see legend to figure 1 for methods summary ; full methods can be found in the supplementary information .
 transcription - coupled repair employs components of the transcription machinery to identify dna lesions and initiate their repair . 
 these repair pathways are complex so their mechanistic features remain poorly understood . 
 bacterial transcription - coupled repair is initiated when rna polymerase stalled at a dna lesion is removed by mfd , an atp - dependent dna translocase [ 13 ] . 
 here we use single - molecule dna nanomanipulation to observe the dynamic interactions of e. coli mfd with rna polymerase elongation complexes stalled by a cyclopyrimidine dimer or by nucleotide starvation . 
 we show that mfd acts by catalyzing two irreversible , atp - dependent steps with different structural , kinetic , and mechanistic features . 
 mfd remains bound to the dna in a long - lived complex that could serve as a marker for sites of dna damage , directing assembly of subsequent dna repair factors . 
 these results provide a framework for considering the kinetics of transcription - coupled repair in vivo , and open the way to reconstruction of complete dna repair pathways at single - molecule resolution . 
Methods Summary
Supplementary Material