Effect of hydroxyurea and 5-fluorodeoxyuridine on deoxyribonucleoside triphosphate pools early in phytohemagglutinin-stimulated human lymphocytes

Strengths and Weaknesses of Cell Synchronization Protocols Based on Inhibition of DNA Synthesis

Synchronous cell populations are commonly used for the analysis of various aspects of cellular metabolism at specific stages of the cell cycle. Cell synchronization at a chosen cell cycle stage is most frequently achieved by inhibition of specific metabolic pathway(s). In this respect, various protocols have been developed to synchronize cells in particular cell cycle stages. In this review, we provide an overview of the protocols for cell synchronization of mammalian cells based on the inhibition of synthesis of DNA building blocks-deoxynucleotides and/or inhibition of DNA synthesis. The mechanism of action, examples of their use, and advantages and disadvantages are described with the aim of providing a guide for the selection of suitable protocol for different studied situations.

Sensitive CometChip assay for screening potentially carcinogenic DNA adducts by trapping DNA repair intermediates

Genotoxicity testing is critical for predicting adverse effects of pharmaceutical, industrial, and environmental chemicals. The alkaline comet assay is an established method for detecting DNA strand breaks, however, the assay does not detect potentially carcinogenic bulky adducts that can arise when metabolic enzymes convert pro-carcinogens into a highly DNA reactive products. To overcome this, we use DNA synthesis inhibitors (hydroxyurea and 1-β-d-arabinofuranosyl cytosine) to trap single strand breaks that are formed during nucleotide excision repair, which primarily removes bulky lesions. In this way, comet-undetectable bulky lesions are converted into comet-detectable single strand breaks. Moreover, we use HepaRG™ cells to recapitulate in vivo metabolic capacity, and leverage the CometChip platform (a higher throughput more sensitive comet assay) to create the 'HepaCometChip', enabling the detection of bulky genotoxic lesions that are missed by current genotoxicity screens. The HepaCometChip thus provides a broadly effective approach for detection of bulky DNA adducts.

Synergistic reduction of HIV-1 infectivity by 5-azacytidine and inhibitors of ribonucleotide reductase

Although many compounds have been approved for the treatment of human immunodeficiency type-1 (HIV-1) infection, additional anti-HIV-1 drugs (particularly those belonging to new drug classes) are still needed due to issues such as long-term drug-associated toxicities, transmission of drug-resistant variants, and development of multi-class resistance. Lethal mutagenesis represents an antiviral strategy that has not yet been clinically translated for HIV-1 and is based on the use of small molecules to induce excessive levels of deleterious mutations within the viral genome. Here, we show that 5-azacytidine (5-aza-C), a ribonucleoside analog that induces the lethal mutagenesis of HIV-1, and multiple inhibitors of the enzyme ribonucleotide reductase (RNR) interact in a synergistic fashion to more effectively reduce the infectivity of HIV-1. In these drug combinations, RNR inhibitors failed to significantly inhibit the conversion of 5-aza-C to 5-aza-2'-deoxycytidine, suggesting that 5-aza-C acts primarily as a deoxyribonucleoside even in the presence of RNR inhibitors. The mechanism of antiviral synergy was further investigated for the combination of 5-aza-C and one specific RNR inhibitor, resveratrol, as this combination improved the selectivity index of 5-aza-C to the greatest extent. Antiviral synergy was found to be primarily due to the reduced accumulation of reverse transcription products rather than the enhancement of viral mutagenesis. To our knowledge, these observations represent the first demonstration of antiretroviral synergy between a ribonucleoside analog and RNR inhibitors, and encourage the development of additional ribonucleoside analogs and RNR inhibitors with improved antiretroviral activity.

Dual anti-HIV mechanism of clofarabine

BACKGROUND

HIV-1 replication kinetics inherently depends on the availability of cellular dNTPs for viral DNA synthesis. In activated CD4(+) T cells and other rapidly dividing cells, the concentrations of dNTPs are high and HIV-1 reverse transcription occurs in an efficient manner. In contrast, nondividing cells such as macrophages have lower dNTP pools, which restricts efficient reverse transcription. Clofarabine is an FDA approved ribonucleotide reductase inhibitor, which has shown potent antiretroviral activity in transformed cell lines. Here, we explore the potency, toxicity and mechanism of action of clofarabine in the human primary HIV-1 target cells: activated CD4(+) T cells and macrophages.

