Ubiquitination of PCNA and the polymerase switch in human cells.

Replicative DNA polymerases are blocked by damage in the template DNA. To get past this damage, the cell employs specialized translesion synthesis (TLS) polymerases, which have reduced stringency and are able to bypass different lesions. For example, DNA polymerase eta (poleta) is able to carry out TLS past UV-induced cyclobutane pyrimidine dimers. How does the cell bring about the switch from replicative to TLS polymerase? We have shown that, in human cells, when the replication machinery is blocked at DNA damage, PCNA, the sliding clamp required for DNA replication, is mono-ubiquitinated and that this modified form of PCNA has increased affinity for poleta. This provides a mechanism for the polymerase switch. In this Extra-View, we discuss the possible signals that might trigger ubiquitination of PCNA, whether PCNA becomes de-ubiquitinated after TLS has been accomplished and the role of the hREV1 protein in TLS. We point out some apparent differences between mechanisms in Saccharomyces cerevisiae and human cells.