Survival and liquid holding recovery in UV-sensitive strains of Saccharomyces cerevisiae after treatment with chemical mutagens and radiation.

Two UV-sensitive mutants of Saccharomyces cerevisiae rad 3 and rad 6 were tested for sensitivity to X-rays, MMS, EMS, HNO2 and DEB. Rad 3 mutant is more sensitive than the wild type strain only to HNO2 and DEB, while rad 6 is cross sensitive both to X-rays and all chemicals tested. Liquid holding recovery (LHR) was studied by comparison of cell survival immediately after mutagen treatment and after 5 days of storage in phosphate buffer. LH greatly increases cell survival of rad 3 mutant after DEB and slightly after EMS, MMS and HNO2, while after UV treatment LH significantly decreases survival of this mutant. LH increases survival of rad 6 mutant after exposure to UV, MMS and HNO2, but decreases survival of DEB-treated cells. Exposure of wild type strain to LH results in an increase of survival after UV, and DEB but not after MMS and HNO2. The results suggest that LHR is a strain- and mutagen-specific phenomenon and cannot be explained within the present knowledge of repair processes in yeast.     

ALKBH1 Is Dispensable for Abasic Site Cleavage during Base Excision Repair and Class Switch Recombination

Potential roles of the abasic site lyase activity associated with AlkB homolog 1 (ALKBH1) were assessed by studies focusing on the two cellular processes that create abasic sites as intermediates: base excision repair and class switch recombination. Alkbh1^−/− pups (lacking exon 3) were born at a lower than expected frequency from heterozygous parents, suggesting a reduced survival rate and non-Mendelian inheritance, and they exhibited a gender bias in favor of males (70% males and 30% females). To study ALKBH1’s potential involvement in DNA repair, fibroblasts were isolated from Alkbh1^−/− mice, spontaneously immortalized and tested for resistance to DNA damaging agents. Alkbh1^−/− and isogenic cells expressing hALKBH1 showed no difference in survival to the DNA damaging agents methyl-methionine sulfate or H₂O₂. This result indicates that ALKBH1 does not play a major role in the base excision repair pathway. To assess ALKBH1’s role in class switch recombination, splenic B cells were isolated from Alkbh1^−/− and Alkbh1^+/+ mice and subjected to switching from IgM to IgG1. No differences were found in IgG1 switching, suggesting that Alkbh1 is not involved in class switch recombination of the immunoglobulin heavy chain during B lymphocyte activation.     

Molecular cloning and functional analysis of a human cDNA encoding an Escherichia coli AlkB homolog, a protein involved in DNA alkylation damage repair.

The Escherichia coli AlkB protein is involved in protecting cells against mutation and cell death induced specifically by SN2-type alkylating agents such as methyl methanesulfonate (MMS). A human cDNA encoding a polypeptide homologous to E.coli AlkB was discovered by searching a database of expressed sequence tags (ESTs) derived from high throughput cDNA sequencing. The full-length human AlkB homolog (hABH) cDNA clone contains a 924 bp open reading frame encoding a 34 kDa protein which is 52% similar and 23% identical to E.coli AlkB. The hABH gene, which maps to chromosome 14q24, was ubiquitously expressed in 16 human tissues examined. When hABH was expressed in E.coli alkB mutant cells partial rescue of the cells from MMS-induced cell death occurred. Under the conditions used expression of hABH in skin fibroblasts was not regulated by treatment with MMS. Our findings show that the AlkB protein is structurally and functionally conserved from bacteria to human, but its regulation may have diverged during evolution.     

The DNA dioxygenase ALKBH2 protects Arabidopsis thaliana against methylation damage

The Escherichia coli AlkB protein (EcAlkB) is a DNA repair enzyme which reverses methylation damage such as 1-methyladenine (1-meA) and 3-methylcytosine (3-meC). The mammalian AlkB homologues ALKBH2 and ALKBH3 display EcAlkB-like repair activity in vitro, but their substrate specificities are different, and ALKBH2 is the main DNA repair enzyme for 1-meA in vivo. The genome of the model plant Arabidopsis thaliana encodes several AlkB homologues, including the yet uncharacterized protein AT2G22260, which displays sequence similarity to both ALKBH2 and ALKBH3. We have here characterized protein AT2G22260, by us denoted ALKBH2, as both our functional studies and bioinformatics analysis suggest it to be an orthologue of mammalian ALKBH2. The Arabidopsis ALKBH2 protein displayed in vitro repair activities towards methyl and etheno adducts in DNA, and was able to complement corresponding repair deficiencies of the E. coli alkB mutant. Interestingly, alkbh2 knock-out plants were sensitive to the methylating agent methylmethanesulphonate (MMS), and seedlings from these plants developed abnormally when grown in the presence of MMS. The present study establishes ALKBH2 as an important enzyme for protecting Arabidopsis against methylation damage in DNA, and suggests its homologues in other plants to have a similar function.     

