(A) Mfd repair function. 1. An RNA polymerase is blocked by UV damage in DNA. 2. Mfd binds to RNA polymerase and DNA, and moves along the DNA to remove the RNA polymerase from the DNA. 3. By removing RNA polymerase, RNA is released and Mfd continues on its way, recruiting more DNA repair factors.
(B) Mfd mutagenesis function. 1. When an RNA polymerase travels through the DNA to read its inscriptions, it causes the accumulation of left-handed braids (corresponding to overtwisted DNA) ahead of the protein and right-handed braids (corresponding to undertwisted DNA behind the protein. 2. When Mfd interacts with an active RNA polymerase and binds to DNA, the right-handed braids are concentrated in the small area between Mfd and the RNA polymerase, causing the DNA to unwind. RNA can then be inserted into the double helix and form a DNA:RNA hybrid, the R-loop.