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Title: Effects of the A.T/T.A degeneracy of pyrrole--imidazole polyamide recognition in the minor groove of DNA.
Author: White S, Baird EE, Dervan PB.
Journal: Biochemistry; 1996 Sep 24; 35(38):12532-7. PubMed ID: 8823190.
Abstract:
Pairing rules have been developed to predict the sequence specificity of minor groove binding polyamides containing pyrrole (Py) and imidazole (Im) amino acids. An Im/Py pair distinguishes G.C from C.G and both of these from A.T/T.A base pairs. A Py/Py pair appears not to distinguish A.T from T.A base pairs. To test the extent of this degeneracy, the affinity and binding orientation of the hairpin polyamide ImPyPy-gamma-PyPyPy-beta-Dp were measured for eight possible five base pair 5'-TG(A,T)(3)-3' match sites. Affinity cleavage experiments using a polyamide with an EDTA.Fe(II) moiety at the carboxy terminus, ImPyPy-gamma-PyPyPy-beta-Dp-EDTA.Fe(II), are consistent with formation of an oriented 1:1 hairpin polyamide complex at all eight 5'-TG(A,T)(3)-3' binding sites [20 mM HEPES, 200 mM NaCl, 50 mg/ml glycogen, pH 7.0, 22 degrees C, 5 mM DTT, 1 mM Fe(II)]. Quantitative DNase I footprint titration experiments reveal that ImPyPy-gamma-PyPyPy-beta-Dp binds all eight 5'-TG(A,T)(3)-3' target sites with only a 12-fold difference in the equilibrium association constants between the strongest site, 5'-TGTTT-3' (Ka = 2.1 x 10(8) M-1), and the weakest site, 5'-TGAAT-3' (Ka = 1.8 x 10(7) M-1) (10 mM Tris.HCl, 10 mM KCl, 10 mM MgCl2, 5 mM CaCl2, pH 7.0, 22 degrees C). This relatively small range indicates that the Py/Py pair is approximately degenerate for recognition of A,T base pairs, affording generality with regard to targeting sequences of mixed A.T/T.A composition.

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