Repair deficient mice reveal mABH2 as the primary oxidative demethylase for repairing 1meA and 3meC lesions in DNA

Two human homologs of the Escherichia coli AlkB protein, denoted hABH2 and hABH3, were recently shown to directly reverse 1-methyladenine (1meA) and 3-methylcytosine (3meC) damages in DNA. We demonstrate that mice lacking functional mABH2 or mABH3 genes, or both, are viable and without overt phenotypes. Neither were histopathological changes observed in the gene-targeted mice. However, in the absence of any exogenous exposure to methylating agents, mice lacking mABH2, but not mABH3 defective mice, accumulate significant levels of 1meA in the genome, suggesting the presence of a biologically relevant endogenous source of methylating agent. Furthermore, embryonal fibroblasts from mABH2-deficient mice are unable to remove methyl methane sulfate (MMS)-induced 1meA from genomic DNA and display increased cytotoxicity after MMS exposure. In agreement with these results, we found that in vitro repair of 1meA and 3meC in double-stranded DNA by nuclear extracts depended primarily, if not solely, on mABH2. Our data suggest that mABH2 and mABH3 have different roles in the defense against alkylating agents.     

DNA repair defect in poly(ADP-ribose) polymerase-deficient cell lines.

To investigate the physiological function of poly(ADP-ribose) polymerase (PARP), we used a gene targeting strategy to generate mice lacking a functional PARP gene. These PARP -/- mice were exquisitely sensitive to the monofunctional-alkylating agent N -methyl- N -nitrosourea (MNU) and gamma-irradiation. In this report, we have analysed the cause of this increased lethality using primary and/or spontaneously immortalized mouse embryonic fibroblasts (MEFs) derived from PARP -/- mice. We found that the lack of PARP renders cells significantly more sensitive to methylmethanesulfonate (MMS), causing cell growth retardation, G2/M accumulation and chromosome instability. An important delay in DNA strand-break resealing was observed following treatment with MMS. This severe DNA repair defect appears to be the primary cause for the observed cytoxicity of monofunctional-alkylating agents, leading to cell death occurring after G2/M arrest. Cell viability following MMS treatment could be fully restored after transient expression of the PARP gene. Altogether, these results unequivocally demonstrate that PARP is required for efficient base excision repair in vivo and strengthens the role of PARP as a survival factor following genotoxic stress.     

Repair of MMS-induced DNA double-strand breaks in haploid cells of Saccharomyces cerevisiae, which requires the presence of a duplicate genome.

Summary The formation and repair of double-strand breaks induced in DNA by MMS was studied in haploid wild type and MMS-sensitive rad6 mutant strains of Saccharomyces cerevisiae with the use of the neutral and alkaline sucrose sedimentation technique. A similar decrease in average molecular weight of double-stranded DNA from 5–6x10⁸ to 1–0.7x10⁸ daltons was observed following treatment with 0.5% MMS in wild type and mutant strains. Incubation of cells after MMS treatment in a fresh drug-free growing medium resulted in repair of double-strand breaks in the wild type strain, but only in the exponential phase of growth. No repair of double-strand breaks was found when cells of the wild type strain were synchronized in G-1 phase by treatment with factor, although DNA single-strand breaks were still efficiently repaired. Mutant rad6 which has a very low ability to repair MMS-induced single-strand breaks, did not repair double-strand breaks regardless of the phase of growth.These results suggest that (1) repair of double-strand breaks requires the ability for single-strand breaks repair, (2) rejoining of double-strand breaks requires the availability of two homologous DNA molecules, this strongly supports the recombinational model of DNA repair.     

