Response of the G2-prophase checkpoint to genotoxic drugs in lymphocytes from healthy individuals.

We analyzed the in vitro effects of the anti-tumoral drugs doxorubicin, cytosine arabinoside and hydroxyurea on the G2-prophase checkpoint in lymphocytes from healthy individuals. At biologically equivalent concentrations, the induced DNA damage activated the corresponding checkpoint. Thus: i) there was a concentration-dependent delay of G2 time and an increase of both the total DNA lesions produced and repaired before metaphase and; ii) G2-checkpoint adaptation took place as chromosome aberrations (CAs) started to appear in the metaphase, indicating the presence of unrepaired double-strand breaks (DSBs) in the previous G2. The checkpoint ATM/ATR kinases are involved in DSB repair, since the recorded frequency of CAs increased when both kinases were caffeine-abrogated. In genotoxic-treated cells about three-fold higher repair activity was observed in relation to the endogenous background level of DNA lesions. The maximum rate of DNA repaired was 3.4 CAs/100 metaphases/hour, this rise being accompanied by a modest 1.3 fold lengthening of late G2 prophase timing. Because of mitotic chromosome condensation, no DSBs repair can take place until the G1 phase of the next cell cycle, when it occurs by DNA non-homologous end joining (NHEJ). Chromosomal rearrangements formed as a consequence of these error-prone DSB repairs ensure the development of genome instability through the DNA-fusion-bridge cycle. Hence, adaptation of the G2 checkpoint supports the appearance of secondary neoplasia in patients pretreated with genotoxic drugs.

Response of the G2-prophase checkpoint to genotoxic drugs in lymphocytes from healthy individuals

We analyzed the in vitro effects of the anti-tumoral drugs doxorubicin, cytosine arabinoside and hydroxyurea on the G2-prophase checkpoint in lymphocytes from healthy individuals. At biologically equivalent concentrations, the induced DNA damage activated the corresponding checkpoint. Thus: i) there was a concentration-dependent delay of G2 time and an increase of both the total DNA lesions produced and repaired before metaphase and; ii) G2-checkpoint adaptation took place as chromosome aberrations (CAs) started to appear in the metaphase, indicating the presence of unrepaired double-strand breaks (DSBs) in the previous G2. The checkpoint ATM/ATR kinases are involved in DSB repair, since the recorded frequency of CAs increased when both kinases were caffeine-abrogated. In genotoxic-treated cells about three-fold higher repair activity was observed in relation to the endogenous background level of DNA lesions. The maximum rate of DNA repaired was 3.4 CAs/100 metaphases/hour, this rise being accompanied by a modest 1.3 fold lengthening of late G2 prophase timing. Because of mitotic chromosome condensation, no DSBs repair can take place until the G1 phase of the next cell cycle, when it occurs by DNA non-homologous end joining (NHEJ). Chromosomal rearrangements formed as a consequence of these error-prone DSB repairs ensure the development of genome instability through the DNA-fusion-bridge cycle. Hence, adaptation of the G2 checkpoint supports the appearance of secondary neoplasia in patients pretreated with genotoxic drugs.

Arabidopsis S6 kinase mutants display chromosome instability and altered RBR1-E2F pathway activity.

The 40S ribosomal protein S6 kinase (S6K) is a conserved component of signalling pathways controlling growth in eukaryotes. To study S6K function in plants, we isolated single- and double-knockout mutations and RNA-interference (RNAi)-silencing lines in the linked Arabidopsis S6K1 and S6K2 genes. Hemizygous s6k1s6k2/++ mutant and S6K1 RNAi lines show high phenotypic instability with variation in size, increased trichome branching, produce non-viable pollen and high levels of aborted seeds. Analysis of their DNA content by flow cytometry, as well as chromosome counting using DAPI staining and fluorescence in situ hybridization, revealed an increase in ploidy and aneuploidy. In agreement with this data, we found that S6K1 associates with the Retinoblastoma-related 1 (RBR1)-E2FB complex and this is partly mediated by its N-terminal LVxCxE motif. Moreover, the S6K1-RBR1 association regulates RBR1 nuclear localization, as well as E2F-dependent expression of cell cycle genes. Arabidopsis cells grown under nutrient-limiting conditions require S6K for repression of cell proliferation. The data suggest a new function for plant S6K as a repressor of cell proliferation and required for maintenance of chromosome stability and ploidy levels.

