Mammalian Hox genes are clustered at four genomic loci. During development, neighbouring genes are coordinately regulated by global enhancer sequences, which control multiple genes at once, as exemplified by the expression of series of contiguous Hoxd genes in either limbs or gut. The link between vertebrate Hox gene transcription and their clustered distribution is poorly understood. Experimental and comparative approaches have revealed that various mechanisms, such as gene clustering or global enhancer sequences, might have constrained this genomic organization and stabilized it throughout evolution. To understand what restricts the effect of a particular enhancer to a precise set of genes, we generated a loxP/Cre-mediated targeted inversion within the HoxD cluster. Mice carrying the inversion showed a reciprocal re-assignment of the limb versus gut regulatory specificities, suggesting the presence of a silencer element with a unidirectional property. This polar silencer appears to limit the number of genes that respond to one type of regulation and thus indicates how separate regulatory domains may be implemented within intricate gene clusters.
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