RESULTS

Clofarabine is a potent HIV-1 inhibitor in both activated CD4(+) T cells and macrophages. Due to its minimal toxicity in macrophages, clofarabine displays a selectivity index over 300 in this nondividing cell type. The anti-HIV-1 activity of clofarabine correlated with a significant decrease in both cellular dNTP levels and viral DNA synthesis. Additionally, we observed that clofarabine triphosphate was directly incorporated into DNA by HIV-1 reverse transcriptase and blocked processive DNA synthesis, particularly at the low dNTP levels found in macrophages.

CONCLUSIONS

Taken together, these data provide strong mechanistic evidence that clofarabine is a dual action inhibitor of HIV-1 replication that both limits dNTP substrates for viral DNA synthesis and directly inhibits the DNA polymerase activity of HIV-1 reverse transcriptase.

Iron depletion results in Src kinase inhibition with associated cell cycle arrest in neuroblastoma cells

Iron is required for cellular proliferation. Recently, using systematic time studies of neuroblastoma cell growth, we better defined the G1 arrest caused by iron chelation to a point in mid-G1, where cyclin E protein is present, but the cyclin E/CDK2 complex kinase activity is inhibited. In this study, we again used the neuroblastoma SKNSH cells lines to pinpoint the mechanism responsible for this G1 block. Initial studies showed in the presence of DFO, these cells have high levels of p27 and after reversal of iron chelation p27 is degraded allowing for CDK2 kinase activity. The initial activation of CDK2 kinase allows cells to exit G1 and enter S phase. Furthermore, we found that inhibition of p27 degradation by DFO is directly associated with inhibition of Src kinase activity measured by lack of phosphorylation of Src at the 416 residue. Activation of Src kinase occurs very early after reversal from the DFO G1 block and is temporally associated with initiation of cellular proliferation associated with entry into S phase. For the first time therefore we show that iron chelation inhibits Src kinase activity and this activity is a requirement for cellular proliferation.

Two cell cycle blocks caused by iron chelation of neuroblastoma cells: separating cell cycle events associated with each block

Studies have presented evidence that besides the well described S phase block, treatment of cancer cell lines with the iron chelator deferrioxamine (DFO) also results in an earlier block in G₁ phase. In this article, measurements of cell cycle regulatory proteins define this block at a very specific point in G₁. DFO treatment results in markedly decreased cyclin A protein levels. Cyclin E levels that accumulate in early to mid‐G₁ are increased in cells treated with DFO as compared to the resting cells. The DFO S phase block is shown after cells are arrested at G₁/S by (aphidicolin) then released into DFO. The same S phase block occurs with DFO treatment of a neuroblastoma cell line relatively resistant to the G₁ DFO block. These experiments clearly differentiate the S phase DFO block from the earlier block pinpointed to a point in mid‐G₁, before G₁/S when cyclin E protein increases but before increased cyclin A synthesis. Apoptosis was observed in cells inhibited by DFO at both cell cycle arrest points.

Iron chelation inhibits cell proliferation at two phases in the cell cycle. The block in G1 phase is now separated from the block in S phase by specific differences in expression of cell cycle regulatory events.

Intracellular deoxyribonucleotide pool imbalance and DNA damage in cells treated with hydroxyurea, an inhibitor of ribonucleotide reductase

Imbalance in the nucleotide pool of mammalian cells has been shown to result in genotoxic damage. The goal of this study was to devise a sensitive, reproducible and simple method for detection of nucleotide pool changes in mammalian cells that could be used for problem-solving activities in drug development, e.g. mechanistic explanation of a positive response in a mammalian in vitro genotoxicity test. The method evaluated in this study is based on ethanol extraction of the total nucleotide pool, heat treatment and filtration, treatment with calf intestine alkaline phosphatase to convert nucleotides to nucleosides and analysis of the nucleosides by high-performance liquid chromatography with ultraviolet detection. The method was applied to measure the intracellular levels of deoxyribonucleotides in mouse lymphoma (ML) L5178Y cells treated with various concentrations of a model compound, hydroxyurea (HU), a ribonucleotide reductase inhibitor. DNA strand breakage and micronuclei formation were assessed in the same experiments. Imbalance of nucleotide pool (i.e. changes in the relative ratios between individual nucleotide pools) in HU-treated ML cells has been observed already at a concentration of 0.01 mmol/l, whereas genotoxic effects became apparent only at higher concentrations of HU (i.e. 0.25 mmol/l and higher) as indicated by formation of DNA strand breaks and micronuclei.

