FIGURE 2.

(A) Segregation of the sister chromatids during chromosome dimer resolution in E. coli. The illustration depicts the central part of a dividing cell in the final steps of chromosome segregation. The closing division septum, the motor domain αβ of FtsK_C (yellow hexameric ring), the unstructured linker domain FtsK_L (Blue ribbon), the KOPS sequences and the XerCD/dif synaptic complex are indicated. Concatenation prevents proper migration of the nascent chain of DNA; the origin regions move toward their respective cell poles, but the rest of the knotted DNA is stretched across and behind the septum. (B) FtsK_C loads onto the KOPS sequences in an oriented manner and translocates toward XerCD/dif complexes. FtsK translocation allows it to reach the XerCD/dif complexes and bring them into proximity; as a consequence, the γ-subdomain of the FtsK_C region activates XerD (Orange sphere) to perform the first strand cleavage. Then, XerC (Green sphere) mediates the second strand cleavage, allowing separation of the sister chromatids from each other. (C) Illustration of the SSR mechanism used by tyrosine recombinases: The OH group of the active residue tyrosine attacks the scissile phosphate forming a 3′-covalent phosphotyrosyl enzyme–DNA covalent intermediate and a free 5′-hydroxyl end. The covalent intermediate is attacked in turn by the other 5′- end to reverse the cleavage reaction and obtain a recombinant product.