Practice question

Within RecBCD trimer, RecC subunit does not possess catalytic domains but functions as scanner for recombination hotspots. Crystal structures show tunnel where duplex DNA threaded before reaching helicase motors, containing specific pocket recognizing Chi octamer 5'-GCTGGTGG-3' in single-stranded form. Recognition occurs when RecBCD unwinds DNA, Chi strand exposed interacts with RecC residues. Binding triggers conformational rearrangement transmitted through flexible linker changing RecB and RecD motor speeds and attenuating nuclease, effectively transforming enzyme from destructive nuclease to RecA loading machinery, explaining RecC mutations abolishing Chi response without affecting helicase activity.