Reinitiation Of DNA Replication Research Articles

Regulation of MCM2-7 function.

Recently published structural and functional analyses of the CMG complex have provided insight into the mechanism of its DNA helicase function and into the distinct roles of its central six component proteins MCM2-MCM7 (MCM2-7). To activate CMG helicase, the two protein kinases CDK and DDK, as well as MCM10, are required. In addition to the initiation of DNA replication, MCM function must be regulated at the DNA replication steps of elongation and termination. Polyubiquitylation of MCM7 is involved in terminating MCM function. Reinitiation of DNA replication in a single cell cycle, which is prevented mainly by CDK, is understood at the molecular level. MCM2-7 gene expression is regulated during cellular aging and the cell cycle, and the expression depends on oxygen concentration. These regulatory processes have been described recently. Genomic structural alteration, which is an essential element in cancer progression, is mainly generated by disruptions of DNA replication fork structures. A point mutation in MCM4 that disturbs MCM2-7 function results in genomic instability, leading to the generation of cancer cells. In this review, I focus on the following points: 1) function of the MCM2-7 complex, 2) activation of MCM2-7 helicase, 3) regulation of MCM2-7 function, 4) MCM2-7 expression, and 5) the role of MCM mutation in cancer progression.

Human CST abundance determines recovery from diverse forms of DNA damage and replication stress

Mammalian CST (CTC1-STN1-TEN1) is a telomere-associated complex that functions in telomere duplex replication and fill-in synthesis of the telomeric C-strand following telomerase action. CST also facilitates genome-wide replication recovery after HU-induced fork stalling by increasing origin firing. CTC1 and STN1 were originally isolated as a DNA polymerase α stimulatory factor. Here we explore how CST abundance affects recovery from drugs that cause different types of DNA damage and replication stress. We show that recovery from HU and aphidicolin induced replication stress is increased by CST over-expression. Elevated CST increases dNTP incorporation and origin firing after HU release and decreases the incidence of anaphase bridges and micronuclei after aphidicolin removal. While the frequency of origin firing after HU release is proportional to CST abundance, the number of cells entering S-phase to initiate replication is unchanged by CST overexpression or STN1 depletion. Instead the CST-related changes in origin firing take place in cells that were already in S-phase at the time of HU addition, indicating that CST modulates firing of late or dormant origins. CST abundance also influences cell viability after treatment with HU, aphidicolin, MMS and camptothecin. Viability is increased by elevated CST and decreased by STN1 depletion, indicating that endogenous CST levels are limiting. However, CST abundance does not affect viability after MMC treatment. Thus, CST facilitates recovery from many, but not all, forms of exogenous DNA damage. Overall our results suggest that CST is needed in stoichiometric amounts to facilitate re-initiation of DNA replication at repaired forks and/or dormant origins.

Homologous Recombination Is a Primary Pathway to Repair DNA Double-Strand Breaks Generated during DNA Rereplication

Re-initiation of DNA replication at origins within a given cell cycle would result in DNA rereplication, which can lead to genome instability and tumorigenesis. DNA rereplication can be induced by loss of licensing control at cellular replication origins, or by viral protein-driven multiple rounds of replication initiation at viral origins. DNA double-strand breaks (DSBs) are generated during rereplication, but the mechanisms of how these DSBs are repaired to maintain genome stability and cell viability are poorly understood in mammalian cells. We generated novel EGFP-based DSB repair substrates, which specifically monitor the repair of rereplication-associated DSBs. We demonstrated that homologous recombination (HR) is an important mechanism to repair rereplication-associated DSBs, and sister chromatids are used as templates for such HR-mediated DSB repair. Micro-homology-mediated non-homologous end joining (MMEJ) can also be used but to a lesser extent compared to HR, whereas Ku-dependent classical non-homologous end joining (C-NHEJ) has a minimal role to repair rereplication-associated DSBs. In addition, loss of HR activity leads to severe cell death when rereplication is induced. Therefore, our studies identify HR, the most conservative repair pathway, as the primary mechanism to repair DSBs upon rereplication.

