The photocleaving properties of were
The photocleaving properties of 1 were studied by performing ΦX174 photocleavage assays. Light irradiation of the mixtures of increasing amounts of 1 with supercoiled DNA caused>90% supercoiled form relaxation and approximately 5% of linearization for the higher concentrations of 1 (1:10bp and 1:2bp). Results of this study might be used in further development of efficient photoactivated DNA cleavers and DNA intercalators that could bind to specific DNA sequence and in a number of potential biological applications.
Uncited Reference [24b].
The basics of translesion DNA synthesis To promote damaged-DNA replication, TLS relies on the Y-family of DNA polymerases (Polη, Polι, Polκ and Rev1) and on the B-family member, Polζ. Either one polymerase, or two TLS polymerases in concert, operate to achieve the bypass of most types of DNA lesions. As depicted in Fig. 1, while TLS across moderate distortions such as UV-induced cyclobutane pyrimidine dimers (CPDs) depends exclusively on Polη, TLS across bulkier adducts including UV-induced 6-4photoproducts (6-4PPs) comprises at least two specialized polymerases, in which Polζ carries out an extension step that follows the lesion bypass step driven by Y-pols . Specialized DNA polymerases have no proofreading activity, their processivity is low and they are highly mutagenic, with a few exceptions as in the case of the Polη when it bypasses CPDs. Polη deficiency in humans causes the xeroderma pigmentosum variant (XPV), with clinical features that resemble those of defective nucleotide excision repair (NER) . Loss of TLS capability also jeopardizes the survival of whole organisms as demonstrated by the embryonic lethality of Polζ deficiency in mouse models . In addition, the overexpression of some Y-family polymerases has been detected in cancer cells, suggesting that dysregulated TLS may contribute to the genesis of human diseases including cancer and to the resistance to chemotherapy . In general, the extent of DNA synthesis by TLS must be tightly regulated to achieve the best balance between cell survival and mutagenesis. In Escherichia coli the DNA stretches synthesized by TLS were shown to be no longer than ∼60 trans-isomer , suggesting an exquisite control of both loading and removal of specialized polymerases at replication forks.
How and when While Y-family DNA polη, κ and ι are recruited to Proliferating Cell Nuclear Antigen (PCNA) through a PCNA interacting protein (PIP) box, Rev1 utilizes its BRCT domain and/or its PAD domain for localization. All Y-family pols have one or two ubiquitin binding domains (UBD), which consolidates their interaction with PCNA at sites for translesion DNA synthesis, as several genotoxic treatments prompt Rad6/Rad18-dependent PCNA mono-ubiquitination at Lys164. Another mechanism that facilitates specialized pol localization to damaged DNA is the direct recruitment to Rev1, which can act as a scaffold protein , . Conversely, it has been postulated that the removal of the ubiquitin moiety from PCNA facilitates the reverse exchange to replicative pols after lesion bypass . PCNA can also be polyubiquitinated to promote non-TLS events but the biological relevance of such modification is not within the scope of this review , . TLS events can take place at or behind the replication fork  (Fig. 1). The initial characterization of polη indicated a post-replicative mode of action . Following the discovery of PCNA ubiquitination, the replication-coupled mode of TLS dominated the field until experiments performed in Saccharomyces cerevisiae demonstrated that TLS events can be postponed to the G2-phase without affecting cell viability , . Currently, it is accepted that both TLS modes aid DNA replication although it is unclear whether this is an arbitrary choice or if signals arising from the DNA lesion or its surroundings are variables that affect such a decision. The post-replicative mode is particularly supported by a paradigm-breaking model that proposes discontinuity of DNA replication in both DNA strands following replication stress , . Interestingly, a novel specialized polymerase with primase activity, PrimPol could be essential for the onset of such discontinuous DNA synthesis events , . It is therefore possible that discontinuous DNA synthesis in both strands and post-replicative TLS are frequent events.