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Segmentation of the vertebrate hindbrain leads to the formation of a series of rhombomeres (r) with distinct identities. Recent studies have uncovered regulatory links between transcription factors governing this process, but little is known of how these relate to molecules mediating cell-cell signalling. The Eph receptor tyrosine kinase gene EphA4 (Sek-1) is expressed in r3 and r5, and function-blocking experiments suggest that it is involved in restricting intermingling of cells between odd- and even-numbered rhombomeres. We have analysed the cis-acting regulatory sequences of the EphA4 gene in transgenic mice and identified a 470 bp enhancer element that drives specific expression in r3 and r5. Within this element, we have identified eight binding sites for the Krox-20 transcription factor that is also expressed in r3 and r5. Mutation of these binding sites abolishes r3/r5 enhancer activity and ectopic expression of Krox-20 leads to ectopic activation of the enhancer. These data indicate that Krox-20 is a direct transcriptional activator of EphA4. Together with evidence that Krox-20 regulates Hox gene expression, our findings reveal a mechanism by which the identity and movement of cells are coupled such that sharply restricted segmental domains are generated.
Theil et al. (Sun,) studied this question.