Maternal embryonic leucine zipper kinase (MELK) reduces replication stress in glioblastoma cells

Maternal embryonic leucine zipper kinase (MELK) belongs to the subfamily of AMP-activated Ser/Thr protein kinases. The expression of MELK is very high in glioblastoma-type brain tumors, but it is not clear how this contributes to tumor growth. Here we show that the siRNA-mediated loss of MELK in U87 MG glioblastoma cells causes a G1/S phase cell cycle arrest accompanied by cell death or a senescence-like phenotype that can be rescued by the expression of siRNA-resistant MELK. This cell cycle arrest is mediated by an increased expression of p21(WAF1/CIP1), an inhibitor of cyclin-dependent kinases, and is associated with the hypophosphorylation of the retinoblastoma protein and the down-regulation of E2F target genes. The increased expression of p21 can be explained by the consecutive activation of ATM (ataxia telangiectasia mutated), Chk2, and p53. Intriguingly, the activation of p53 in MELK-deficient cells is not due to an increased stability of p53 but stems from the loss of MDMX (mouse double minute-X), an inhibitor of p53 transactivation. The activation of the ATM-Chk2 pathway in MELK-deficient cells is associated with the accumulation of DNA double-strand breaks during replication, as demonstrated by the appearance of γH2AX foci. Replication stress in these cells is also illustrated by an increased number of stalled replication forks and a reduced fork progression speed. Our data indicate that glioblastoma cells have elevated MELK protein levels to better cope with replication stress during unperturbed S phase. Hence, MELK inhibitors hold great potential for the treatment of glioblastomas as such or in combination with DNA-damaging therapies.

Remodeling of the metabolome during early frog development

A rapid series of synchronous cell divisions initiates embryogenesis in many animal species, including the frog Xenopus laevis. After many of these cleavage cycles, the nuclear to cytoplasmic ratio increases sufficiently to somehow cause cell cycles to elongate and become asynchronous at the mid-blastula transition (MBT). We have discovered that an unanticipated remodeling of core metabolic pathways occurs during the cleavage cycles and the MBT in X. laevis, as evidenced by widespread changes in metabolite abundance. While many of the changes in metabolite abundance were consistently observed, it was also evident that different female frogs laid eggs with different levels of at least some metabolites. Metabolite tracing with heavy isotopes demonstrated that alanine is consumed to generate energy for the early embryo. dATP pools were found to decline during the MBT and we have confirmed that maternal pools of dNTPs are functionally exhausted at the onset of the MBT. Our results support an alternative hypothesis that the cell cycle lengthening at the MBT is triggered not by a limiting maternal protein, as is usually proposed, but by a decline in dNTP pools brought about by the exponentially increasing demands of DNA synthesis.

Hydroxyurea arrests DNA replication by a mechanism that preserves basal dNTP pools

The relationship between dNTP levels and DNA synthesis was investigated using alpha factor-synchronized yeast treated with the ribonucleotide reductase inhibitor hydroxyurea (HU). Although HU blocked DNA synthesis and prevented the dNTP pool expansion that normally occurs at G1/S, it did not exhaust the levels of any of the four dNTPs, which dropped to about 80% of G1 levels. When dbf4 yeast that are ts for replication initiation were allowed to preaccumulate dNTPs at 37 degrees C before being released to 25 degrees C in the presence of HU, they synthesized 0.3 genome equivalents of DNA and then arrested as dNTPs approached sub-G1 levels. Accumulation of dNTPs at G1/S was not a prerequisite for replication initiation, since dbf4 cells incubated in HU at 25 degrees C were able to replicate when subsequently switched to 37 degrees C in the absence of HU. The replication arrest mechanism was not dependent on the Mec1/Rad53 pathway, since checkpoint-deficient rad53 cells also failed to exhaust basal dNTPs when incubated in HU. The persistence of basal dNTP levels in HU-arrested cells and partial bypass of the arrest in cells that had preaccumulated dNTPs suggest that cells have a mechanism for arresting DNA chain elongation when dNTP levels are not maintained above a critical threshold.