Markers of cell division cycle in glioblastoma: significance in prediction of treatment response and patient prognosis

Objective. To investigate whether expression of regulatory components of the cell division cycle can be used independently to predict survival and response to adjuvant therapy in glioblastomas. Method. A tissue micro-array, constructed using glioblastomas (n = 66), was stained using antibodies against minichromosome maintenance protein-2 (Mcm-2), expressed throughout the cell-division cycle; geminin, a protein that prevents re-initiation of DNA replication; and cyclin A, an S-phase cyclin. A semi-quantitative labelling index (LI) was calculated using an average of 18 high-power fields (hpf) in three replicate cores. The patients were divided into two groups: Group 1 (n = 50) underwent surgery and radiotherapy with 24 patients receiving temozolomide, and Group 2 (n = 16) received surgical treatment only. Results. The LIs (median +/− IQR) for Group 1 were as follows: Mcm-2, 36.7% (22.9%–51.8%); geminin, 7.8% (5.8%–10.5%); and cyclin A, 4.2% (2.4%–6.9%). Elevated LIs, higher than the median, for geminin and cyclin A correlated with prolonged survival when the tumours received adjuvant therapy (Kaplan–Meier curves, p = 0.0046 and p = 0.0063 for geminin and cyclin A, respectively). Linear regression analysis revealed positive correlations with survival for Mcm-2 (p = 0.0376), geminin (p = 0.0006) and cyclin A (p = 0.004). In Group 2, there was no relationship between the patient survival and the LI for any marker. Conclusions. Geminin and cyclin A, each show potential as independent prognostic markers in glioblastomas receiving adjuvant therapy. This may reflect the fact that both geminin and cyclin A estimate proliferating tumour cell subpopulations sensitive to radio/chemotherapy. These markers could provide valuable prognostic information, even in small biopsies, especially if combined with O6MGMT expression and 1p;19q deletion status.

Abstract P5-06-01: Active pak6 induces aneuploidy in breast cancer cells.

Abstract The natural history of breast cancer remains largely undefined. More than 50% of clinical specimens exhibit aneuploidy. Among these aneuploid cancers, nearly half have chromosome copy numbers approaching tetraploid. The high incidence of aneuploidy in advanced breast cancer is thought to be a byproduct of chromosome mis-segregation resulting from mutations in tumor suppressor genes regulating cell cycle checkpoints and corresponding to disease progression. However, recent large-scale sequencing projects have revealed that most tumors contain approximately 14–20 different mutant genes, suggesting heterogeneity exists throughout the genomes of cancer cells. This notion revives the concept that aneuploidy resulting from chromosome mis-segregation or other similar mechanisms may play an active role leading to transformation and disease progression. A novel p21-activated kinase 6 (PAK6) was recently identified to be an androgen receptor (AR) and estrogen receptor (ER) interacting protein. An estrogen-stimulated PAK6 activation was observed in BT474 breast cancer cells. This activation is mediated by estrogen-stimulated activation of PKA. At the cellular level, active PAK6 promotes transformation to metastatic phenotypes in MCF7 breast cancer cells as evidenced by increases in cell motility, matrigel invasion, and soft agar growth. Further characterization of MCF7 cells expressing a constitutively active PAK6 mutant revealed an accumulation of cells with higher ploidy as compared to that of parental cells. Using a proteomic approach to determine the PAK6 downstream target, we identified that Cdt1, a DNA pre-replicative complex subunit, is overexpressed in response to PAK6 activation. In eukaryotes, the expression and stability of Cdt1 are strictly regulated to ensure only one round of DNA replication in each cell cycle. Deregulated overexpression of Cdt1 has previously been demonstrated to induce aneuploidy and malignant transformation due to re-initiation of DNA replication within the same S-phase. The identification of Cdt1 overexpression in response to PAK6 activation provides a plausible mechanism for the observed accumulation of higher ploidy cells in MCF7 cells expressing active PAK6. Taken together these findings suggest a direct link between hormonal signals, DNA replication, and maintenance of genomic integrity. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-06-01.

