Practice question

RecA forms helical nucleoprotein filament on single-stranded DNA generated at lesion-stalled forks, with ATP stabilizing active extended conformation termed RecA*. Activated filament functions as coprotease enhancing intrinsic autoproteolytic activity of LexA, UmuD, and phage lambda repressor. LexA self-cleavage occurs between alanine 84 and glycine 85 within flexible linker connecting N-terminal DNA-binding domain and C-terminal dimerization domain. Cleavage splits repressor, rendering fragments unable to dimerize and bind SOS box. Consequently operons derepress without RecA directly cutting LexA. Signal transduction links recombinational sensor to transcriptional response essential for stress adaptation.