Isolation of a classical MHC class I cDNA from an amphibian. Evidence for only one class I locus in the Xenopus MHC.
The amphibian Xenopus is an ectothermic vertebrate in which the MHC has been studied extensively at the functional, biochemical, and genetic levels. A cDNA clone corresponding to the MHC class la gene (Xela-UAA1f) of Xenopus laevis was isolated by screening a cDNA phage library with oligonucleotides based on NH2-terminal protein sequence. Three pieces of evidence support its status as a class la gene: 1) Previous biochemical data suggested that only one polymorphic class la molecule is expressed per MHC haplotype in X. laevis. NH2-terminal sequencing of the class I protein encoded by the f haplotype showed a single unambiguous sequence of the first 22 amino acids; the deduced protein sequence of the cDNA clone matches precisely to this peptide sequence; 2) Genes that hybridized to the cDNA clone segregated perfectly with the serologically typed MHC in two family studies; and 3) There is a strong conservation of amino acids in the peptide-binding region that have been shown in mammals to dock peptides at their NH2- and COOH-termini. In contrast to all other species that have been examined, there appears to be only one class I locus present in the MHC of X. laevis. Xenopus speciates by allopolyploidization, and there are Xenopus species with different levels of ploidy (2n-12n). Functionally, the MHC has been shown to be "diploidized" in most Xenopus species. As in previous studies with MHC class II and HSP70 probes, there is a trend toward maintaining a diploid number of class la genes in all Xenopus species regardless of their chromosome number, probably accomplished through a deletional mechanism. Thus, there is a strong pressure in Xenopus to maintain very few MHC-linked class I genes, exemplified both by the number of class I genes per MHC haplotype and by the number of class la genes per organism.
Shum
B P
BP
Department of Microbiology and Immunology, University of Miami School of Medicine, FL 33101.
Avila
D
D
Du Pasquier
L
L
Kasahara
M
M
Flajnik
M F
MF
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002874:Chromosome Mapping; D018076:DNA, Complementary / Q000235:genetics / Q000302:isolation & purification; D005805:Genes, MHC Class I; D008040:Genetic Linkage; D015395:Histocompatibility Antigens Class I / Q000737:chemistry / Q000302:isolation & purification; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes; D014981:Xenopus / Q000235:genetics / Q000276:immunology*
D018076:DNA, Complementary; D015395:Histocompatibility Antigens Class I; D015345:Oligonucleotide Probes