Final answer:
The UvrAB complex is responsible for recognizing a thymine dimer in the nucleotide excision repair system.
Step-by-step explanation:
In the process of nucleotide excision repair, the proteins responsible for recognizing the distortion caused by a thymine dimer and initiating repair are UvrA and UvrB, collectively known as UvrAB complex. When DNA is exposed to UV light, adjacent thymines can bond together, forming a thymine dimer, which induces a bulge in the DNA helix. The UvrAB complex scans the DNA for such distortions. Upon recognition of a thymine dimer, UvrA dissociates and UvrB recruits UvrC, which makes cuts on both sides of the dimer. Subsequently, UvrD (a helicase) removes the damaged DNA. The correct nucleotides are then replaced by DNA Polymerase I, and the strand is sealed by DNA Ligase. Therefore, the answer to the question 'In the nucleotide excision repair system, which of the following proteins is responsible for recognizing a thymine dimer to be repaired?' is (a) UvrAB.