File:Chi recomb model (for Wiki).tif

English: Molecular model for the RecBCD pathway of recombination. This model is based on reactions of DNA and RecBCD with ATP in excess over Mg2+ ions. Step 1: RecBCD binds to a double-stranded DNA end. Step 2: RecBCD unwinds DNA. RecD is a fast helicase on the 5’-ended strand, and RecB is a slower helicase on the 3’-ended strand (that with an arrowhead). This produces two single-stranded (ss) DNA tails and one ss loop. The loop and tails enlarge as RecBCD moves along the DNA. Step 3: The two tails anneal to produce a second ss DNA loop, and both loops move and grow. Step 4: Upon reaching the Chi hotspot sequence (5’ GCTGGTGG 3’; red dot) RecBCD nicks the 3’-ended strand. Further unwinding produces a long 3’-ended ss tail with Chi near its end. Step 5: RecBCD loads RecA protein onto the Chi tail. At some undetermined point, the RecBCD subunits disassemble. Step 6: The RecA-ssDNA complex invades an intact homologous duplex DNA to produce a D-loop, which can be resolved into intact, recombinant DNA in two ways. Step 7: The D-loop is cut and anneals with the gap in the first DNA to produce a Holliday junction. Resolution of the Holliday junction (cutting, swapping of strands, and ligation) at the open arrowheads by some combination of RuvABC and RecG produces two recombinants of reciprocal type. Step 9: The 3’ end of the Chi tail primes DNA synthesis, from which a replication fork can be generated. Resolution of the fork at the open arrowheads produces one recombinant (non-reciprocal) DNA, one parental-type DNA, and one DNA fragment. Based on Figure 1 in Amundsen SK et al (December 15, 2007). “Intersubunit signaling in RecBCD enzyme, a complex protein machine regulated by Chi hot spots”. Genes Dev: 21(24): 3296–3307. doi: 10.1101/gad.1605807