Translesion synthesis mechanisms depend on the nature of DNA damage in UV-irradiated human cells

International audience; Ultraviolet-induced 6-4 photoproducts (6-4PP) and cyclobutane pyrimidine dimers (CPD) can be tolerated by translesion DNA polymerases (TLS Pols) at stalled replication forks or by gap-filling. Here, we investigated the involvement of Pol, Rev1 and Rev3L (Pol catalytic subunit) in the specific bypass of 6-4PP and CPD in repair-deficient XP-C human cells. We combined DNA fiber assay and novel methodologies for detection and quantification of singlestranded DNA (ssDNA) gaps on ongoing replication forks and postreplication repair (PRR) tracts in the human genome. We demonstrated that Rev3L, but not Rev1, is required for postreplicative gapfilling, while Pol and Rev1 are responsible for TLS at stalled replication forks. Moreover, specific photolyases were employed to show that in XP-C cells, CPD arrest replication forks, while 6-4PP are responsible for the generation of ssDNA gaps and PRR tracts. On the other hand, in the absence of Pol or Rev1, both types of lesion block replication forks progression. Altogether, the data directly show that, in the human genome, Pol and Rev1 bypass CPD and 6-4PP at replication forks, while only 6-4PP are also tolerated by a Pol-dependent gap-filling mechanism, independent of S phase.