XIdx, a dominant negative regulator of bHLH function in early Xenopus embryos.

We have identified a divergent member of the Xenopus Id family, XIdx, which disrupts binding of myogenic factor/E-protein complexes to DNA in vitro and inhibits transactivation of the E-box regulated cardiac actin gene by MyoD in embryonic tissue. XIdx transcripts accumulate from the early neurula stage in discrete domains of the anterior neural plate and subsequently identify regions of the developing nervous system, including the eye rudiments and the rhombencephalon. These results suggest that bHLH proteins and their Id protein regulators may participate in patterning of embryonic neural tissue. Phylogenetic analysis indicates that XIdx is the product of a novel Id gene and is distinct from XId2, which is expressed primarily in the developing pronephros.