Extended Data Fig. 16. Role of Mfd in TCR (see also Movie 3 and Extended Data Fig 15).

We propose that the modest contribution of Mfd to NER (Fig. 4a) is due to its ability to terminate multiple queuing ECs in front of TCRCs, thereby helping to “clean up” space between the TCRCs and DNA lesions at highly expressed genes. a,b, UvrA of the pre-TCRC facilitates Mfd recruitment and/or its transition to a processive translocase (Fig. 1d). Mfd then translocates forward (downstream of the TCRC) to “push” and terminate multiple ECs between the TCRC and CPD (red “TT”). This directionality ensures that Mfd preferentially terminates non-TCR complexes, thereby facilitating TCRC access to the sites of damage. c, TCR proceeds as described in Extended Data Fig. 14. d, Mfd continues to be recruited during the recovery phase, even after most repair has been completed (Fig. 1d). These additional Mfd molecules can now also reactivate backtracked complexes, hence the role of Mfd in facilitating transcription recovery post-UV ¹¹ This model explains why a delay in NER in the Mfd-deficient cells occurs only within the most highly transcribed (most congested) regions and why NER of less actively transcribed regions is indifferent to, or even compromised by, Mfd activity^35,48. It also explains why the overexpression of Mfd is so detrimental to NER (Extended Data Fig. 8b): excessive Mfd would prematurely terminate both ECs and TCRCs, thereby abolishing repair. Finally, the model also explains why mfd cells become more sensitive to genotoxic stress in the presence of Rho inhibitor BCM (Extended Data Fig. 17). Rho, like Mfd, can terminate ECs that obscure the lesion sites from TCRCs⁴⁰. If both termination factors were inactivated, there is no obvious solution to this problem.