Residual current histograms due to single nucleotide substitutions in an otherwise poly-dA hairpin tail. (*A*) For comparison purposes the top panel summarizes the averaged Gaussian mean and width of *I*_{res} of the homopolymer hairpin tails at 180 mV (with fit values in text). Colors of black, blue, red, and green are used to help separate effects on *I*_{res} due to dA, dG, dC, and dT, respectively. The residual current markedly changes with the position, *x*, of a single nucleotide dN_{x}, within an otherwise poly-dA homopolymer hairpin (hp) tail. (*B*) When the nucleotide substitution is adjacent to the double-stranded terminus, *x* = 1, the residual current deviates to resemble the hompolymer values associated with the substituted nucleotide. The dT_{1} substitution most closely resembles *I*_{dT}. (*C*) At *x* = 2, the nucleotide substitution also causes residual current is closer to the homopolymer associated with the substituted nucleotide. The dC_{2} substitution is closest to *I*_{dC}. (*d*) With any substitution at *x* = 3, *I*_{res} is only slightly different from*I*_{dA}, suggesting that MspA is primarily sensitive to the two nt after the hairpin duplex. A dG_{x} substitution at *x* = 1, 2, or 3, does not significantly influence the current, as may be expected given the relative closeness of *I*_{dG} and *I*_{dA}.

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