Human immunodeficiency virus type 1 DNA sequences genetically damaged by hypermutation are often abundant in patient peripheral blood mononuclear cells and may be generated during near-simultaneous infection and activation of CD4(+) T cells

G-to-A hypermutation has been sporadically observed in human immunodeficiency virus type 1 (HIV-1) proviral sequences from patient peripheral blood mononuclear cells (PBMC) and virus cultures but has not been systematically evaluated. PCR primers matched to normal and hypermutated sequences were used in conjunction with an agarose gel electrophoresis system incorporating an AT-binding dye to visualize, separate, clone, and sequence hypermutated and normal sequences in the 297-bp HIV-1 protease gene amplified from patient PBMC. Among 53 patients, including individuals infected with subtypes A through D and at different clinical stages, at least 43% of patients harbored abundant hypermutated, along with normal, protease genes. In 70 hypermutated sequences, saturation of G residues in the GA or GG dinucleotide context ranged from 20 to 94%. Levels of other mutants were not elevated, and G-to-A replacement was entirely restricted to GA or GG, and not GC or GT, dinucleotides. Sixty-nine of 70 hypermutated and 3 of 149 normal sequences had in-frame stop codons. To investigate the conditions under which hypermutation occurs in cell cultures, purified CD4(+) T cells from normal donors were infected with cloned NL4-3 virus stocks at various times before and after phytohemagglutinin (PHA) activation. Hypermutation was pronounced when HIV-1 infection occurred simultaneously with, or a few hours after, PHA activation, but after 12 h or more after PHA activation, most HIV-1 sequences were normal. Hypermutated sequences generated in culture corresponded exactly in all parameters to those obtained from patient PBMC. Near-simultaneous activation and infection of CD4(+) T cells may represent a window of susceptibility where the informational content of HIV-1 sequences is lost due to hypermutation.

Effects of deoxyribonucleosides and cell-stimulation on frequencies of ultraviolet-B-induced micronuclei in human peripheral blood lymphocytes

In the present study the involvement of deoxyribonucleotides (dNTPs) in the clastogenicity of ultraviolet-B (UVB) in unstimulated peripheral blood lymphocytes (G(0)-PBLs) was investigated. This was studied by analyzing the frequency of UVB-induced micronuclei (MN), either after adding a cocktail of the four deoxyribonucleosides to the PBLs immediately after exposure to UVB, or by stimulating the cells before exposure. In total, PBLs obtained from two different donors were investigated. For both donors, it could be demonstrated that addition of deoxyribonucleosides to UVB-irradiated G(0)-PBLs resulted in a significant reduction of the clastogenic effect of UVB. A gradual reduction of the clastogenic effect of UVB could also be realized by irradiating PBLs that were progressively more stimulated with the lectin PHA before exposure. The latter finding is explained by upregulation of intracellular pool sizes of dNTPs in stimulated PBLs.

HIV genetic variation is directed and restricted by DNA precursor availability

The effects of deoxynucleoside triphosphate (dNTP) imbalances on the fidelity of human immunodeficiency virus type 1 (HIV-1) replication were investigated. Using detergent permeabilized virions and biased dNTP concentrations different types of hypermutants were readily produced. However, the mutant spectrum was different from naturally occurring hypermutants demonstrating that the host cell may restrict variation. Using a genetic screen based on the blue/white beta-galactosidase complementation assay, G --> A hypermutants were recovered from HIV-infected thymidine treated U937 cells. Furthermore, hypermutants were recovered from 1 to 2% of resting or activated peripheral blood mononuclear cells indicating that small proportions of primary cells had distorted intracellular [dTTP] and [dCTP]. Such imbalances may underlie a proportion of somatic and germline point mutations and shape to some extent the evolution of mammalian and viral genomes.

Restriction and enhancement of human immunodeficiency virus type 1 replication by modulation of intracellular deoxynucleoside triphosphate pools

Human immunodeficiency virus type 1 (HIV-1) replication is shown to be sensitive to the intracellular concentration of deoxynucleoside triphosphate substrates. Addition of thymidine to established cell lines resulted in a dramatic reduction of virus production. The effect could be substantially alleviated by addition of deoxycytidine, which, alone, enhanced viral titers by a factor of 2 to 3. Hydroxyurea treatment abolished HIV-1 replication in peripheral blood mononuclear cells and could be reversed by deoxyadenosine. These data show that HIV-1 replication occurs under suboptimal DNA precursor conditions.

