DNA Replication in the Face of (In)surmountable Odds

Abstract

"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material… Whether a specific enzyme is required to carry out the polymerization… remains to be seen." 1 ABSTRACTWe describe here a model for sequential recruitment of various enzymatic systems that maintain DNA replication fidelity in cells with damaged bases, especially those formed by ultraviolet (UV) irradiation. Systems of increasing complexity but decreasing fidelity are recruited to restore replication of damaged DNA. The first and most accurate response is nucleotide excision repair (NER) that is cell cycle-independent; next come various delaying cell cycle checkpoints that provide an extended time window for NER. These delay the onset of the S phase at the G 1 /S boundary, and inhibit the initiation of individual replicating units (replicons and clusters of replicons) within the S phase. When checkpoints fail to operate completely, DNA replication forks must negotiate damage and the loss of coding information on the parental DNA strands. Replication can be resumed using bypass polymerases, or alternative bypass mechanisms. Finally, if all else fails, replication forks may degrade to double strand breaks and recombinational processes then allow their reconstruction. A network of signaling kinases modulates the efficiency of many damage responsive proteins to tailor their activities and subcellular localizations by phosphorylation and dephosphorylation.

Activation of the Fanconi Anemia/BRCA Pathway and Recombination Repair in the Cellular Response to Solar Ultraviolet Light

“…DNA polymerase D, mutated in xeroderma pigmentosum variant, serves that function for DNA photoproducts (46,47). Recent reports indicate that repair recombination might be required for the resolution of stalled replication forks at DNA photoproducts should they occur when nucleotide excision repair and translesional DNA synthesis have failed (48,49). Replication forks stalled at DNA photoproducts have been suggested to degrade to DSBs, in particular, in xeroderma pigmentosum variant cells (49,50).…”

“…Recent reports indicate that repair recombination might be required for the resolution of stalled replication forks at DNA photoproducts should they occur when nucleotide excision repair and translesional DNA synthesis have failed (48,49). Replication forks stalled at DNA photoproducts have been suggested to degrade to DSBs, in particular, in xeroderma pigmentosum variant cells (49,50). Although we excluded the role of directly UV-induced DSBs (see Discussion above), it remains a possibility that such degraded replication forks mediate the activation of the FA/BRCA pathway.…”

Rad18 Regulates DNA Polymerase κ and Is Required for Recovery from S-Phase Checkpoint-Mediated Arrest

“…This result is consistent with the presence of persistent stalled forks resulting from BPDE treatment in a Rad18-null background. Stalled replication forks are prone to breakage if left unprotected (10). If Rad18/Pol-mediated TLS contributes to protection or elimination of stalled forks, we expected that defective TLS of BPDE-adducted DNA might result in increased formation of DNA DSBs.…”

p53 suppression overwhelms DNA polymerase η deficiency in determining the cellular UV DNA damage response

“…In XP-V cells that lack the translesional DNA Pol η, UV damage to DNA causes an asymmetric replication fork arrest due to a replication block on the leading strand of DNA [41][42][43][44][45][46]. These structures trigger the activation of intra-S-phase checkpoints [44].…”

“…These structures trigger the activation of intra-S-phase checkpoints [44]. These arrested replication forks contain long (several kB) regions of single-stranded DNA, which are covered by RPA which in turn binds p53, ATRIP/ATR and other proteins [42,[47][48][49].…”