Molecular Genetics

Facts

• Among hemoglobins with abnormal function, several have a or b chains slightly longer at the C-terminus than normal chains. The longer chains were thought due to defects in termination.
• Some were due to mutations in stop codons (Hb Constant Spring).
• Others were due to frameshifts due to deletion (Hb Wayne) or insertion (HbTak, Hb Cranston (not shown)
• Nonsense mutations and frameshift mutations leading to truncated globin chains were not found among the mutations producing abnormal globins.

As nThrValLysLeuGluProArgStop
AA-UACCGUUAAGCUGGAGCCUCGGUAG Hba Wayne
AAAUACCGUUAAGCUGGAGCCUCGGUAG Hba normal
LysTyrArgStop

LysTyrArgGlnAlaGlyAlaSerVal...
AAAUACCGUCAAGCUGGAGCCUCGGUAG Hba Constant Spring
AAAUACCGUUAAGCUGGAGCCUCGGUAG Hba normal
LysTyrArgStop

HisLysTyrHisThrLysLeuAlaPheLeuLeuPheAsnPheTyrStop
CACAAGUAUCACACUAAGCUCGCUUUCUUGCUGUCCAAUUUCUAUUAAAG  Hbb Tak
CACAAGUAUCA--CUAAGCUCGCUUUCUUGCUGUCCAAUUUCUAUUAAAG  Hbb normal
HisLysTyrHis  Stop

Interpretations

• Polypeptides with slightly longer C-terminal sequences or with slightly altered C-terminal sequences are often partly functional.
• Substitution or frameshift mutations that cause the by-pass of the normal stop codon do not drastically increase the length of the chain because non-coding regions contain abundant termination codons in all three reading frames.
• Nonsense mutations or frameshift mutations other than near the C-terminus produce totally non-functional proteins and are thus null mutations. Such mutations in globin genes produce thalassemias (insufficient amounts of globin chains produced).

Further information

• Test are available for abnormal globin chains.
• Termination codons that arise from mutation are in a different context than wild-type termination codons. These nonsense codons trigger degradation of the mRNA bearing them. This degradation is termed nonsense-mediated decay and is part of a larger system that removes aberrant mRNAs, mRNA surveillance (ref). The protein factor Upf1p is a slow ATPase. If translation stops before ATP is hydrolyzed, the mRNA is protected. Hydrolysis of the ATP before termination leads to a conformational change that leads to mRNA instability. Presumably, termination codons in an improper context cause an extended pause before termination actually occurs.
• mRNA's with longer open reading frames, such as that for the Constant Spring Hb, may also be less stable.  In the case of Constant Spring mRNA, translation displaces a complex (alpha-complex) that binds to the normally 3' non-translated region and slows the deadenylation process (ref).  More rapid deadenylation leads to less mRNA stability.

This is page 2413 of Molecular Genetics by Ulrich Melcher, © 1997-2000, 2002, 2005