Figure 5.

Read-ahead uracil detector. (a) Blocking of a running polymerase actively extending a primer, by a second polymerase bound to deoxyuridine in the single-stranded segment of the template, would give a gap between the last incorporated nucleotide (arrowed) and the template deoxyuridine (U), larger than the “footprint” of the polymerase. (b) In the read-ahead model, specific stalling of a running polymerase at a deoxyuridine upstream in the single-stranded segment of the template would give a gap between the last incorporated nucleotide (arrowed) and the template deoxyuridine (U), smaller than the “footprint” of the polymerase, as is observed.