Cloning and chromosome mapping of human retinoid X receptor beta: selective amino acid sequence conservation of a nuclear hormone receptor in mammals.

The murine retinoid X receptor beta (mRXR beta) is a nuclear hormone receptor that activates transcription of murine major histocompatibility complex (MHC) class I genes in response to retinoic acid. In this study, the human RXR beta gene was mapped onto the short arm or centromeric region of chromosome 6 (6pter-q13), which also harbors the MHC. Chromosomal localization was performed by Southern hybridization of genomic DNA from human rodent cell hybrids with the mRXR beta gene as a probe. In addition, a full-length cDNA clone encoding a human RXR beta was isolated by nucleic acid screening of a human cDNA library with a fragment of the mRXR beta gene as a probe. Comparison of the nucleotide-coding sequences of the human and the murine RXR beta revealed a predominance of third base substitutions, resulting in selective conservation of the predicted amino acid sequence of the proteins. The overall sequence homology was 97.6% on the amino acid level as opposed to 91.6% on the nucleotide level. In Northern hybridization experiments with the human cDNA as a probe, RXR beta gene transcripts were detected in a variety of human tumor cell lines, regardless of whether these cell lines expressed MHC class I genes.