Cell Division Cycle Progression Research Articles

Statins decrease the expression of c-Myc protein in cancer cell lines.

Statins are potent inhibitors of the mevalonate/cholesterol biosynthetic pathway and are widely prescribed for the prevention of cardiovascular diseases. Here, we carried out a comprehensive analysis of the effects of three statins, simvastatin, atorvastatin, and lovastatin, on six different cancer cell lines that include a P-glycoprotein-expressing, multidrug resistant variant of an ovarian cancer cell line. Incubation of all cancer cell lines with statins resulted in suppression of cell proliferation without inducing apoptotic cell death. The cell proliferation arrest could be reversed upon transfer of cells to statin-free growth media as well as by the supplementation of the growth media with mevalonate. Further analysis suggested that statins induced cell cycle arrest at G0/G1 phase in four cancer cell lines and the loss of c-Myc protein in three cancer cell lines. The c-Myc expression and the progression of cell division cycle were restored upon the addition of mevalonate to the culture media containing statins. Finally, cells incubated with statins contained an increased level of phosphorylated histone H2AX, an observation previously correlated to cellular senescence. Together, these data demonstrate that statins inhibit the mevalonate pathway which is tightly coupled to oxidative branch of the pentose phosphate pathway, c-Myc expression, cell division cycle progression, and cellular senescence. Implications of these observations in the application of statins as cancer therapeutics are discussed.

Abstract 1088: Elucidating the regulation of delayed-early gene targets of sustained MAPK signaling

Abstract RAS and its downstream effector, BRAF, are commonly mutated proto-oncogenes in many types of human cancer. Mutationally activated RAS or BRAF signal through the MEK→ERK MAP kinase (MAPK) pathway to regulate key cancer cell hallmarks such as cell division cycle progression, reduced programmed cell death, and enhanced cell motility. Amongst the list of RAS/RAF-regulated genes are those encoding integrins, alpha-beta heterodimeric transmembrane proteins that regulate cell adhesion to the extracellular matrix. Altered integrin expression has been linked to the acquisition of more aggressive behavior by melanoma, lung, and breast cancer cells leading to diminished survival of cancer patients. We have previously documented the ability of the RAS-activated MAPK pathway to induce the expression of ITGB3 encoding integrin β3 in several different cell types. RAS/RAF-mediated induction of ITGB3 mRNA requires sustained, high-level activation of RAF→MEK→ERK signaling mediated by oncogene activation and is classified as “delayed-early”, in that it is sensitive to the protein synthesis inhibitor cycloheximide. However, to date, the regulatory mechanisms that allow for induced ITGB3 downstream of sustained, high-level activation of MAPK signaling remains obscure. We have identified over 300 DEGs, including those expressing additional cell surface proteins, that display similar regulatory characteristics as ITGB3. We use integrin β3 as a model to test our hypothesis that there is a different mechanism of regulation for delayed-early genes (DEG) compared to the canonical regulation of Immediate-Early genes. There are three regions in the chromatin upstream of the ITGB3 that become more accessible during RAF activation. We are relating the chromatin changes seen during RAF activation to active enhancer histone marks. To elucidate the essential genes of this regulation process, we are employing the use of a genome-wide CRISPR knockout screen. The work presented from this abstract will help elucidate the regulatory properties of oncogenic progression in BRAF mutated cancers that could lead to the identification of biomarkers. Citation Format: Kali J. Dale, Martin McMahon. Elucidating the regulation of delayed-early gene targets of sustained MAPK signaling [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1088.

Menadione reduces CDC25B expression and promotes tumor shrinkage in gastric cancer.

Background:Gastric cancer is one of the most incident types of cancer worldwide and presents high mortality rates and poor prognosis. MYC oncogene overexpression is a key event in gastric carcinogenesis and it is known that its protein positively regulates CDC25B expression which, in turn, plays an essential role in the cell division cycle progression. Menadione is a synthetic form of vitamin K that acts as a specific inhibitor of the CDC25 family of phosphatases.Methods:To better understand the menadione mechanism of action in gastric cancer, we evaluated its molecular and cellular effects in cell lines and in Sapajus apella, nonhuman primates from the new world which had gastric carcinogenesis induced by N-Methyl-N-nitrosourea. We tested CDC25B expression by western blot and RT-qPCR. In-vitro assays include proliferation, migration, invasion and flow cytometry to analyze cell cycle arrest. In in-vivo experiments, in addition to the expression analyses, we followed the preneoplastic lesions and the tumor progression by ultrasonography, endoscopy, biopsies, histopathology and immunohistochemistry.Results:Our tests demonstrated menadione reducing CDC25B expression in vivo and in vitro. It was able to reduce migration, invasion and proliferation rates, and induce cell cycle arrest in gastric cancer cell lines. Moreover, our in-vivo experiments demonstrated menadione inhibiting tumor development and progression.Conclusions:We suggest this compound may be an important ally of chemotherapeutics in the treatment of gastric cancer. In addition, CDC25B has proven to be an effective target for investigation and development of new therapeutic strategies for this malignancy.

