Regulation of Rad6/Rad18 Activity During DNA Damage Tolerance

Abstract

Replicative polymerases (pols) cannot accommodate damaged template bases, and these pols stall when such offenses are encountered during S phase. Rather than repairing the damaged base, replication past it may proceed via one of two DNA damage tolerance (DDT) pathways, allowing replicative DNA synthesis to resume. In translesion DNA synthesis (TLS), a specialized TLS pol is recruited to catalyze stable, yet often erroneous, nucleotide incorporation opposite damaged template bases. In template switching, the newly synthesized sister strand is used as a damage-free template to synthesize past the lesion. In eukaryotes, both pathways are regulated by the conjugation of ubiquitin to the PCNA sliding clamp by distinct E2/E3 pairs. Whereas monoubiquitination by Rad6/Rad18 mediates TLS, extension of this ubiquitin to a polyubiquitin chain by Ubc13-Mms2/Rad5 routes DDT to the template switching pathway. In this review, we focus on the monoubiquitination of PCNA by Rad6/Rad18 and summarize the current knowledge of how this process is regulated.

Stability of the human polymerase δ holoenzyme and its implications in lagging strand DNA synthesis

“…During S-phase, the template DNA may be compromised by modifications that replicative pols cannot accommodate (2). For instance, the lagging strand pol δ in humans cannot replicate past common byproducts of lipid peroxidation and synthesis on the afflicted Okazaki fragment abruptly stops (44,45).…”

“…In humans, TLS involves the monoubiquitination of PCNA and at least seven TLS pols with varying fidelities. However, remarkably low error rates are observed in vivo after exposure to various DNA-damaging agents, indicating a highly efficient process (2,46). Currently, the mechanism for pol exchange, the role of monoubiquitinated PCNA in particular, is under scrutiny.…”

“…Each subunit consists of two independent domains connected by an interdomain connecting loop (IDCL). The "front" face of the homotrimeric PCNA ring contains all IDCLs and is a platform for interaction with the eukaryotic replicative pols, e and δ, which are responsible for the faithful replication of the leading and lagging strands, respectively (1,2). Specifically, the well-conserved PCNA-interacting peptide (PIP) box within replicative pols makes extensive contact with an IDCL of PCNA and displays conserved residues that "plug" into the proximal hydrophobic patches.…”

“…The clamp loader, replication factor C (RFC), recognizes these hybrid primers abutted by RPA and loads PCNA onto each such that the front face of the clamp is oriented toward the 3′ end of the nascent primer/template (P/T) junction where DNA synthesis will initiate. An incoming pol δ subsequently captures the loaded PCNA ring, forming a holoenzyme, and DNA synthesis initiates (2,5).…”

Differential expression profile analysis of DNA damage repair genes in CD133+/CD133− colorectal cancer cells

“…E3 ubiquitin-protein ligase RAD18, which is involved in the post-replication repair of UV-damaged DNA, and helicase-like transcription factor, which functions in the error-free post-replication repair of damaged DNA and the maintenance of genomic stability (53). HLTF is a SWI2/SNF2-family ATP-dependent chromatin remodeling enzyme that acts in the error-free branch of DNA damage tolerance (DDT), a cellular mechanism that enables replication of damaged DNA while leaving damage repair for a later time (54).…”