We appreciate the reviewer’s comment and have repeated the experiments as requested. We now space the two biotinylated T residues by 6 bp on each strand (this is the spacing used on the lagging strand of the first submission). The results are presented in Figure 5B, C, E and Figure 6A (for lagging blocks) and Figure 5—figure supplement 1 and Figure 6B (for leading blocks). Figure 5 has been expanded by taking one figure from the supplement and adding it to the main figure (now parts D and E). The closer spaced blocks on the leading strand are more inhibitory than the 14bp spacing in the earlier manuscript, and therefore the more exact comparison was important to the outcome, and we thank the reviewers for this suggestion.