Abstract 2153: Active PAK6 induces aneuploidy in breast cancer cells

Abstract The natural history of breast cancer remains largely undefined. More than 50% of the clinical specimen exhibits aneuploidy. Among these aneuploid cancers, 50% has chromosome copy number near tetraploid. The high incidence of aneupoidy in advanced breast cancer is thought to be a byproduct of chromosome mis-segregation resulting from mutations of checkpoint-regulating tumor suppressor genes associated with disease progression. However, new large-scale sequencing projects reveal that most tumors contain approximately 14-20 different mutant genes, suggesting that genomes and karyotypes of cancer cells are equally heterogeneous. This notion revives the old concept that aneuploidy resulting from chromosome mis-segregation or other similar mechanisms may play an active role leading to transformation and disease progression. A novel p21-activated kinase 6 (PAK6) was recently identified to be an androgen receptor (AR) and estrogen receptor (ER) interacting protein. An estrogen-stimulated PAK6 activation was observed in breast cancer BT474 cells. This activation is mediated by estrogen-stimulated activation of PKA. At the cellular level, active PAK6 promotes breast cancer MCF7 cell metastatic phenotypes including increased cell motility, matrigel invasion and soft agar growth. Further characterization of breast cancer MCF7 cells expressing PAK6 constitutive active mutant revealed an accumulation of cells with higher ploidy as compared to that of parental MCF7. Using a proteomic approach to determine the PAK6 downstream target, we identified that Cdt1, a DNA pre-replicative complex (pre-RC) subunit, is overexpressed in response to PAK6 activation. In eukaryotes, the expression and stability of Cdt1 are strictly regulated to ensure only one round of DNA replication in each cell cycle. Deregulated overexpression of Cdt1 has previously been demonstrated to induce aneuploidy and malignant transformation due to re-initiation of DNA replication within the same S-phase. The identification of Cdt1 overexpression in response to PAK6 activation provides a plausible mechanism for the observed accumulation of higher ploidy cells in MCF7 expressing active PAK6. These findings directly link the hormonal signals to the DNA replication process and maintenance of genomic integrity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2153. doi:1538-7445.AM2012-2153

The priB Gene of Klebsiella pneumoniae Encodes a 104-Amino Acid Protein That Is Similar in Structure and Function to Escherichia coli PriB

Primosome protein PriB is a single-stranded DNA-binding protein that serves as an accessory factor for PriA helicase-catalyzed origin-independent reinitiation of DNA replication in bacteria. A recent report describes the identification of a novel PriB protein in Klebsiella pneumoniae that is significantly shorter than most sequenced PriB homologs. The K. pneumoniae PriB protein is proposed to comprise 55 amino acid residues, in contrast to E. coli PriB which comprises 104 amino acid residues and has a length that is typical of most sequenced PriB homologs. Here, we report results of a sequence analysis that suggests that the priB gene of K. pneumoniae encodes a 104-amino acid PriB protein, akin to its E. coli counterpart. Furthermore, we have cloned the K. pneumoniae priB gene and purified the 104-amino acid K. pneumoniae PriB protein. Gel filtration experiments reveal that the K. pneumoniae PriB protein is a dimer, and equilibrium DNA binding experiments demonstrate that K. pneumoniae PriB's single-stranded DNA-binding activity is similar to that of E. coli PriB. These results indicate that the PriB homolog of K. pneumoniae is similar in structure and in function to that of E. coli.

Spotlight on Geminin

In the previous issue of Breast Cancer Research, Gardner and co-workers describe a novel interaction between Geminin, a protein that prevents reinitiation of DNA replication, and Topoisomerase IIα (TopoIIα), an enzyme essential for removing catenated intertwines between sister chromatids. Geminin facilitates the action of TopoIIα, thereby promoting termination of DNA replication at the same time it inhibits initiation. In this manner, Geminin ensures that cells duplicate their genome once, but only once, each time they divide. Remarkably, either depletion of Geminin or over-expression of Geminin inhibits the action of TopoIIα, thereby making Geminin an excellent target for cancer chemotherapy.

Factors engaged in reactivation of DNA replication in the nuclei of growing mouse oocytes introduced into the cytoplasm of parthenogenetic one-cell embryos

Mammalian primary oocytes are arrested in the post-replicative G2 phase of the cell cycle. In contrast to other G2 nuclei, the nucleus of the growing mouse oocyte can reinitiate DNA synthesis after transfer by cell fusion under favorable cytoplasmic conditions, created by the parthenogenetic one-cell embryo. In the present study, we used the cell hybrid system to analyze the distribution of proteins involved in DNA re-replication in the oocyte nucleus. We show that this process is preceded by an extensive rearrangement of the insoluble fractions of minichromosome maintenance (MCM) proteins (Mcm2, -6 and 7). We also demonstrate that Cdc6 protein is present in primary growing mouse oocytes freshly isolated from the ovary, in a soluble and insoluble form. In contrast to MCM proteins, the insoluble fraction of Cdc6 was not rearranged in oocyte nuclei reinitiating DNA replication in hybrid cells. The rearrangement of MCM proteins and reinitiation of DNA synthesis occurred in the nuclei, in which the nuclear envelope remained intact. Reinitiation of DNA replication in the oocyte nucleus was sensitive to the inhibition of both CDK activity and polyadenylation of maternal mRNAs, indicating a role of proteins synthesized de novo by the embryo. These results allow us to understand better the mechanisms involved in the reinitiation of DNA replication in growing oocytes.