DNA precursor asymmetries, replication fidelity, and variable genome evolution

Balanced pools of deoxyribonucleoside triphosphates (dNTPs) are essential for DNA replication to occur with maximum fidelity. Conditions that create biased dNTP pools stimulate mutagenesis, as well as other phenomena, such as recombination or cell death. In this essay we consider the effective dNTP concentrations at replication sites under normal conditions, and we ask how maintenance of these levels contributes toward the natural fidelity of DNA replication. We focus upon two questions. (1) In prokaryotic systems, evidence suggests that replication is driven by small, localized, rapidly replenished dNTP pools that do not equilibrate with the bulk dNTP pools in the cell. Since these pools cannot be analyzed directly, what indirect approaches can illuminate the nature of these replication-active pools? (2) In eukaryotic cells, the normal dNTP pools are highly asymmetric, with dGTP being the least abundant nucleotide. Moreover, the composition of the dNTP pools changes as cells progress through the cell cycle. To what extent might these natural asymmetries contribute toward a recently described phenomenon, the differential rate of evolution of different genes in the same genome?

Deoxynucleotide pools and DNA synthesis in resting and PHA-stimulated human lymphocytes treated with mutagens

In resting and PHA-stimulated PBL treated with uv light or MMS we measured the sizes of the dTTP and dATP pools and the variation of ATP content taken as an indicator of cytotoxicity. The effects on DNA synthesis were examined by measuring DNA repair (unscheduled DNA synthesis) in resting PBL and the inhibition of DNA replication in stimulated PBL. While both treatments affected DNA synthesis, only MMS perturbed dNTP pools and decreased the intracellular concentration of ATP. All the effects were more evident in cycling than in resting lymphocytes.

Consequences of the depletion of cellular deoxynucleoside triphosphate pools on the excision-repair process in cultured human fibroblasts

DNA excision repair requires the insertion of bases into gaps in the DNA which arise during the removal of damaged sites from the chromatin. The number of bases required is dependent on the amount of damage and the patch size of repair in response to the particular type of damage. In cells in which the ability to synthesize deoxynucleoside triphosphates (dNTPs) has been compromised, repair cannot proceed to completion following doses of DNA-damaging agents which induce repair that requires greater than the steady-state level of dNTPs. Repair is thus not equally sensitive to depletion of dNTPs when measured in rapidly cycling cells with relatively high dNTP pools or in non-cycling cells with significantly smaller pools. Critical depletion of dNTPs results in the production of long-lived DNA strand breaks at repairing sites and reduction in the number of sites initiating repair. On the other hand, elevation of dNTP pools to 10-50-fold normal levels did not inhibit repair. This indicates that dNTP pool depletion but not general pool-imbalance affects DNA excision repair.

Hypersensitivity of Drosophila mei-41 mutants to hydroxyurea is associated with reduced mitotic chromosome stability

6 mutant alleles of the mei-41 locus in Drosophila melanogaster are shown to cause hypersensitivity to hydroxyurea in larvae. The strength of that sensitivity is directly correlated with the influence of the mutant alleles on meiosis in that: alleles exhibiting a strong meiotic effect (mei-41D2, mei-41D5, mei-41D7) are highly sensitive; alleles with negligible meiotic effects (mei-41(104)D1, mei-41(104)D2) are moderately sensitive and an allele which expresses meiotic effects only under restricted conditions (mei-41D9) has an intermediate sensitivity. This sensitivity is not a general feature of strong postreplication repair-deficient mutants, because mutants with that phenotype from other loci do not exhibit sensitivity (mus(2)205A1, mus(3)302D1, mus(3)310D1). The observed lethality is not due to hypersensitivity of DNA synthesis in mei-41 larvae to hydroxyurea as assayed by tritiated thymidine incorporation. Lethality is, however, potentially attributable to an abnormal enhancement of chromosomal aberrations by hydroxyurea in mutant mei-41 larvae. Both in vivo and in vitro exposure of neuroblast cells to hydroxyurea results in an increase in 3 types of aberrations which is several fold higher in mei-41 tissue. Since hydroxyurea disrupts DNA synthesis, these results further implicate the mei-41 locus in DNA metabolism and provide an additional tool for an elucidation of its function. The possible existence of additional genes of this nature is suggested by a more modest sensitivity to hydroxyurea which has been detected in two stocks carrying mutagen-sensitive alleles of alternate genes.

