LOOP-TAG: massively-parallel measurement of length-dependent protein-mediated DNA looping probabilities by tagmentation in vitro.

We are interested in DNA stiffness in vitro and in vivo . Classical approaches for measuring this parameter in vitro involve tedious T4 DNA ligase-mediated cyclization kinetics experiments performed for one DNA fragment length at a time to establish effective local end-end concentrations ( J -factors). Protein-mediated DNA looping is a more relevant biological reaction. An efficient approach to determine length-dependent protein-mediated DNA J - loop values in vitro has been lacking. We describe LOOP-TAG, a massively-parallel method. We leverage intramolecular DNA looping and tagmentation catalyzed by Tn5 transposase (Tnp) tethered to the terminus of a ∼1,000-bp bead-bound duplex DNA. We detected loops terminating with Tnp. Counts of the resulting DNA looping-dependent bead-bound tagmentation products from deep sequencing determine DNA loop length probabilities at single-base pair resolution for all bead-bound fragments. When normalized to DNA fragment length probabilities from tagmentation using known concentrations of free Tnp, length-dependent J - loop values are directly determined. We demonstrate LOOP-TAG for Tnp tethered in two ways and in the presence of DNA-kinking architectural DNA binding protein Nhp6A. Intermediate length J-loop values are consistent with expectations of wormlike chain theory after accounting for strong sequence-dependence of Tnp. DNA loops are shifted to smaller sizes in the presence of Nhp6A, as predicted. (200/200 words).