Crystal Structures of the Human RNA Demethylase Alkbh5 Reveal Basis for Substrate Recognition

N⁶-Methylation of adenosine is the most ubiquitous and abundant modification of nucleoside in eukaryotic mRNA and long non-coding RNA. This modification plays an essential role in the regulation of mRNA translation and RNA metabolism. Recently, human AlkB homolog 5 (Alkbh5) and fat mass- and obesity-associated protein (FTO) were shown to erase this methyl modification on mRNA. Here, we report five high resolution crystal structures of the catalytic core of Alkbh5 in complex with different ligands. Compared with other AlkB proteins, Alkbh5 displays several unique structural features on top of the conserved double-stranded β-helix fold typical of this protein family. Among the unique features, a distinct “lid” region of Alkbh5 plays a vital role in substrate recognition and catalysis. An unexpected disulfide bond between Cys-230 and Cys-267 is crucial for the selective binding of Alkbh5 to single-stranded RNA/DNA by bringing a “flipping” motif toward the central β-helix fold. We generated a substrate binding model of Alkbh5 based on a demethylation activity assay of several structure-guided site-directed mutants. Crystallographic and biochemical studies using various analogs of α-ketoglutarate revealed that the active site cavity of Alkbh5 is much smaller than that of FTO and preferentially binds small molecule inhibitors. Taken together, our findings provide a structural basis for understanding the substrate recognition specificity of Alkbh5 and offer a foundation for selective drug design against AlkB members.     

Defective flap endonuclease 1 activity in mammalian cells is associated with impaired DNA repair and prolonged S phase delay.

Flap endonuclease 1 (FEN-1) is a 5'-3' flap exo-/endonuclease that plays an important role in Okazaki fragment maturation, nonhomologous end joining of double-stranded DNA breaks, and long patch base excision repair. Here, we demonstrate that the wild type FEN-1 binds tightly to chromatin in conjunction with proliferating cell nuclear antigen (PCNA) recruitment after MMS treatment, and the nuclease-defective FEN-1 increased the sensitivity of the cells to methylmethane sulfonate (MMS) and to UV light but not to ionizing radiation. In contrast, the cells expressing the nuclease-defective and PCNA binding-defective double mutant FEN-1 exhibited sensitivities similar to those in the cells expressing the wild type FEN-1. MMS treatment caused a prolonged delay of S phase progression and impairment in colony-forming activity of cells expressing nuclease-defective FEN-1. A comet assay demonstrated that DNA repair after MMS or UV treatment was impaired in the cells expressing nuclease-deficient FEN-1 but not in the cells with double-mutated FEN-1. Taken together, these findings suggest that FEN-1 plays an essential role in the DNA repair processes in mammalian cells and that this activity of FEN-1 is PCNA-dependent.     

Novel AlkB dioxygenases--alternative models for in silico and in vivo studies

Background

ALKBH proteins, the homologs of Escherichia coli AlkB dioxygenase, constitute a direct, single-protein repair system, protecting cellular DNA and RNA against the cytotoxic and mutagenic activity of alkylating agents, chemicals significantly contributing to tumor formation and used in cancer therapy. In silico analysis and in vivo studies have shown the existence of AlkB homologs in almost all organisms. Nine AlkB homologs (ALKBH1–8 and FTO) have been identified in humans. High ALKBH levels have been found to encourage tumor development, questioning the use of alkylating agents in chemotherapy. The aim of this work was to assign biological significance to multiple AlkB homologs by characterizing their activity in the repair of nucleic acids in prokaryotes and their subcellular localization in eukaryotes.

Methodology and Findings

Bioinformatic analysis of protein sequence databases identified 1943 AlkB sequences with eight new AlkB subfamilies. Since Cyanobacteria and Arabidopsis thaliana contain multiple AlkB homologs, they were selected as model organisms for in vivo research. Using E. coli alkB ⁻ mutant and plasmids expressing cyanobacterial AlkBs, we studied the repair of methyl methanesulfonate (MMS) and chloroacetaldehyde (CAA) induced lesions in ssDNA, ssRNA, and genomic DNA. On the basis of GFP fusions, we investigated the subcellular localization of ALKBHs in A. thaliana and established its mostly nucleo-cytoplasmic distribution. Some of the ALKBH proteins were found to change their localization upon MMS treatment.

