Figure 7.

Multiple NHEJ pathways in mycobacteria. Two alternative pathways of Ku-dependent blunt DSB repair are depicted in A and B. (A) In wild-type cells, the NHEJ apparatus is assembled at the DSB via contacts between the Ku homodimer (which encircles the DNA as a toroidal clamp) and the POL domain of LigD. The choice between faithful and unfaithful DSB repair reflects a kinetic balance between immediate sealing by the LIG domain (in the context of a synaptic complex with the other DSB end), nontemplated single nucleotide addition by the LigD POL domain, and end resection by an unknown mycobacterial nuclease. (B) Absent the LigD LIG domain (or when LigD ligase activity is suppressed), LigC provides backup sealing activity for blunt and 5′ overhang NHEJ. Because all blunt/5′overhang NHEJ requires the POL domain, we surmise that LigC interacts with the Ku–POL complex. Virtually all NHEJ events that proceed via this pathway are mutagenic. (C) M. smegmatis has a Ku-independent NHEJ pathway specific to sticky 3′ overhang DSBs that is nonmutagenic and, being active in ΔligB/C/D cells, apparently relies on LigA. It is not known if other accessory factors participate in this novel pathway.