Hepatocytes, rather than leukocytes reverse DNA damage in vivo induced by whole body gamma-irradiation of mice, as shown by the alkaline comet assay.

DNA damage repair was assessed in quiescent (G0) leukocytes and in hepatocytes of mice, after 1 and 2 hours recovery from a single whole body y-irradiation with 0.5, 1 or 2 Gy. Evaluation of single-strand breaks (SSB) and alkali-labile sites together were carried out by a single-cell electrophoresis at pH>13.0 (alkaline comet assay). In non-irradiated (control) mice, the constitutive, endogenous DNA damage (basal) was around 1.5 times higher in leukocytes than in hepatocytes. Irradiation immediately increased SSB frequency in both cell types, in a dose-dependent manner. Two sequential phases took place during the in vivo repair of the radio-induced DNA lesions. The earliest one, present in both hepatocytes and leukocytes, further increased the SSB frequency, making evident the processing of some primary lesions in DNA bases into the SSB repair intermediates. In a second phase, SSB frequency decreased because of their removal. In hepatocytes, such a frequency regressed to the constitutive basal level after 2 hours recovery from either 0.5 or 1 Gy. On the other hand, the SSB repair phase was specifically abrogated in leukocytes, at the doses and recovery times analyzed. Thus, the efficiency of in vivo repair of radio-induced DNA damage in dormant cells (lymphocytes) is quite different from that in hepatocytes whose low proliferation activity accounts only for cell renewal.

Nuclear actin in plants.

Actins constitute a wide family of proteins that are major components of the cytoskeleton. Animal cells have nuclear G-actin forms that assemble into several nuclear macromolecular complexes and are substrates for myosin I beta (NMI). The nuclear actin related proteins (ARPs) are part of the chromatin-remodelling complex, while nuclear acting binding proteins (nABPs) comprise either nuclear forms of cytoplasmic ABPs (as NMI) or specific nABPs. No evidence of the presence of nuclear actin exists in plants, which lack orthologues of the main animal structural nABPs. Here we prove the presence of actin forms with different solubility, and their associated protein NMI in the plant nucleus, as components of the transcription complexes and the nucleoskeleton. For this, WB and confocal immunofluorescence with antibodies against human actin and NMI were used.

Triplex configuration in the nick-free DNAs that constitute the chromosomal scaffolds in grasshopper spermatids.

After applying proper deoxyribonucleic acid (DNA) probes, fluorescence in situ hybridization (FISH) showed that the 8/9 centromeres-one per chromatid of the male haploid complement (X0) of Pyrgomorpha conica grasshopper-colocalized at the spermatid blunt end, where the spermatozoa flagellum inserts. A bundle of aligned 4',6-diamidino-2-phenylindole-positive chromatid scaffolds, which formed the central spermatid core, was observed after DNA breakage detection followed by FISH. Modular nature of scaffold DNA was occasionally evident. The technique also showed that in the early spermatid, the chromatid scaffolds lacked any DNA nick, whereas abundant breaks accumulated in the surrounding loops. Moreover, immunodetection showed that scaffold DNA participated in the formation of triplex DNA, while this configuration was absent from the loops. During spermatid maturation, triplex DNA disappeared from the scaffold in parallel with loop retraction, while protamines replace histones. Thus, the presence of triplex DNA in the chromatid scaffold correlates with the anchoring of expanded DNA loops to it. After loop retraction, the scaffolds of all chromatids coiled as a single unit in the spermatid head. This cooperative coiling produced enlargement and tilting of the distal telomeric signals, which were distributed along the spermatid head according to the length of each chromosome. We propose that specific DNA sequences dispersed throughout the whole chromatid fold forward and backward coaxially to chromatid length, forming individual scaffold modules whose linear assembly accounts for the minimum length of each individual chromatid. Finally, the core of the grasshopper male spermatid should be considered as a single chromosome in which the DNA scaffolds of the whole set of the nonhomologous chromosomes of the haploid complement are interconnected. This pattern of chromatin organization applies probably to other elongated spermatids.

