CHARACTERIZATION AND RETINOIC ACID RESPONSIVENESS OF THE MURINE HOXD4TRANSCRIPTION UNIT

We have characterized the transcription unit of a murine Hox gene in t he fourth paralogous group, Hoxd4, We have identified two HoxdB transc ription start sites by S1 analysis, The upstream promoter (P2) is 5.2 kilobase pairs upstream from the coding region, while the downstream p romoter (P1) is 1.1 kilobase pairs distant, Both promoters bear a clus ter of start sites, Multiple transcripts were identified by Northern b lot, originating from both promoters and multiple polyadenylation sign als. Expression of P1 transcripts in the neural tube shows an anterior border at the rhombomere 6/7 boundary, corresponding to previous repo rts (Gaunt, S. J., Krumlauf, R,, and Duboule, D. (1989) Development 10 7, 131-141; Morrison, A., Moroni, M, C,, Ariza-McNaughton, L,, Krumlau f, R,, and Mavilio, F, (1996) Development 122, 1895-1907), A more post erior boundary in the central nervous system was observed for P2 trans cripts, We observed strong expression up to somite 6 and weak expressi on in somite 5, correlating with the phenotype of Hoxd4 null mutant mi ce (Horan, G, S. B,, Nagy Kovacs, E,, Behringer, R, R,, and Feathersto ne, RI, S, (1995) Dev. Biol. 169, 359-372), In response to retinoic ac id, expression from P1 in the hindbrain was anteriorized after 4 or 24 h of treatment, P2 transcripts seemed to be less responsive and/or to have an indirect response to retinoic acid, The long 5'-untranslated region found in all Hoxd4 transcripts suggests that translation does n ot occur by a classical ribosome scanning mechanism.