Sequence divergence in the 3' untranslated regions of human zeta- and alpha-globin mRNAs mediates a difference in their stabilities and contributes to efficient alpha-to-zeta gene development switching.
The developmental stage-specific expression of human globin proteins is characterized by a switch from the coexpression of zeta- and alpha-globin in the embryonic yolk sac to exclusive expression of alpha-globin during fetal and adult life. Recent studies with transgenic mice demonstrate that in addition to transcriptional control elements, full developmental silencing of the human zeta-globin gene requires elements encoded within the transcribed region. In the current work, we establish that these latter elements operate posttranscriptionally by reducing the relative stability of zeta-globin mRNA. Using a transgenic mouse model system, we demonstrate that human zeta-globin mRNA is unstable in adult erythroid cells relative to the highly stable human alpha-globin mRNA. A critical determinant of the difference between alpha- and zeta-globin mRNA stability is mapped by in vivo expression studies to their respective 3' untranslated regions (3'UTRs). In vitro messenger ribonucleoprotein (mRNP) assembly assays demonstrate that the alpha- and zeta-globin 3'UTRs assemble a previously described mRNP stability-determining complex, the alpha-complex, with distinctly different affinities. The diminished efficiency of alpha-complex assembly on the zeta 3'UTR results from a single C-->G nucleotide substitution in a crucial polypyrimidine tract contained by both the human alpha- and zeta-globin mRNA 3'UTRs. A potential pathway for accelerated zeta-globin mRNA decay is suggested by the observation that its 3'UTR encodes a shortened poly(A) tail. Based upon these data, we propose a model for zeta-globin gene silencing in fetal and adult erythroid cells in which posttranscriptional controls play a central role by providing for accelerated clearance of zeta-globin transcripts.
Russell
J E
JE
Department of Genetics, University of Pennsylvania School of Medicine, Philadelphia 19104, USA. jeruss@mail.med.upenn.edu
Morales
J
J
Makeyev
A V
AV
Liebhaber
S A
SA
D000225:Adenine / Q000378:metabolism; D000818:Animals; D001483:Base Sequence; D004912:Erythrocytes / Q000166:cytology; D004920:Erythropoiesis; D018507:Gene Expression Regulation, Developmental; D005812:Genes, Switch; D005914:Globins / Q000235:genetics*; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011061:Poly A / Q000378:metabolism; D014176:Protein Biosynthesis; D012323:RNA Processing, Post-Transcriptional; D012333:RNA, Messenger / Q000378:metabolism*
D012333:RNA, Messenger; D011061:Poly A; D005914:Globins; D000225:Adenine