Determination of left right asymmetric expression of nodal by a left side-specific enhancer with sequence similarity to a lefty-2 enhancer

The nodal gene is expressed on the left side of developing mouse embryos an d is implicated in left/right (L-R) axis formation. The transcriptional reg ulatory regions of nodal have now been investigated by transgenic analysis. A node-specific enhancer was detected in the upstream region (-9.5 to -8.7 kb) of the gene. Intron 1 was also shown to contain a left side-specific e nhancer (ASE) that was able to direct transgene expression in the lateral p late mesoderm and prospective floor plate on the left side. A 3.5-kb region of nodal that contained ASE responded to mutations in iv, inv, and lefty-1 , all genes that act upstream of nodal. The same 3.5- kb region also direct ed expression in the epiblast and visceral endoderm at earlier stages of de velopment. Characterization of deletion constructs delineated ASE to a 340- bp region that was both essential and sufficient for asymmetric expression of nodal. Several sequence motifs were found to be conserved between the no dal ASE and the lefty-2 ASE, some of which appeared to be essential for nod al ASE activity. These results suggest that similar transcriptional mechani sms underlie the asymmetric expression of nodal and of lefty-2 as well as t he earlier expression of nodal in the epiblast and endoderm.