Changes in deoxynucleoside triphosphate pools induced by inhibitors and modulators of ribonucleotide reductase

Changes in dNTP pools have been studied by a number of investigators, in a wide range of cell types. The in vitro pertubations in dNTP pool levels induced, in particular, by deoxynucleosides which act as allosteric modulators, are not totally consistent with current 'in vitro models' of ribonucleotide reductase function. This problem has also been addressed by Henderson et al. (1980) who stress the profusion of such models. Possible explanations, apart from the technical problems of the range of different experimental conditions (e.g. concentration of modulator used, time of incubation, etc.) for the various cell lines include: Modulators presumably have unpredictable 'network' effects by inhibiting or stimulating many other enzymes involved in the de novo and salvage synthesis of purines and pyrimidines. It is possible there are two separate forms of ribonucleotide reductase, one specifically reducing CDP/UDP, the other ADP/GDP. This, in particular, would explain the lack of decrease in dCTP levels after elevation of the dATP pool. There may be variations in ribonucleotide function which in vivo are cell specific, e.g. in thymic-derived compared with non-T-cell types. Peculiarities of T-cells include: Their ability to elevate their dNTP pools on exposure to very low exogenous concentration of deoxynucleoside. This may reflect very low rates of dNTP catabolism. The biological response of T-cells to elevation of the dATP or dGTP pool is reflected by a G1 block compared to an S phase block in cell-cycle progression in non-T-cell lines. The possibility that, in thymic cells, ribonucleotide reduction is restricted to ADP/GDP while pyrimidine dNTPs are synthesized by salvage pathways. As well, possible variation in the pool localization of dNTPs depending on production by either de novo or salvage synthesis could produce dNTP pool changes not clearly in accord with in vitro models. Clearly, the solution to these problems (although not easy) requires systematic comparative study, using cells of various origin (particularly T vs non-T), of dNTP pool responses to deoxynucleoside modulators, with an attempt to explore the factors described above. However, in the detailed pursuit of such an analysis the concept, that these variations in the control of nucleotide metabolism in T and non-T-cell systems may reflect quite significant differences in growth control and cell-cycle progression, should not be lost.

Deoxyribonucleoside triphosphate pools in human diploid fibroblasts and their modulation by hydroxyurea and deoxynucleosides

Deoxyribonucleoside triphosphate (dNTP) pool levels were examined in synchronized and unsynchronized log phase cultures and in quiescent cultures of human diploid foreskin fibroblasts. dNTP levels were in good agreement with those previously published for human HeLa and lymphoblastic leukemia cells. dCTP and dGTP levels showed only a modest lowering in quiescent as compared to log-phase cells, but dATP and dTTP levels were reduced dramatically in quiescent cultures. Cells synchronized by serum starvation and assayed at the peak DNA synthetic phase (18-21 hr post release) showed substantially higher pools of all four dNTPs. Hydroxyurea treatment reduced only purine dNTPs in both log phase and confluent cells while increasing dTTP and dCTP pools. The effects of deoxynucleosides on dNTP pools were also examined and are discussed in light of current models regarding regulation of purified ribonucleotide reductase formulated from in vitro studies.

The role of deoxynucleoside triphosphate pools in the inhibition of DNA-excision repair and replication in human cells by hydroxyurea

The effects of hydroxyurea (HU) on the DNA-excision repair process in human cells has been systematically examined. It is demonstrated that HU induces DNA single-strand break accumulation in a dose-dependent fashion in ultraviolet-irradiated and MMS-treated confluent but not log-phase fibroblasts and that these breaks are clearly the consequence of the inhibition by HU of the enzyme, ribonucleotide reductase. The breaks form rapidly, are stable for at least 10 h and largely disappear by 20 h. The production of these DNA-strand breaks is antagonized by a combined treatment of 10 microM deoxyadenosine, deoxycytidine and deoxyguanosine whereas thymidine potentiates strand-break formation at low HU concentrations. It is also confirmed that HU, while inhibiting replicative synthesis has no apparent inhibitory effect on unscheduled DNA synthesis (UDS) although the increased uptake of labeled DNA precursors into HU-treated cells makes it difficult to assess the actual effects on the repair-synthetic process. Analysis of the effects of HU on deoxynucleoside triphosphate pool levels and the demonstration of the failure of the HU block to replicative synthesis to be reversed by high (1 mM) concentrations of added deoxynucleosides lend support to the notion of compartmentalized dNTP pools for repair and replication.