Hepatocytes, rather than leukocytes reverse DNA damage in vivo induced by whole body y-irradiation of mice, as shown by the alkaline comet assay

DNA damage repair was assessed in quiescent (G0) leukocytes and in hepatocytes of mice, after 1 and 2 hours recovery from a single whole body y-irradiation with 0.5, 1 or 2 Gy. Evaluation of single-strand breaks (SSB) and alkali-labile sites together were carried out by a single-cell electrophoresis at pH>13.0 (alkaline comet assay). In non-irradiated (control) mice, the constitutive, endogenous DNA damage (basal) was around 1.5 times higher in leukocytes than in hepatocytes. Irradiation immediately increased SSB frequency in both cell types, in a dose-dependent manner. Two sequential phases took place during the in vivo repair of the radio-induced DNA lesions. The earliest one, present in both hepatocytes and leukocytes, further increased the SSB frequency, making evident the processing of some primary lesions in DNA bases into the SSB repair intermediates. In a second phase, SSB frequency decreased because of their removal. In hepatocytes, such a frequency regressed to the constitutive basal level after 2 hours recovery from either 0.5 orí Gy. On the other hand, the SSB repair phase was specifically abrogated in leukocytes, at the doses and recovery times analyzed. Thus, the efficiency of in vivo repair of radio-induced DNA damage in dormant cells (lymphocytes) is quite different from that in hepatocytes whose low proliferation activity accounts only for cell renewal.

G2 checkpoint-dependent DNA repair and its response to catalase in Down syndrome and control lymphocyte cultures.

The amount of DNA lesions repaired in G2 and also G2 timing are controlled by the DNA damage-dependent checkpoint. Down syndrome (DS) lymphocytes showed twice as much constitutive DNA damage in G2 than control ones, when recording it as chromosomal aberrations in metaphase, after caffeine-induced checkpoint abrogation. During G2, DS lymphocytes repaired 1.5 times more DNA lesions than control ones. However the DS cells displayed a decreased threshold for checkpoint adaptation, as the spontaneous override of the G2 to mitosis transition block induced by the checkpoint took place in the DS cells when they had three times more DNA lesions than controls. Catalase addition to cultures scavenges hydrogen peroxide diffused from cells, resulting in subsequent intracellular depletion (Antunes and Cadenas, 2000). The intracellular H2O2 level seemed to regulate the G2 checkpoint. Thus, in controls, H2O2 depletion (induced by 3.2-50 microg/mL catalase) prevented its functioning: chromosomal damage increased while G2 shortened. Conversely, in the DS lymphocytes, 12.5 microg/mL catalase lengthened G2 and decreased chromosomal damage, in spite that the amount of DNA repaired in G2 was half of that repaired in the catalase-free DS lymphocytes.

The G2 checkpoint activated by DNA damage does not prevent genome instability in plant cells.

Root growth, G2 length, and the frequency of aberrant mitoses and apoptotic nuclei were recorded after a single X-ray irradiation, ranging from 2.5 to 40 Gy, in Allium cepa L. root meristematic cells. After 72 h of recovery, root growth was reduced in a dose-dependent manner from 10 to 40 Gy, but not at 2.5 or 5 Gy doses. Flow cytometry plus TUNEL (TdT-mediated dUTP nick end labeling) showed that activation of apoptosis occurred only after 20 and 40 Gy of X-rays. Nevertheless, irrespective of the radiation dose, conventional flow cytometry showed that cells accumulated in G2 (4C DNA content). Simultaneously, the mitotic index fell, though a mitotic wave appeared later. Cell accumulation in G2 was transient and partially reversed by caffeine, thus it was checkpoint-dependent. Strikingly, the additional G2 time provided by this checkpoint was never long enough to complete DNA repair. Then, in all cases, some G2 cells with still-unrepaired DNA underwent checkpoint adaptation, i.e., they entered into the late mitotic wave with chromatid breaks. These cells and those produced by the breakage of chromosomal bridges in anaphase will reach the G1 of the next cell cycle unrepaired, ensuring the appearance of genome instability.

CK2 phosphorylation weakens 90 kDa MFP1 association to the nuclear matrix in Allium cepa.

