Structure and expression of the human θl globin gene

Abstract

The recently identified θ-globin gene subfamily conists of the θ1-globin gene located downstream from the α1 -globin gene1–5, and several other members including at least one truncated, processed pseudogene ψθ2 (refs 1,6). Unlike the θ1 -globin genes of the rabbit3 and galago4, the structure of these genes in the orang-utan and baboon1,5 and their flanking regions show no apparent defects that would prevent their expression. Both θ1 -globin genes are split into three exons with the potential to code for a polypeptide of length 141 amino acids. Besides differing by 26% in replace-ment-site substitutions, the θ1 and α1 -globin genes of the orang-utan and baboon also differ in their promoter structures, in the use of TGA versus TAA as the termination codon, and in the use of AGTAAA versus AATAAA as the polyadenylation signal1,5. In contrast, the two θ1,-globin genes from primates only differ by 1.7% in the replacement-site substitutions5. Here we present the complete DNA sequence of a cloned θ1 -globin gene of humans, and show that it contains no apparent defects that would abolish its expression. Furthermore, by primer extension of single-stranded oligonucleotide probes, we show that the θ1 -globin gene of humans is transcribed in an erythroleukemia cell line K562. Three messenger RNA species were detected, with 5′-ends mapping to ∼70 base pairs (bp) downstream from a TAT A promoter sequence, at 8 bp downstream from a GGGCGG promoter sequence and at 40 bp upstream from the ATG inititrion codon, respectively. Haemin treatment of the K562 cells slightly enhances the level of the longest θ1-transcript. Our results provide strong evidence that the θ1 -globin gene of humans is transcriptionally active in cells of erythroid orign, and suggests the presence of a functional θ1-polypeptide in specific cells, possibly those of early erythroid tissue.