SEGMENTAL EXPRESSION OF THE EPHA4 (SEK-1) RECEPTOR TYROSINE KINASE INTHE HINDBRAIN IS UNDER DIRECT TRANSCRIPTIONAL CONTROL OF KROX-20

Segmentation of the vertebrate hindbrain leads to the formation of a s eries of rhombomeres (r) with distinct identities, Recent studies have uncovered regulatory links between transcription factors governing th is process, but little is known of how these relate to molecules media ting 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 betw een odd-and even-numbered rhombomeres. We have analysed the cis-acting regulatory sequences of the EphA4 gene in transgenic mice and identif ied a 470 bp enhancer element that drives specific expression in r3 an d 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 enhan cer, These data indicate that Krox-20 is a direct transcriptional acti vator 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.