A NOVEL TYPE OF RETINOIC ACID RESPONSE ELEMENT IN THE 2ND INTRON OF THE MOUSE H2K(B) GENE IS ACTIVATED BY THE RAR RXR HETERODIMER/

We have identified and characterized a novel retinoic acid (RA) respon se element (Hi-RARE) in the second intron of the mouse major histocomp atibility H2K(b) gene. The Hi-RARE sequence is conserved in all mouse classical and Q class I genes, in MHC class I genes of the rat, Rhesus macaque, cat and in the vast majority of human classical and non-clas sical class I genes. The Hi-RARE sequence lies within a regulatory ele ment responsible for constitutive expression of a 5' enhancerless H2K( b) gene in the Ltk(-) fibroblasts. Hi-RARE consists of two inverted pa lindromic RARE consensus sites (5'-PuGGTCA-3') separated by an 8 nt sp acer. Mutational analysis revealed that both inverted palindromic hexa nucleotide motifs are indispensable functional sites for the 9-cis RA response. The Hi-RARE sequence confers 9-cis RA inducibility to a hete rologous promoter. The inducibility is further augmented in embryonal carcinoma cells by the expression of recombinant retinoic acid recepto rs (RARs) and the retinoid X receptors (RXRs). In vitro, the recombina nt RAR/RXR heterodimer creates DNA-protein complex with the Hi-RARE se quence. Treatment of P19 embryonal carcinoma cells with 9C-RA induces the Hi-RARE binding activity of nuclear proteins that proved to be RAR (or RAR-like)/RXR heterodimer. Thus the Hi-RARE represents a new type of RA response element with a role in the modulation of the expressio n of MHC class I family genes.