Nucleotide excision repair in E. coli. The NER pathway uses several proteins, including UvrA (red), UvrB (purple), and UvrC (green), that recognize the lesion and make incisions on either side, allowing UvrD (helicase II) to displace a section of lesion-containing DNA. The single-strand gap is filled in by Pol I, and the DNA is sealed by ligase. A transcription-coupled repair (TCR) path can also be taken, in which RNA polymerase stalls at the lesion on the coding strand (not shown). After the RNA polymerase is displaced, the reaction proceeds as shown here, using UvrA–D, Pol I, and ligase.