Exon-intron organization of Xenopus MHC class II β chain genes
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- Kobari, F., Sato, K., Shum, B.P. et al. Immunogenetics (1995) 42: 376. doi:10.1007/BF00179399
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The ampibian Xenopus laevis is the most primative vertebrate in which the major histocompatibility complex (MHC) has been defined at the biochemical, functional, and molecular genetic levels. We previously described the isolation and characterization of cDNA clones encoding X. laevis MHC class II β chains. In the present study, genomic clones encoding class II β chains were isolated from X. laevis homozygous for the MHC f haplotype. Three class II β chain genes, designed Xela-DAB, Xela-DBB, and Xela-DCB, were identified. Seqeunce analysis of these genes showed that Xela-DBB and Xela-DCB corresponding to the previously characterized cDNA clones F3 and F8, respectively, whereas Xela-DAB encodes a third, hitherti unidentified class II β chain of the MHC f haplotype. As a representative of X. laevis class II β chain genes, the Xela-DAB gene underwent detailed structural analysis. In addition,the nucleotide sequence of Xela-DABf cDNA clones was determined. The Xela-DAB gene is made up of a least six exons, with an exon-intron organization similar to that of a typical mammalian class II β chain gene. The 5′-flanking region of the Xela-DAB gene contains transcriptional control elements known as X1, X2, and Y, but lacks typical TATA or CCAAT boxes. A notable feature of the X. laevis class II β chain genes is that sizes of the introns are larger than those of their mammalian counterparts. As assessed by northern blot analysis, the three class II β chain genes had similar expression patterns, with the highest level of transcription detected in the intestine. Identification of the Xela-DAB,-DBB, and -DCB genes is consistent with our previous observations, which suggested that the MHC of the tettraploid frog X. laevis is diploidized at the genomic level and contains three class II β chain genes per haplotype that cross-hybridize to one another under reduced stringency conditions.