Conclusions

Our in vivo studies showed highly specific activity of cyanobacterial AlkB proteins towards lesions and nucleic acid type. Subcellular localization and translocation of ALKBHs in A. thaliana indicates a possible role for these proteins in the repair of alkyl lesions. We hypothesize that the multiplicity of ALKBHs is due to their involvement in the metabolism of nucleo-protein complexes; we find their repair by ALKBH proteins to be economical and effective alternative to degradation and de novo synthesis.     

A new role for translation initiation factor 2 in maintaining genome integrity

Escherichia coli translation initiation factor 2 (IF2) performs the unexpected function of promoting transition from recombination to replication during bacteriophage Mu transposition in vitro, leading to initiation by replication restart proteins. This function has suggested a role of IF2 in engaging cellular restart mechanisms and regulating the maintenance of genome integrity. To examine the potential effect of IF2 on restart mechanisms, we characterized its influence on cellular recovery following DNA damage by methyl methanesulfonate (MMS) and UV damage. Mutations that prevent expression of full-length IF2-1 or truncated IF2-2 and IF2-3 isoforms affected cellular growth or recovery following DNA damage differently, influencing different restart mechanisms. A deletion mutant (del1) expressing only IF2-2/3 was severely sensitive to growth in the presence of DNA-damaging agent MMS. Proficient as wild type in repairing DNA lesions and promoting replication restart upon removal of MMS, this mutant was nevertheless unable to sustain cell growth in the presence of MMS; however, growth in MMS could be partly restored by disruption of sulA, which encodes a cell division inhibitor induced during replication fork arrest. Moreover, such characteristics of del1 MMS sensitivity were shared by restart mutant priA300, which encodes a helicase-deficient restart protein. Epistasis analysis indicated that del1 in combination with priA300 had no further effects on cellular recovery from MMS and UV treatment; however, the del2/3 mutation, which allows expression of only IF2-1, synergistically increased UV sensitivity in combination with priA300. The results indicate that full-length IF2, in a function distinct from truncated forms, influences the engagement or activity of restart functions dependent on PriA helicase, allowing cellular growth when a DNA-damaging agent is present.     

Suppression of Escherichia coli alkB mutants by Saccharomyces cerevisiae genes.

The alkB gene is one of a group of alkylation-inducible genes in Escherichia coli, and its product protects cells from SN2-type alkylating agents such as methyl methanesulfonate (MMS). However, the precise biochemical function of the AlkB protein remains unknown. Here, we describe the cloning, sequencing, and characterization of three Saccharomyces cerevisiae genes (YFW1, YFW12, and YFW16) that functionally complement E. coli alkB mutant cells. DNA sequence analysis showed that none of the three gene products have any amino acid sequence homology with the AlkB protein. The YFW1 and YFW12 proteins are highly serine and threonine rich, and YFW1 contains a stretch of 28 hydrophobic residues, indicating that it may be a membrane protein. The YFW16 gene turned out to be allelic with the S. cerevisiae STE11 gene. STE11 is a protein kinase known to be involved in pheromone signal transduction in S. cerevisiae; however, the kinase activity is not required for MMS resistance because mutant STE11 proteins lacking kinase activity could still complement E. coli alkB mutants. Despite the fact that YFW1, YFW12, and YFW16/STE11 each confer substantial MMS resistance upon E. coli alkB cells, S. cerevisiae null mutants for each gene were not MMS sensitive. Whether these three genes provide alkylation resistance in E. coli via an alkB-like mechanism remains to be determined, but protection appears to be specific for AlkB-deficient E. coli because none of the genes protect other alkylation-sensitive E. coli strains from killing by MMS.     

Phosphorylation of the delta-opioid receptor regulates its beta-arrestins selectivity and subsequent receptor internalization and adenylyl cyclase desensitization

