DNA replication across blocking lesions occurs by translesion DNA synthesis (TLS), involving a multitude of mutagenic DNA polymerases that operate to protect the mammalian genome. Using a quantitative TLS assay, we identified three main classes of TLS in human cells: two rapid and error-free, and the third slow and error-prone. A single gene, REV3L, encoding the catalytic subunit of DNA polymerase f (polf), was found to have a pivotal role in TLS, being involved in TLS across all lesions examined, except for a TT cyclobutane dimer. Genetic epistasis siRNA analysis indicated that discrete two-polymerase combinations with polf dictate error-prone or error-free TLS across the same lesion. These results highlight the central role of polf in both error-prone and error-free TLS in mammalian cells, and show that bypass of a single lesion may involve at least three different DNA polymerases, operating in different two-polymerase combinations.
Replication across unrepaired DNA lesions in mammalian cells is effected primarily by specialized, low fidelity DNA polymerases. We studied translesion DNA synthesis (TLS) across a benzo[a]pyrene-guanine (BP-G) adduct, a major mutagenic DNA lesion generated by tobacco smoke. This was done using a quantitative assay that measures TLS indirectly, by measuring the recovery of gapped plasmids transfected into cultured mammalian cells. Analysis of PolK ؉/؉ mouse embryo fibroblasts (MEFs) showed that TLS across the BP-G adduct occurred with an efficiency of 48 ؎ 4%, which is an order of magnitude higher than in Escherichia coli. In PolK ؊/؊ MEFs, bypass was 16 ؎ 1%, suggesting that at least twothirds of the BP-G adducts in MEFs were bypassed exclusively by polymerase (pol ). In contrast, pol was not required for bypass across BP-G in a human XP-V cell line. Analysis of misinsertion specificity across BP-G revealed that bypass was more error-prone in MEFs lacking pol . Expression of pol from a plasmid introduced into PolK ؊/؊ MEFs restored both the extent and fidelity of bypass across BP-G. Pol was not required for bypass of a synthetic abasic site. In vitro analysis demonstrated efficient bypass across BP-G by both pol and pol , suggesting that the biological role of pol in TLS across BP-G is due to regulation of TLS and not due to an exclusive ability to bypass this lesion. These results indicate that BP-G is bypassed in mammalian cells with relatively high efficiency and that pol bypasses BP-G in vivo with higher efficiency and higher accuracy than other DNA polymerases.Genomic DNA is constantly subject to damage caused by both external agents, such as sunlight, and endogenous chemicals, such as reactive oxygen species. Most of this damage is eliminated by error-free DNA repair mechanisms, thereby restoring the DNA to its native sequence (1). However, a significant number of lesions escape repair and might therefore interfere with DNA replication and gene expression. Such interference can be mitigated by DNA damage tolerance mechanisms, primarily translesion DNA synthesis (TLS 1 ; also termed translesion replication) (2-5) and postreplicative recombinational repair (1, 6 -8). The key components in TLS are low fidelity DNA polymerases that specialize in lesion bypass (9 -12). These proteins were conserved in evolution and are present in organisms ranging from Escherichia coli to humans (13). Humans contain at least four specialized DNA polymerases belonging to the Y superfamily (pol , pol , pol , and REV1) as well as several from other polymerase families (e.g. pol (14), pol (15, 16), and pol (16, 17)). Many of these polymerases have been implicated in TLS in vitro (18 -24). However, there is a paucity of information about the efficiency and fidelity with which they support lesion bypass in living cells.Pol has a well established biological role in TLS, since it is mutated in all patients examined with the variant form of the hereditary disease xeroderma pigmentosum (10,18). This disease is characterized by sensitivi...
DP178, a synthetic peptide corresponding to a segment of the transmembrane envelope glycoprotein (gp41) of human immunodeficiency virus, type 1 (HIV-1), is a potent inhibitor of viral infection and virus-mediated cell-cell fusion. Nevertheless, DP178 does not contain gp41 coiled-coil cavity binding residues postulated to be essential for inhibiting HIV-1 entry. We find that DP178 inhibits phospholipid redistribution mediated by the HIV-1 envelope glycoprotein at a concentration 8 times greater than that of solute redistribution (the IC 50 values are 43 and 335 nM, respectively). In contrast, C34, a synthetic peptide which overlaps with DP178 but contains the cavity binding residues, did not show this phenomenon (11 and 25 nM, respectively). The ability of DP178 to inhibit membrane fusion at a post-lipid mixing stage correlates with its ability to bind and oligomerize on the surface of membranes. Furthermore, our results are consistent with a model in which DP178 inhibits the formation of gp41 viral hairpin structure at low affinity, whereas C34 inhibits its formation at high affinity: the failure to form the viral hairpin prevents both lipid and solute from redistributing between cells. However, our data also suggest an additional membrane-bound inhibitory site for DP178 in the ectodomain of gp41 within a region immediately adjacent to the membrane-spanning domain. By binding to this higher affinity site, DP178 inhibits the recruitment of several gp41-membrane complexes, thus inhibiting fusion pore formation.The first step in HIV-1 1 infection involves the binding of the viral envelope glycoproteins gp120-gp41 to CD4 (1-3) and subsequently to a co-receptor (4 -8) (for recent review, see Refs. 9 -11). Consequently, gp41 undergoes conformational changes that mediate the fusion between the viral and the cellular membranes or between infected and healthy cells (12, 13). Gallaher and co-workers (14, 15) postulated a model of gp41, identifying a fusion peptide followed by a leucine/isoleucine zipper-like sequence (N-helix) and an amphipathic helical segment (C-helix) in the viral glycoprotein. The indispensability of the fusion peptide for viral infection was confirmed by sitedirected mutagenesis (16,17). Furthermore, gp41 was found to contain a protease-resistant core consisting of the postulated N-and C-helices (18). Specifically, peptides corresponding to these sequences co-crystallized as a six-helix bundle in which the N-and C-helices are arranged in a three-hairpin structure (19 -21). Three N peptides form a coiled-coil, and the C peptides are packed in an antiparallel manner into highly conserved, hydrophobic grooves on the surface of the coiled-coil. Recently, the solution and crystal structures of the ectodomain of the Simian immunodeficiency virus gp41, consisting of those two helices as well as the loop connecting them, confirmed the interplay of the N-and C-helices (22, 23). Remarkably, the coiled coil is a common motif found in many diverse viral membrane fusion proteins (24), as well as in proteins invol...
Regulation of mutation rates is critical for maintaining genome stability and controlling cancer risk. A special challenge to this regulation is the presence of multiple mutagenic DNA polymerases in mammals. These polymerases function in translesion DNA synthesis (TLS), an error-prone DNA repair process that involves DNA synthesis across DNA lesions. We found that in mammalian cells TLS is controlled by the tumor suppressor p53, and by the cell cycle inhibitor p21 via its PCNA-interacting domain, to maintain a low mutagenic load at the price of reduced repair efficiency. This regulation may be mediated by binding of p21 to PCNA and via DNA damage-induced ubiquitination of PCNA, which is stimulated by p53 and p21. Loss of this regulation by inactivation of p53 or p21 causes an out of control lesion-bypass activity, which increases the mutational load and might therefore play a role in pathogenic processes caused by genetic instability.
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