The recruitment of SOX/OCT complexes and the differential activity of HOXA1 and HOXB1 modulate the Hoxb1 auto-regulatory enhancer function

Regionally restricted expression patterns of Hox genes in developing embryo s rely on auto-, cross-, and para-regulatory transcriptional elements. One example is the Hoxb1 auto-regulatory element (b1-ARE), which drives express ion of Hoxb1 in the fourth rhombomere of the hindbrain, We previously showe d that HOXB1 and PBX1 activate transcription from the b1-ARE: by binding to sequences required for the expression of a reporter gene in rhombomere 4 i n vivo, We now report: that in embryonal carcinoma cells, which retain char acteristics of primitive neuroectodermal cells, the b1-ARE displays higher basal and HOX/PBX-induced activities than in other cell backgrounds. We hav e identified a bipartite-binding site for SOX/OCT heterodimers within the b 1-ARE that accounts for its cell context-specific activity and is required for maximal transcriptional activity of HOX/PBX complexes in embryonal carc inoma cells. Furthermore, we found that in an embryonal carcinoma cell back ground, HOXB1 has a significantly higher transcriptional activity than its paralog HOXA1. We map the determinants for this differential activity withi n the HOXB1 N-terminal transcriptional activation domain. By using analysis in transgenic and HOXA1 mutant mice, we extended these findings on the dif ferential activities of HOXA1 and HOXB1 in vivo, and we demonstrated that t hey are important for regulating aspects of HOXB1 expression in the hindbra in. We found that mutation of the SOX/OCT site and targeted inactivation of Hoxa1 both impair the response of the b1-ARE to retinoic acid in transgeni c mice. Our results show that Hoxa1 is the primary mediator of the response of b1-ARE to retinoic acid in vivo and that this function is dependent on the binding of SOX/OCT heterodimers to the b1-ARE. These results uncover no vel functional differences between Hox paralogs and their modulators.