Transducing positional information to the Hox genes: critical interaction of cdx gene products with position-sensitive regulatory elements

Studies of pattern formation in the vertebrate central nervous system indic ate that anteroposterior positional information is generated in the embryo by signalling gradients of an as yet unknown nature, We searched for transc ription factors that transduce this information to the Hox genes. Based on the assumption that the activity levels of such factors might vary with pos ition along the anteroposterior axis, we devised an in vivo assay to detect responsiveness of cis-acting sequences to such differentially active facto rs. We used this assay to analyze a Herbs regulatory element, and detected the most pronounced response in a short stretch of DNA containing a cluster of potential CDX binding sites, We show that differentially expressed DNA binding proteins are present in gastrulating embryos that bind to these sit es in vitro, that cdx gene products are among these, and that binding site mutations that abolish binding of these proteins completely destroy the abi lity of the regulatory element to drive regionally restricted expression in the embryo. Finally, we show that ectopic expression of cdx gene products anteriorizes expression of reporter transgenes driven by this regulatory el ement, as well as that of the endogenous Hoxb8 gene, in a manner that is co nsistent with them being essential transducers of positional information. T hese data suggest that, in contrast to Drosophila Caudal, vertebrate cdx ge ne products transduce positional information directly to the Hox genes, act ing through CDX binding sites in their enhancers. This may represent the an cestral mode of action of caudal homologues, which are involved in anteropo sterior patterning in organisms with widely divergent body plans and modes of development.