MFP1 is a conserved plant coiled-coil protein located on the stroma side of the chloroplast thylakoids, as well as in the nuclear matrix. It displays species-specific variability in the number of genes, proteins, and expression. Allium cepa has two nuclear proteins antigenically related to MFP1 with different M(r), pI, distribution, and expression, but only the 90 kDa MFP1 protein is a nuclear matrix component that associates with both the nucleoskeletal filaments and a new category of nuclear bodies. The 90 kDa AcMFP1 migrates in two-dimensional blots as two sets of spots. The hypo-phosphorylated forms (pI approximately 9.5) are tightly bound to the nuclear matrix, while high ionic strength buffers release the more acidic hyper-phosphorylated ones (pI approximately 8.5), suggesting that the protein is post-translationally modified, and that these modifications control its attachment to the nuclear matrix. Dephosphorylation by exogenous alkaline phosphatase and phosphorylation by exogenous CK2, as well as specific inhibition and stimulation of endogenous CK2 with heparin and spermine and spermidine, respectively, revealed that the protein is an in vitro and in vivo substrate of this enzyme, and that CK2 phosphorylation weakens the strength of its binding to the nuclear matrix. In synchronized cells, the nuclear 90 kDa AcMFP1 phosphorylation levels vary during the cell cycle with a moderate peak in G2. These results provide the first evidence for AcMFP1 in vivo phosphorylation, and open up further research on its nuclear functions.

The G2 checkpoint activated by DNA damage does not prevent genome instability in plant cells

Root growth, G2 length, and the frequency of aberrant mitoses and apoptotic nuclei were recorded after a single X-ray irradiation, ranging from 2.5 to 40 Gy, in Allium cepa L. root meristematic cells. After 72 h of recovery, root growth was reduced in a dose-dependent manner from 10 to 40 Gy, but not at 2.5 or 5 Gy doses. Flow cytometry plus TUNEL (TdT-mediated dUTP nick end labeling) showed that activation of apoptosis occurred only after 20 and 40 Gy of X-rays. Nevertheless, irrespective of the radiation dose, conventional flow cytometry showed that cells accumulated in G2 (4C DNA content). Simultaneously, the mitotic index fell, though a mitotic wave appeared later. Cell accumulation in G2 was transient and partially reversed by caffeine, thus it was checkpoint-dependent. Strikingly, the additional G2 time provided by this checkpoint was never long enough to complete DNA repair. Then, in all cases, some G2 cells with still-unrepaired DNA underwent checkpoint adaptation, i.e., they entered into the late mitotic wave with chromatid breaks. These cells and those produced by the breakage of chromosomal bridges in anaphase will reach the G1 of the next cell cycle unrepaired, ensuring the appearance of genome instability.

Proteasome activity is required for centromere separation independently of securin degradation in human cells.

Loss of centromere cohesion during anaphase in human cells is regulated by the spindle assembly checkpoint and is thought to depend on a ubiquitin ligase, the Anaphase Promoting Complex/Cyclosome (APC). APC-Cdc20 adds ubiquitin chains to securin inducing its destruction by the proteasome and these events correlate with the loss of sister chromatid cohesion and the onset of anaphase. But whether securin destruction is necessary and sufficient for anaphase initiation is not clear. Therefore, we asked if proteasome activity is needed for anaphase onset in human cells that lack securin. We find that even in the absence of securin, a metaphase block with cohered sister centromeres can be enforced in the absence of proteasome activity. Therefore, other targets of the proteasome must be degraded to allow anaphase onset.

Roles of nibrin and AtM/ATR kinases on the G2 checkpoint under endogenous or radio-induced DNA damage.

Checkpoint response to DNA damage involves the activation of DNA repair and G2 lengthening subpathways. The roles of nibrin (NBS1) and the ATM/ATR kinases in the G2 DNA damage checkpoint, evoked by endogenous and radio-induced DNA damage, were analyzed in control, A-T and NBS lymphoblast cell lines. Short-term responses to G2 treatments were evaluated by recording changes in the yield of chromosomal aberrations in the ensuing mitosis, due to G2 checkpoint adaptation, and also in the duration of G2 itself. The role of ATM/ATR in the G2 checkpoint pathway repairing chromosomal aberrations was unveiled by caffeine inhibition of both kinases in G2. In the control cell lines, nibrin and ATM cooperated to provide optimum G2 repair for endogenous DNA damage. In the A-T cells, ATR kinase substituted successfully for ATM, even though no G2 lengthening occurred. X-ray irradiation (0.4 Gy) in G2 increased chromosomal aberrations and lengthened G2, in both mutant and control cells. However, the repair of radio-induced DNA damage took place only in the controls. It was associated with nibrin-ATM interaction, and ATR did not substitute for ATM. The absence of nibrin prevented the repair of both endogenous and radio-induced DNA damage in the NBS cells and partially affected the induction of G2 lengthening.