Checkpoint effects and telomere amplification during DNA re-replication in fission yeast

BackgroundAlthough much is known about molecular mechanisms that prevent re-initiation of DNA replication on newly replicated DNA during a single cell cycle, knowledge is sparse regarding the regions that are most susceptible to re-replication when those mechanisms are bypassed and regarding the extents to which checkpoint pathways modulate re-replication. We used microarrays to learn more about these issues in wild-type and checkpoint-mutant cells of the fission yeast, Schizosaccharomyces pombe.ResultsWe found that over-expressing a non-phosphorylatable form of the replication-initiation protein, Cdc18 (known as Cdc6 in other eukaryotes), drove re-replication of DNA sequences genome-wide, rather than forcing high level amplification of just a few sequences. Moderate variations in extents of re-replication generated regions spanning hundreds of kilobases that were amplified (or not) ~2-fold more (or less) than average. However, these regions showed little correlation with replication origins used during S phase. The extents and locations of amplified regions in cells deleted for the checkpoint genes encoding Rad3 (ortholog of human ATR and budding yeast Mec1) and Cds1 (ortholog of human Chk2 and budding yeast Rad53) were similar to those in wild-type cells. Relatively minor but distinct effects, including increased re-replication of heterochromatic regions, were found specifically in cells lacking Rad3. These might be due to Cds1-independent roles for Rad3 in regulating re-replication and/or due to the fact that cells lacking Rad3 continued to divide during re-replication, unlike wild-type cells or cells lacking Cds1. In both wild-type and checkpoint-mutant cells, regions near telomeres were particularly susceptible to re-replication. Highly re-replicated telomere-proximal regions (50–100 kb) were, in each case, followed by some of the least re-replicated DNA in the genome.ConclusionThe origins used, and the extent of replication fork progression, during re-replication are largely independent of the replication and DNA-damage checkpoint pathways mediated by Cds1 and Rad3. The fission yeast pattern of telomere-proximal amplification adjacent to a region of under-replication has also been seen in the distantly-related budding yeast, which suggests that subtelomeric sequences may be a promising place to look for DNA re-replication in other organisms.

The effect of methylation on DNA replication in Salmonella enterica serovar typhimurium

The DNA adenine methylase of Salmonella typhimurium methylates adenine at GATC sequences. Strains deficient in this methylase are not well transformed by methylated plasmids, but unmethylated plasmids transform them at high frequencies. Hemimethylated daughter molecules accumulate after the transformation of dam − strains with fully methylated plasmids, suggesting that hemimethylation prevents DNA replication. It will also be shown that plasmids isolated from dam − bacteria are hemimethylated by restriction enzyme digestion. These results may explain why newly formed daughter molecules are not substrates for immediate reinitiation of DNA replication in dam − bacteria. To cite this article: A. Aloui et al., C. R. Biologies 330 (2007).

Androgen receptor as a licensing factor for DNA replication in androgen-sensitive prostate cancer cells

Androgen receptor (AR) protein expression and function are critical for survival and proliferation of androgen-sensitive (AS) prostate cancer cells. Besides its ability to function as a transcription factor, experimental observations suggest that AR becomes a licensing factor for DNA replication in AS prostate cancer cells and thus must be degraded during each cell cycle in these cells to allow reinitiation of DNA replication in the next cell cycle. This possibility was tested by using the AS human prostate cancer cell lines, LNCaP, CWR22Rv1, and LAPC-4. These studies demonstrated that AR levels fluctuate both within and between various phases of the cell cycle in each of these AS lines. Consistent with its licensing ability, AR is degraded during mitosis via a proteasome-dependent pathway in these AS prostate cancer cells. In contrast, proteasome-dependent degradation of AR during mitosis is not observed in AR-expressing but androgen-insensitive human prostate stromal cells, in which AR does not function as a licensing factor for DNA replication. To evaluate mitotic degradation of AR in vivo, the same series of human AS prostate cancers growing as xenografts in nude mice and malignant tissues obtained directly from prostate cancer patients were evaluated by dual Ki-67 and AR immunohistochemistry for AR expression in mitosis. These results document that AR is also down-regulated during mitosis in vivo. Thus, AS prostate cancer cells do not express AR protein during mitosis, either in vitro or in vivo, consistent with AR functioning as a licensing factor for DNA replication in AS prostate cancer cells.

