FIG. 1.

Exonuclease removal of dCMP with primers with or without frayed termini. The wild-type holoenzyme (100 nM) was preincubated with 90 nM 26-mer-45-mer terminated with dCMP correctly base paired (○) or frayed at the 3′ terminus (eight mismatches; •) and mixed with Mg²⁺ to start the reaction. The concentration of the substrate (26-mer primer) as a function of time was fit to a double exponential equation. Reactions of both the correctly base paired and frayed DNA were biphasic. The correctly paired dCMP DNA was excised at rates of 0.86 ± 0.35 s⁻¹ and 0.016 ± 0.004 s⁻¹ with amplitudes of 21 ± 3 nM and 70 ± 3 nM, respectively. The frayed dCMP-terminated DNA was excised at rates of 1.6 ± 0.5 s⁻¹ and 0.1 ± 0.07 s⁻¹ with amplitudes of 60 ± 10 nM and 25 ± 10 nM, respectively.