DNA damage occurs frequently in all organisms as a consequence of both endogenous metabolic processes and exogenous DNA-damaging agents. Organisms have evolved several repair and tolerance mechanisms that remove and tolerate DNA damage and coordinate cell cycle progression. In the S phase of the cell cycle, replication stress occurs when an active fork encounters DNA lesions or proteins that are tightly bound to the DNA. These obstacles pose a threat to the integrity of the replication fork and are thus a potential source of genome instability that can contribute to tumorigenesis and aging in humans. Confronted with this risk, cells have developed fundamental DNA damage response mechanisms in order to faithfully complete DNA replication. Our group uses Escherichia coli and budding yeast as model systems to examine the cellular responses to DNA damage, with a special emphasis on the mechanisms that maintain genome integrity during DNA replication.
(1) Molecular mechanisms of the post-replication repair (PRR) pathway.
The UV spectrum present in sunlight is a potent and ubiquitous carcinogen that is responsible for most of the skin cancers in humans. In the natural environment, organisms are exposed to chronic low-dose UV light (CLUV), as opposed to the acute high doses that are commonly used in laboratory experiments. Hence, to clarify the biological significance of specific DNA damage response pathways, understanding the cellular response to CLUV exposure is an important approach that complements the more traditional laboratory approaches. An experimental assay that was recently developed to analyze CLUV-induced DNA damage responses was used to show that the PCNA polyubiquitination-dependent error-free PRR pathway plays a critical role in tolerance to CLUV exposure. We are currently analyzing in more detail the role(s) the error-free PRR pathway plays upon CLUV exposure. In addition, we are examining the molecular and structural foundations of PRR functions by combining genetic, biochemical and structural approaches.
Fig. 1: DNA damage tolerance pathway. The RAD6 pathway consists of at least two different Rad6-Rad18 dependent mechanisms, which include translesion DNA synthesis and Rad5-dependent template switching.