Final answer:
The correct answer to the question is (b) UvrABC complex, LexA. This pertains to the bacterial SOS response where DNA damage leads to the degradation of LexA, allowing synthesis of UmuC/UmuD for DNA repair.
Step-by-step explanation:
Single-stranded DNA damage activates an enzyme which degrades a transcriptional repressor, allowing UmuC/UmuD to be synthesized. The correct pairing for the blanks in this statement would be (b) UvrABC complex, LexA. This mechanism is part of the SOS response, a bacterial DNA repair system. Upon DNA damage, the UvrABC complex, specifically UvrA and UvrB proteins, detects distortions in the DNA helix, while UvrC is an endonuclease that cleaves the damaged site. This system is tightly regulated; the LexA protein represses the transcription of SOS genes. When single-stranded DNA accumulates, due to replication stalling at sites of damage, the RecA protein binds to it and facilitates cleavage of LexA, leading to derepression and allowing the expression of SOS genes, including those for UmuC and UmuD, which are involved in translesion DNA synthesis.