Isolation and nucleotide sequence of the rabbit globin gene cluster psi zeta-alpha 1-psi alpha. Absence of a pair of alpha-globin genes evolving in concert.

Abstract

A cloned 13.3-kilobase (kb) region of rabbit genomic DNA contains a cluster of alpha-like globin genes arranged 5'-psi zeta-(3.6 kb)-alpha 1-(2.2 kb)-psi alpha-3'. Genomic blot hybridization data show that this is the major alpha-like globin gene cluster in rabbits, although a second alpha-globin gene (alpha 2) is also detected. Repetitive sequences from the C family of short repeats flank the psi zeta gene, and a new repetitive element, the F repeat is located 3' to psi alpha. The sequence was determined for a 4024-base pair (bp) segment that extends from 149 bp 5' to the cap site of alpha 1 to 207 bp 3' to psi alpha. Gene alpha 1 is functional and encodes one of the major allelic variants of rabbit alpha-globin. This gene has very short introns (77 bp in intron 1 and 83 bp in intron 2) and an unusual ATA box in the 5' flanking region (CTTAAA), which does function to promote transcription by RNA polymerase II in a cell-free system. Short (4 or 9 bp) G + C-rich repeats are interspersed throughout the flanking regions and the introns. Gene psi alpha cannot encode a globin polypeptide, and it is probably inactive. This is shown by the absence of a normal globin gene promoter, the replacement of the 5' untranslated sequence by tandem repeats of the sequence GCCCGCCGC, frameshift deletions in exon 2, and the modification of the polyadenylation signal to AGTAAA. The intergenic region between alpha 1 and psi alpha is very G + C rich (65.7% G + C) and contains many short, tandem repeats. Gene psi zeta hybridizes specifically to a human zeta-globin gene probe, but a partial sequence reveals frameshift mutations that probably make psi zeta a pseudogene. Mammalian alpha-globin gene clusters vary in the presence or absence of pseudogenes, and if present, the position of the pseudogene differs in various gene clusters. Among the mammalian alpha-like gene clusters so far analyzed, the rabbit gene cluster is unique in the absence of duplicated alpha-globin genes that are undergoing concerted evolution.