The mouse Rxrb gene encoding RXRβ: genomic organization and two mRNA isoforms generated by alternative splicing of transcripts initiated from CpG island promoters

Abstract

Two major isoforms of retinoid X receptor β (RXRβ; H-2RIIBP), encoded by the Rxrb gene, have been identified in the mouse. Northern analysis of Rxrb mRNA showed two close bands of 2.8 and 2.6 kb in many tissues and cell lines. They are designated as m Rxrβ1 and m Rxrβ2, respectively. Some rapidly growing cell lines and spleen tissue had about twofold more Rxrβ1 mRNA than Rxrβ2 whereas most adult tissues had similar amounts of both β 1 and β 2. Amino acid (aa) sequences deduced from cDNAs show an extra N-terminal domain of 72 aa for RXRβ1 that is well conserved between mouse and human, but not found in RXRβ2. These isoforms are generated from separate exons transcribed from different CpG island promoters and spliced into the common acceptor site in the transactivation domain by an alternative splicing. The Rxrb gene contains an intron in the midst of the first zinc-finger coding region. This is different from the retinoic acid receptor (RAR) and other nuclear receptor superfamily genes that contain an intron between the first and the second zinc-finger coding regions. These results, together with their unique ability to form heterodimers with other members of the superfamily, suggest a distinct phylogenic position for the Rxr genes.