genes required for correct ventricular 
differentiation, the 250 base pair transgene was bred into both retinoid X 
receptoralpha and Nkx2-5 null backgrounds. The transgene was expressed in both 
mutant backgrounds, despite the absence of endogenous myosin light chain-2 
ventricular transcript in Nkx2-5 null embryos. Ventricular specification, as 
judged by transgene expression, appeared to occur normally in both mutants. 
Thus, the HF-1 element, directs chamber-specific transcription of a transgene 
reporter independently of retinoid X receptoralpha and Nkx2-5, and defines a 
minimal combinatorial pathway for ventricular chamber gene expression. The 
patterned expression of this transgene may provide a model system in which to 
investigate the cues that dictate anterior-posterior (right ventricle/left 
ventricle) gradients during mammalian heart development.