In the current study, we investigated the role of receptor phosphorylation and beta-arrestins in delta-opioid receptor (DOR) signaling and trafficking by using a DOR mutant in which all Ser/Thr residues in the C terminus were mutated to Ala (DTS). We demonstrated that the DOR agonist D-[Pen(2),Pen(5)]enkephalin could induce receptor internalization and adenylyl cyclase (AC) desensitization of DTS, but with comparatively slower kinetics than those observed with wild type DOR. Blockade of the internalization of DTS by the dominant-negative mutant dynamin, dynamin K44E, did not affect AC desensitization. However, depletion of beta-arrestins almost totally blocked both internalization and AC desensitization of DTS. A BRET assay suggested that DOR phosphorylation promotes receptor selectivity for beta-arrestin 2 over beta-arrestin 1. Furthermore, in mouse embryonic fibroblast (MEF) cells lacking either beta-arrestin 1 (beta arr1(-/-)) or beta-arrestin 2 (beta arr2(-/-)), agonist-induced DTS desensitization and internalization were similar to that observed in wild type MEFs. In contrast, although DOR internalization decreased in both beta arr1(-/-) MEFs and beta arr2(-/-) MEFs, DPDPE-induced DOR desensitization was significantly reduced in beta arr2(-/-) MEFs, but not in beta arr1(-/-) MEFs. Additionally, the BRET assay suggested that depletion of phosphorylation did not influence the stability of the receptor-beta-arrestin complex. Consistent with this observation, DTS did not recycle after internalization, which is like wild type DOR. Taken together, these results indicate that receptor phosphorylation confers DOR selectivity for beta-arrestin 2 without affecting the stability of the receptor-beta-arrestin complex and the fate of the internalized receptor.     

Mice lacking Alkbh1 display sex-ratio distortion and unilateral eye defects

Background

Eschericia coli AlkB is a 2-oxoglutarate- and iron-dependent dioxygenase that reverses alkylated DNA damage by oxidative demethylation. Mouse AlkB homolog 1 (Alkbh1) is one of eight members of the newly discovered family of mammalian dioxygenases.

Methods and Findings

In the present study we show non-Mendelian inheritance of the Alkbh1 targeted allele in mice. Both Alkbh1^−/− and heterozygous Alkbh1^+/− offspring are born at a greatly reduced frequency. Additionally, the sex-ratio is considerably skewed against female offspring, with one female born for every three to four males. Most mechanisms that cause segregation distortion, act in the male gametes and affect male fertility. The skewing of the sexes appears to be of paternal origin, and might be set in the pachythene stage of meiosis during spermatogenesis, in which Alkbh1 is upregulated more than 10-fold. In testes, apoptotic spermatids were revealed in 5–10% of the tubules in Alkbh1^−/− adults. The deficiency of Alkbh1 also causes misexpression of Bmp2, 4 and 7 at E11.5 during embryonic development. This is consistent with the incompletely penetrant phenotypes observed, particularly recurrent unilateral eye defects and craniofacial malformations.

Conclusions

Genetic and phenotypic assessment suggests that Alkbh1 mediates gene regulation in spermatogenesis, and that Alkbh1 is essential for normal sex-ratio distribution and embryonic development in mice.     

Compensatory activation of ERK1/2 in Atg5-deficient mouse embryo fibroblasts suppresses oxidative stress-induced cell death

Despite of the increasing evidence that oxidative stress may induce non-apoptotic cell death or autophagic cell death, the mechanism of this process is unclear. Here, we report a role and a down-stream molecular event of Atg5 during oxidative stress-induced cell death. Compared to wild type (WT) cells, Atg5-deficient mouse embryo fibroblasts (Atg5-/- MEFs) and Atg5 knockdown HT22 neuronal cells were more resistant to cell death induced by H2O2. On the contrary, Atg5-/- MEFs were as sensitive to tumor necrosis factor (TNF)-alpha and cycloheximide as WT cells, and were more sensitive to cell death triggered by amino acid-deprivation than WT MEFs. Treatment with H2O2 induced the recruitment of a GFP-LC3 fusion protein and conversion of LC3 I to LC3 II, correlated with the extent of autophagosome formation in WT cells, but much less in Atg5-deficient cells. Among stress kinases, ERK1/2 was markedly activated in Atg5-/- MEFs and Atg5 knockdown HT22 and SH-SY5Y neuronal cells. The inhibition of ERK1/2 by MEK1 inhibitor (PD98059) or dominant negative ERK2 enhanced the susceptibility of Atg5-/- MEFs to H2O2-induced cell death. Further, reconstitution of Atg5 sensitized Atg5-/- MEFs to H2O2 and suppressed the activation of ERK1/2. These results suggest that the inhibitory effect of Atg5 deficiency on cell death is attributable by the compensatory activation of ERK1/2 in Atg5-/- MEFs during oxidative stress-induced cell death.     