Roles of polo-like kinase 1 in the assembly of functional mitotic spindles.

BACKGROUND: The stable association of chromosomes with both poles of the mitotic spindle (biorientation) depends on spindle pulling forces. These forces create tension across sister kinetochores and are thought to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint. Polo-like kinase 1 (Plk1) has been implicated in regulating centrosome maturation, mitotic entry, sister chromatid cohesion, the anaphase-promoting complex/cyclosome (APC/C), and cytokinesis, but it is unknown if Plk1 controls chromosome biorientation. RESULTS: We have analyzed Plk1 functions in synchronized mammalian cells by RNA interference (RNAi). Plk1-depleted cells enter mitosis after a short delay, accumulate in a preanaphase state, and subsequently often die by apoptosis. Spindles in Plk1-depleted cells lack focused poles and are not associated with centrosomes. Chromosomes attach to these spindles, but the checkpoint proteins Mad2, BubR1, and CENP-E are enriched at many kinetochores. When Plk1-depleted cells are treated with the Aurora B inhibitor Hesperadin, which silences the spindle checkpoint by stabilizing microtubule-kinetochore interactions, cells degrade APC/C substrates and exit mitosis without chromosome segregation and cytokinesis. Experiments with monopolar spindles that are induced by the kinesin inhibitor Monastrol indicate that Plk1 is required for the assembly of spindles that are able to generate poleward pulling forces. CONCLUSIONS: Our results imply that Plk1 is not essential for mitotic entry and APC/C activation but is required for proper spindle assembly and function. In Plk1-depleted cells spindles may not be able to create enough tension across sister kinetochores to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint.

Regulation of sister chromatid cohesion between chromosome arms.

Sister chromatid separation in anaphase depends on the removal of cohesin complexes from chromosomes. In vertebrates, the bulk of cohesin is already removed from chromosome arms during prophase and prometaphase, whereas cohesin remains at centromeres until metaphase, when cohesin is cleaved by the protease separase. In unperturbed mitoses, arm cohesion nevertheless persists throughout metaphase and is principally sufficient to maintain sister chromatid cohesion. How arm cohesion is maintained until metaphase is unknown. Here we show that small amounts of cohesin can be detected in the interchromatid region of metaphase chromosome arms. If prometaphase is prolonged by treatment of cells with microtubule poisons, these cohesin complexes dissociate from chromosome arms, and arm cohesion is dissolved. If cohesin dissociation in prometaphase-arrested cells is prevented by depletion of Plk1 or inhibition of Aurora B, arm cohesion is maintained. These observations imply that, in unperturbed mitoses, small amounts of cohesin maintain arm cohesion until metaphase. When cells lacking Plk1 and Aurora B activity enter anaphase, chromatids lose cohesin. This loss is prevented by proteasome inhibitors, implying that it depends on separase activation. Separase may therefore be able to cleave cohesin at centromeres and on chromosome arms.

p53-independent checkpoint controls in a plant cell model.

Allium cepa L. meristems were used as a plant model to study the p53-independent control of S and G2 phases by checkpoint pathways, in eukaryotic cells. Checkpoint blocks were induced at early and mid S by hydroxyurea. After their spontaneous override, cells became accumulated in G2-prophase, giving rise later on to a delayed mitotic wave. Cell growth was maintained during the checkpoint blocks, as the delayed mitoses were larger in size than the control ones. Under continuous hydroxyurea treatment, the delayed mitotic was formed by two subpopulations: normal mitoses corresponding to cells having properly recovered from the checkpoint block, and abnormal ones resulting from checkpoint adaptation. These latter cells displayed broken chromatids as they had unduly overriden the G2 checkpoint block, without completing DNA repair. The frequency of the checkpoint-adapted mitoses increased with the hydroxyurea concentration from 0.25 to 1.0 mM. However, from 1 mM hydroxyurea upwards, some of the cells lost their competence for checkpoint adaptation. Therefore, the dose of a genotoxic agent that still allows G2 checkpoint adaptation should always be applied in order to get rid of uncontrolled proliferating cells. This is specially suitable for cells lacking a functional p53 protein.

