Hairpin-duplex equilibrium reflected in the A→B transition in an undecamer quasi-palindrome present in the locus control region of the human β-globin gene cluster

Abstract

Our recent work on an A→G single nucleotide polymorphism (SNP) at the quasi-palindromic sequence d(TGGGG[A/G]CCCCA) of HS4 of the human β-globin locus control region in an Indian population showed a significant association between the G allele and the occurrence of β-thalassemia. Using UV-thermal denaturation, gel assay, circular dichroism (CD) and nuclease digestion experiments we have demonstrated that the undecamer quasi- palindromic sequence d(TGGGGACCCCA) (HPA11) and its reported polymorphic (SNP) version d(TGG GGGCCCCA) (HPG11) exist in hairpin-duplex equilibria. The biphasic nature of the melting profiles for both the oligonucleotides persisted at low as well as high salt concentrations. The HPG11 hairpin showed a higher T_m than HPA11. The presence of unimolecular and bimolecular species was also shown by non-denaturating gel electrophoresis experiments. The CD spectra of both oligonucleotides showed features of the A- as well as B-type conformations and, moreover, exhibited a concentration dependence. The disappearance of the 265 nm positive CD signal in an oligomer concentration-dependent manner is indicative of an A→B transition. The results give unprecedented insight into the in vitro structure of the quasi-palindromic sequence and provide the first report in which a hairpin-duplex equilibrium has been correlated with an A→B interconversion of DNA. The nuclease-dependent degradation suggests that HPG11 is more resistant to nuclease than HPA11. Multiple sequence alignment of the HS4 region of the β-globin gene cluster from different organisms revealed that this quasi-palindromic stretch is unique to Homo sapiens. We propose that quasi-palindromic sequences may form stable mini- hairpins or cruciforms in the HS4 region and might play a role in regulating β-globin gene expression by affecting the binding of transcription factors.