Functional specialization of Chlamydomonas reinhardtii cytosolic thioredoxin h1 in the response to alkylation-induced DNA damage

DNA damage occurs as a by-product of intrinsic cellular processes, like DNA replication, or as a consequence of exposure to genotoxic agents. Organisms have evolved multiple mechanisms to avoid, tolerate, or repair DNA lesions. To gain insight into these processes, we have isolated mutants hypersensitive to DNA-damaging agents in the green alga Chlamydomonas reinhardtii. One mutant, Ble-1, showed decreased survival when it was treated with methyl methanesulfonate (MMS), bleomycin, or hydrogen peroxide (H2O2) but behaved like the wild type when it was exposed to UVC irradiation. Ble-1 carries an extensive chromosomal deletion that includes the gene encoding cytosolic thioredoxin h1 (Trxh1). Transformation of Ble-1 with a wild-type copy of Trxh1 fully corrected the MMS hypersensitivity and partly restored the tolerance to bleomycin. Trxh1 also complemented a defect in the repair of MMS-induced DNA strand breaks and alkali-labile sites. In addition, a Trxh1-beta-glucuronidase fusion protein translocated to the nucleus in response to treatment with MMS. However, somewhat surprisingly, Trxh1 failed to correct the Ble-1 hypersensitivity to H2O2. Moreover, Trxh1 suppression by RNA interference in a wild-type strain resulted in enhanced sensitivity to MMS and DNA repair defects but no increased cytotoxicity to H2O2. Thioredoxins have been implicated in oxidative-stress responses in many organisms. Yet our results indicate a specific role of Chlamydomonas Trxh1 in the repair of MMS-induced DNA damage, whereas it is dispensable for the response to H2O2. These observations also suggest functional specialization among cytosolic thioredoxins since another Chlamydomonas isoform (Trxh2) does not compensate for the lack of Trxh1.     

BID-D59A is a potent inducer of apoptosis in primary embryonic fibroblasts

The proapoptotic activity of BID seems to solely depend upon its cleavage to truncated tBID. Here we demonstrate that expression of a caspase-8 non-cleavable (nc) BID-D59A mutant or expression of wild type (wt) BID induces apoptosis in Bid -/-, caspase-8 -/-, and wt primary MEFs. Western blot analysis indicated that no cleavage products appeared in cells expressing ncBID. ncBID was as effective as wtBID in inducing cytochrome c release, caspase activation, and apoptosis. ncBID and wtBID (nc/wtBID) were much less effective than tBID in localizing to mitochondria and in inducing cytochrome c release, but only slightly less effective in inducing apoptosis. Studies with Apaf-1- and caspase-9-deficient primary MEFs indicated that both proteins were essential for nc/wtBID and for tBID-induced apoptosis. Most importantly, expression of non-apoptotic levels of either ncBID or wtBID in Bid -/- MEFs induced a similar and significant enhancement in apoptosis in response to a variety of death signals, which was accompanied by enhanced localization of BID to mitochondria and cytochrome c release. Thus, these results implicate full-length BID as an active player in the mitochondria during apoptosis.     

Poly(ADP-ribose) polymerase 1 is not strictly required for infection of murine cells by retroviruses

The DNA-breaking and -joining steps initiating retroviral integration are well understood, but the later steps, thought to be carried out by cellular DNA repair enzymes, have not been fully characterized. Poly(ADP-ribose) polymerase 1 (PARP-1) has been proposed to play a role late during retroviral integration, because infection by human immunodeficiency virus (HIV)-based vectors was reported to be strongly inhibited in PARP-1-deficient fibroblasts. PARP-1, a nuclear enzyme, binds tightly to nicked DNA and synthesizes poly(ADP-ribose) as an early response to DNA damage. To investigate the role of PARP-1 in retroviral integration, we infected wild-type and PARP-1-deficient mouse embryonic fibroblasts (MEFs) separately with two HIV type 1-derived, vesicular stomatitis virus G-pseudotyped lentivirus vectors. Surprisingly, infection of both wild-type and PARP-1-deficient cells was observed with both vectors. Marker gene transduction and provirus formation by one vector was reduced by 45 to 75% compared to the wild type, but the other vector was unaffected by the PARP-1 mutant. In addition, PARP-1-deficient MEFs infected with Moloney murine leukemia virus showed no decrease in virus output after infection compared to the wild type. We conclude that PARP-1 cannot be strictly required for retroviral infection because replication steps, including integration, can proceed efficiently in its absence.