Ubiquitin, SUMO and the Maintenance of Genome Stability

Figure 1: PCNA modifications govern the processing of DNA damage during replication. Ubiquitin, black circles; SUMO, white circles marked “S”; PCNA, yellow.

DNA is susceptible to a variety of insults from exogenous and endogenous sources. Dealing with damage during DNA replication is particularly important because the replication machinery cannot cope with defective templates. In order to avoid a permanent arrest in this situation, cells have developed mechanisms of damage bypass. In contrast to DNA repair systems, which usually rely on the excision and subsequent re-synthesis of the damaged region to restore the original sequence information, DNA damage bypass mechanisms allow the replication machinery to “skip over” lesions without their actual removal. They ensure the completion of DNA replication on damaged templates and are therefore essential for survival of a cell in the presence of genotoxic agents. As lesion bypass is often associated with damage-induced mutations, however, the pathway is also a potential source of genome instability in itself and therefore needs to be tightly controlled. Our research aims at understanding the mechanisms and signals by which the ubiquitin and SUMO systems promote damage tolerance and limit the accumulation of unwanted mutations.

DNA damage bypass, also called postreplication repair, is controlled via posttranslational modification of the replication factor PCNA (Figure 1). Whereas monoubiquitylation activates translesion synthesis by damage-tolerant DNA polymerases, polyubiquitylation via lysine 63-linked chains is required for an error-free pathway of damage avoidance possibly involving template switching. In budding yeast, PCNA is also sumoylated in a damage-independent manner, which prevents unwanted recombination events during replication and allows the ubiquitin-dependent reactions to proceed upon replication fork stalling.

Using a combination of molecular and cellular biology, biochemistry and classical genetics, we are investigating:

  • the factors that influence the choice of bypass pathway and determine the efficiency and accuracy of damage processing,
  • the molecular signals that trigger the each of the modifications at the appropriate time within the cell,
  • the mechanisms by which the ubiquitin and SUMO conjugation factors recognise and modify their substrates, and
  • the coordination of damage bypass with DNA replication, chromatin dynamics and other pathways of genome maintenance.

We are using mainly budding yeast, Saccharomyces cerevisiae, as a model organism for which sophisticated genetic tools are available, but some of our projects involve mammalian cell culture or Xenopus laevis egg extracts.