Cdc6 chromatin affinity is unaffected by serine-54 phosphorylation, S-phase progression, and overexpression of cyclin A.

Ectopically expressed Cdc6 is translocated from the nucleus during S phase in a cyclin A-Cdk2-dependent process, suggesting that reinitiation of DNA replication is prevented by removal of phosphorylated Cdc6 from chromatin after origin firing. However, whether endogenous Cdc6 translocates during S phase remains controversial. To resolve the questions regarding regulation of endogenous Cdc6, we cloned the cDNA encoding the Chinese hamster Cdc6 homolog and specifically focused on analyzing the localizations and chromatin affinities of endogenous and exogenous proteins during S phase and following overexpression of cyclin A. In agreement with other reports, ectopically expressed Cdc6 translocates from the nucleus during S phase and in response to overexpressed cyclin A. In contrast, using a combination of biochemical and immunohistochemical assays, we show convincingly that endogenous Cdc6 remains nuclear and chromatin bound throughout the entire S period, while Mcm5 loses chromatin affinity during S phase. Overexpression of cyclin A is unable to alter the nuclear localization of Cdc6. Furthermore, using a phosphospecific antibody we show that phosphoserine-54 Cdc6 maintains a high affinity for chromatin during the S period. Considering recent in vitro studies, these data are consistent with a proposed model in which Cdc6 is serine-54 phosphorylated during S phase and functions as a chromatin-bound signal that prevents reformation of prereplication complexes.

The function of RecQ helicase gene family (especially BLM) in DNA recombination and joining

Bloom syndrome is a rare autosomal recessive genetic disorder characterized by lupus-like erythematous telangiectasias of the face, sun sensitivity, stunted growth, and immunodeficiency. Chromosome instability syndromes have a common feature, being associated at high frequency with neoplasia. BS is considered as one of the chromosome instability syndromes since the fibroblasts or lymphocytes of BS patients show excessive spontaneous chromosome instability. The causative gene of BS (BLM) was identified as a RecQ helicase homologue. In this review, we showed the characteristic phenotypes of BS, especially two Japanese siblings. In the latter of the review, the functional domains of BLM, those are nuclear localization signal and the interacting proteins such as ATM, are shown. Several lines of reports indicates that BLM helicase is involved in the re-initiation of DNA replication at sites where replication forks have arrested or collapsed. To elucidate the precise function of RecQ helicase in DNA repair and replication aims not only to improve our understanding of the molecular basis for tumorigenesis, but also to extend the range of potential therapeutic targets.

CIBA Medal Lecture. Regulatory roles of the nuclear membrane.

The nuclear envelope regulates the trafficking of macromolecules between the nucleus and the cytoplasm, Two layers of membrane are perforated by nuclear pore complexes that provide regulated aqueous channels for transport of proteins and ribonucleoproteins. This lecture will consider two aspects of the regulatory roles of the nuclear membrane. First, the import of nuclear proteins from the cytoplasm into the nucleus will be discussed with emphasis on nuclear localization signals and their cytosolic receptors, the importin family. Secondly, involvement of the nuclear envelope in coupling DNA replication to the cell cycle will be considered. Each time the nuclear envelope breaks down during mitosis, the chromosomes are exposed to cytoplasmic factors. I shall argue that this exposure is exploited in Xenopus eggs and embryos to prevent the re-initiation of DNA replication within a single cell cycle. I shall also compare this mechanism with the coupling of DNA replication to the cell cycle in other organisms, revealing a remarkable conservation of the proteins involved in spite of differences in replication regulatory mechanisms.

Regulation mechanism for initiation of chromosomal DNA replication in Escherichia coli.

Re-initiation of DNA replication from a newly replicated origin has to be suppressed so that DNA replication couples with cell division. In this review, I show our original model for the suppression, DNA replication-dependent inactivation of initiator protein for DNA replication. I also describe recent studies suggesting that membrane acidic phospholipids are involved in the regulation of initiation of DNA replication in cells.

Interaction of the S phase regulator cdc18 with cyclin-dependent kinase in fission yeast.

