Fig. 7
Mechanisms for Chi-stimulated recombination by RecBCD in DSB repair. (A) A mechanism based on biochemical experiments done at ATP and Mg2+ concentrations where the enzyme has helicase and moderate nuclease activity (Dillingham and Kowalczykowski 2008). (a) A RecBCD heterotrimer binds to a blunt, or nearly blunt, end of dsDNA. (b) The enzyme catalyzes ATP hydrolysis, unwinds the dsDNA, and degrades both unwound strands, although to different products. (c) RecBCD undergoes a change of state when it encounters a Chi sequence (green rectangle) oriented as shown in the figure. (d) RecBCD continues to unwind the dsDNA beyond Chi but degradation of the 3′-terminated strand is suppressed. The Chi-modified RecBCD loads RecA protein onto this DNA strand to form a RecA-ssDNA filament which can initiate strand exchange reactions such as shown in previous figures. (B) Alternative mechanism of Chi-stimulated recombination by RecBCD. The mechanism is based in part on biochemical experiments done at ATP and Mg2+ concentrations where the enzyme has helicase activity but very little nuclease activity (Smith 2012). (e) A RecBCD heterotrimer binds to a blunt, or nearly blunt, end of dsDNA. (f) The enzyme catalyzes ATP hydrolysis and unwinds the dsDNA but the conditions are such that the DNA is not degraded. (g) RecBCD nicks the Chi-containing strand close to the Chi sequence. (h) RecBCD continues to unwind the DNA beyond Chi and loads RecA onto the 3′-terminated strand. RecA can then initiate strand exchange with a homologous DNA molecule