GENE TRANSPOSITIONS IN THE HOXD COMPLEX REVEAL A HIERARCHY OF REGULATORY CONTROLS

Vertebrate Hox genes are activated following a temporal sequence that reflects their linear order in the clusters. We introduced two Herd tr anscription units, labeled with lacZ, to an ectopic 5' position in the HoxD complex. Early expression of the relocated genes was delayed and resembled that of the neighboring Hoxd-13. At later stages, locus-dep endent expression in distal limbs and the genital eminence was observe d, indicating that common regulatory mechanisms are used for several g enes. These experiments also illustrated that neighboring genes can sh are the same cis-acting sequence and that moving genes around in the c omplex induces novel regulatory interferences. These results suggest t hat high order regulation controls the activation of Hox genes and hig hlight three important constraints responsible for the conservation of Hox gene clustering.