The fission yeast gene cdc18(+) is required for entry into S phase and for coupling mitosis to the successful completion of S phase. Cdc18 is a highly unstable protein that is expressed only once per cell cycle at the G1/S boundary. Overexpression of Cdc18 causes a mitotic delay and reinitiation of DNA replication, suggesting that the inactivation of Cdc18 plays a role in preventing rereplication within a given cell cycle. In this paper, we present evidence that Cdc18 is associated with active cyclin-dependent kinase in vivo. We have expressed Cdc18 as a glutathione S-transferase fusion in fission yeast and demonstrated that the fusion protein is functional in vivo. We find that the Cdc18 fusion protein copurifies with a kinase activity capable of phosphorylating histone H1 and Cdc18. The activity was identified by a variety of methods as the cyclin-dependent kinase containing the product of the cdc2(+) gene. The amino terminus of Cdc18 is required for association with cyclin-dependent kinase, but the association does not require the consensus cyclin-dependent kinase phosphorylation sites in this region. Additionally, both G1/S and mitotic forms of cyclin-dependent kinase phosphorylate and interact with Cdc18. These interactions between Cdc18 and cyclin-dependent kinases suggest mechanisms by which cyclin-dependent kinases could activate the initiation of DNA replication and could prevent rereplication.

Cdc18+ regulates initiation of DNA replication in Schizosaccharomyces pombe.

In the fission yeast Schizosaccharomyces pombe the cdc18'+gene is required both for initiation of DNA replication and for coupling mitosis to the completion of S phase. Cells lacking Cdc18 fail to enter S phase but still undergo nuclear division. Expression of cdc18+ is sufficient to drive a G1-arrested cdc10ts mutant into the S phase of the cell cycle, indicating that cdc18+ represents a critical link between passage through START and the initiation of DNA replication. Here we show that Cdcl8 is a highly unstable protein that is expressed only once per cell cycle at the boundary between GI and S phase. De novo synthesis of Cdc18 is required before, but not after, the initiation of DNA replication, indicating that Cdc18 function is not necessary once the initiation event has occurred. Overproduction of the protein results in an accumulation of cells with DNA content of greater than 2C and delays mitosis, suggesting that Cdc18 is sufficient to cause reinitiation of DNA replication within a given cell cycle. Our data indicate that the synthesis of Cdc18 protein is a critical rate-limiting step in the initiation of DNA replication during each cell cycle. The extreme lability of the protein may contribute to the prevention of reinitiation.

Distinct modes of cyclin E/cdc2c kinase regulation and S-phase control in mitotic and endoreduplication cycles of Drosophila embryogenesis.

Drosophila cyclin E (DmcycE) is required in embryos for S phase of mitotic and endoreduplication cycles. Here, we describe regulatory differences characteristic for these two cell cycle types. While DmcycE transcript levels decline in DmcycE mutant cells programmed for mitotic proliferation, they are maintained and no longer restricted to transient pulses in DmcycE mutant cells programmed for endoreduplication. Moreover, DmcycE expression in endoreduplicating cells is down-regulated by ectopic expression of a heat-inducible cyclin E transgene. DmcycE expression in endoreduplicating tissues, therefore, is restricted by a negative feedback to the transient pulse triggering entry into S-phase. Conversely, during mitotic cycles, where S phase entry is not only dependent on cyclin E but also on progression through M phase, cyclin E and associated Dmcdc2c kinase activity are present throughout the cell cycle. Reinitiation of DNA replication during the G2 phase of the mitotic cell cycle, therefore, is prevented by cyclin E/Dmcdc2c kinase-independent regulation. Observations in cyclin A mutants implicate G2 cyclins in this regulation. Our results suggest molecular explanations for the different rules governing S phase during mitotic and endoreduplication cycles.

Developmental Progression of DNA Puffs in Sciara coprophila: Amplification and Transcription

DNA amplification for two major DNA puffs (II/2B and II/9A) of the fungus fly Sciara coprophila increases steeply from 17 to 19 days after hatching (18°C), resulting in almost 20-fold more DNA at these loci in mature larval salivary glands than in adult tissues. At 19 days after hatching when gene amplification reaches a plateau, there is a burst in the amount of mRNA encoded by these two DNA puffs. Expansion of the two puffs coincides with the increase in transcription rather than reinitiation of DNA replication. In contrast, an RNA puff (III/9B) undergoes no DNA amplification, and the burst in RNA produced by this locus occurs slightly later, at 20 days after hatching. The progression of cytological puffing and associated molecular events of DNA amplification and transcription correlate with features of the external phenotype of the larvae, especially the number of pigment granules in the eyespots that are the anlage to the adult eyes. The sequential and coordinated regulation of DNA puff replication and transcription is discussed.