A plant cyclin B2 is degraded early in mitosis and its ectopic expression shortens G2-phase and alleviates the DNA-damage checkpoint.

Mitotic progression is timely regulated by the accumulation and degradation of A- and B-type cyclins. In plants, there are three classes of A-, and two classes of B-type cyclins, but their specific roles are not known. We have generated transgenic tobacco plants in which the ectopic expression of a plant cyclin B2 gene is under the control of a tetracycline-inducible promoter. We show that the induction of cyclin B2 expression in cultured cells during G2 phase accelerates the entry into mitosis and allows cells to override the replication checkpoint induced by hydroxyurea in the simultaneous presence of caffeine or okadaic acid, drugs that are known to alleviate checkpoint control. These results indicate that in plants, a B2-type cyclin is a rate-limiting regulator for the entry into mitosis and a cyclin B2-CDK complex might be a target for checkpoint control pathways. The cyclin B2 localization and the timing of its degradation during mitosis corroborate these conclusions: cyclin B2 protein is confined to the nucleus and during mitosis it is only present during a short time window until mid prophase, but it is effectively degraded from this timepoint onwards. Although cyclin B2 is not present in cells arrested by the spindle checkpoint in metaphase, cyclin B1 is accumulating in these cells. Ectopic expression of cyclin B2 in developing plants interferes with differentiation events and specifically blocks root regeneration, indicating the importance of control mechanisms at the G2- to M-phase transition during plant developmental processes.

A topoisomerase II-dependent checkpoint in G2-phase plant cells can be bypassed by ectopic expression of mitotic cyclin B2.

DNA topoisomerase II is required for mitotic chromosome condensation and segregation. Here we characterize the effects of inhibiting DNA topoisomerase II activity in plant cells using the non-DNA damaging topoisomerase II inhibitor ICRF-193. We report that ICRF-193 abrogated chromosome condensation in cultured alfalfa (Medicago sativa L.) and tobacco (Nicotiana tabaccum L.) mitoses and led to bridged chromosomes at anaphase. Moreover, ICRF-193 treatment delayed entry into mitosis, increasing the frequency of cells having a pre-prophase band of microtubules, a marker of late G2 and prophase, and delaying the activation of cyclin-dependent kinase. These data suggest the existence of a late G2 checkpoint in plant cells that is activated in the absence of topoisomerase II activity. To determine whether the checkpoint-induced delay was a result of reduced cyclindependent kinase activity, mitotic cyclin B2 was ectopically expressed. Cyclin B2 bypassed the ICRF-193-induced delay before mitosis, and correspondingly, reduced the frequency of interphase cells with a pre-prophase band. These data provide evidence that plant cells possess a topoisomerase II-dependent G2 cell cycle checkpoint that transiently inhibits mitotic CDK activation and entry into mitosis, and that is overridden by raising the level of CDK activity through the ectopic expression of a plant mitotic cyclin.

Dynamics of replication foci and nuclear matrix during S phase in Allium cepa L. cells.

The sequential organisation of replication foci during S phase in onion ( Allium cepa) and their relationship to the nuclear matrix were investigated. To discern their structural features and temporal firing sequence, immunodetection of 5-bromo-2'-deoxyuridine (BrdU) was carried out after in vivo feeding in synchronised cells released from a 14-h-long hydroxyurea block. Replication foci consisted of small replication granules, called replisomes, which clustered together. Analysis of synchronous binucleate cells that maintained in their two nuclei the specular symmetry of distribution of sister chromosomes in anaphase, showed that replication starts in small replication foci at the telomeric pole (pattern I), though the telomeres themselves formed large foci that were late-replicating. The rDNA replication foci (pattern II) also become replicated in early S phase. Replication of large foci, including the heterochromatin (IV), occurred in late S phase and finished at the centromeric nuclear pole (pattern V). Labelling of proliferating cell nuclear antigen (PCNA) in nuclear matrices, prepared from S-phase nuclei after extensive DNase digestion, demonstrated that replication foci were always stably anchored to the nuclear matrix. Thus, association with the nucleoskeleton is not exclusively mediated by the replicating or nascent DNA. The overlapping of patterns I, II and III in the nuclear matrix, in contrast to the results of BrdU localisation in nuclei, suggests that PCNA becomes associated with the nuclear matrix before the replication foci are operative, and remains bound during replication.