Melatonin alleviates aluminum-induced root growth inhibition by interfering with nitric oxide production in Arabidopsis

Root growth inhibition is the primary symptom of aluminum (Al) toxicity. In this study, the roles of nitric oxide (NO) and melatonin, two ubiquitous signal molecules involved in diverse plant physiological processes and stress responses, were investigated in Arabidopsis responses to Al toxicity. The results showed that nitric reductase- and nitric oxide synthase-dependent NO production contributed to Al-induced root growth inhibition through interfering with the root cell cycle progression and the quiescent center cell homeostasis. The Al-induced NO production was decreased by exogenous melatonin, which was associated with abolishment of Al-induced root elongation and cell cycle arresting, and with the quiescent center activity returning to the Al-free control levels. The expression of AtSNAT (Arabidopsis thalianaserotonin N-acetyltransferase) was down-regulated by Al, which coincided with decreased melatonin accumulation in Arabidopsis. Pharmacological and genetic analyses further confirmed that a decrease in melatonin synthesis enhanced NO production and Al sensitivity. Taken together, the results presented here suggest that melatonin alleviates Al-induced root growth inhibition by interfering with NO-mediated reduction of cell division cycle progression and the quiescent center cellular activity in Arabidopsis roots.

Genetic and Epigenetic Control of CDKN1C Expression: Importance in Cell Commitment and Differentiation, Tissue Homeostasis and Human Diseases.

The CDKN1C gene encodes the p57Kip2 protein which has been identified as the third member of the CIP/Kip family, also including p27Kip1 and p21Cip1. In analogy with these proteins, p57Kip2 is able to bind tightly and inhibit cyclin/cyclin-dependent kinase complexes and, in turn, modulate cell division cycle progression. For a long time, the main function of p57Kip2 has been associated only to correct embryogenesis, since CDKN1C-ablated mice are not vital. Accordingly, it has been demonstrated that CDKN1C alterations cause three human hereditary syndromes, characterized by altered growth rate. Subsequently, the p57Kip2 role in several cell phenotypes has been clearly assessed as well as its down-regulation in human cancers. CDKN1C lies in a genetic locus, 11p15.5, characterized by a remarkable regional imprinting that results in the transcription of only the maternal allele. The control of CDKN1C transcription is also linked to additional mechanisms, including DNA methylation and specific histone methylation/acetylation. Finally, long non-coding RNAs and miRNAs appear to play important roles in controlling p57Kip2 levels. This review mostly represents an appraisal of the available data regarding the control of CDKN1C gene expression. In addition, the structure and function of p57Kip2 protein are briefly described and correlated to human physiology and diseases.

Ribosomal protein NtRPL17 interacts with kinesin-12 family protein NtKRP and functions in the regulation of embryo/seed size and radicle growth.

We previously reported that a novel motor protein belonging to the kinesin-12 family, NtKRP, displays critical roles in regulating embryo and seed size establishment. However, it remains unknown exactly how NtKRP contributes to this developmental process. Here, we report that a 60S ribosomal protein NtRPL17 directly interacts with NtKRP. The phenotypes of NtRPL17 RNAi lines show notable embryo and seed size reduction. Structural observations of the NtRPL17-silenced embryos/seeds reveal that the embryo size reduction is due to a decrease in cell number. In these embryos, cell division cycle progression is delayed at the G2/M transition. These phenotypes are similar to that in NtKRP-silenced embryos/seeds, indicating that NtKRP and NtRPL17 function as partners in the same regulatory pathway during seed development and specifically regulate cell cycle progression to control embryo/seed size. This work reveals that NtRPL17, as a widely distributed ribosomal protein, plays a critical role in seed development and provides a new clue in the regulation of seed size. Confirmation of the interaction between NtKRP and NtRPL17 and their co-function in the control of the cell cycle also suggests that the mechanism might be conserved in both plants and animals.

Let's get fISSical: fast in silico synchronization as a new tool for cell division cycle analysis.

Cell cycle progression is a question of fundamental biological interest. The coordinated duplication and segregation of all cellular structures and organelles is however an extremely complex process, and one which remains only partially understood even in the most intensively researched model organisms. Trypanosomes are in an unusual position in this respect - they are both outstanding model systems for fundamental questions in eukaryotic cell biology, and pathogens that are the causative agents of three of the neglected tropical diseases. As a failure to successfully complete cell division will be deleterious or lethal, analysis of the cell division cycle is of relevance both to basic biology and drug design efforts. Cell division cycle analysis is however experimentally challenging, as the analysis of phenotypes associated with it remains hypothesis-driven and therefore biased. Current methods of analysis are extremely labour-intensive, and cell synchronization remains difficult and unreliable. Consequently, there exists a need - both in basic and applied trypanosome biology - for a global, unbiased, standardized and high-throughput analysis of cell division cycle progression. In this review, the requirements - both practical and computational - for such a system are considered and compared with existing techniques for cell cycle analysis.

Identification of YPL014W (Cip1) as a novel negative regulator of cyclin-dependent kinase in Saccharomyces cerevisiae.

Cyclin-dependent kinases drive cell division cycle progression in eukaryotic cells. In the model eukaryotic organism Saccharomyces cerevisiae (budding yeast), a single cyclin-dependent kinase, Cdk1, is essential and sufficient to drive the cell cycle. Misregulated CDK activity induces unscheduled proliferation as well as genomic instability, which are hallmarks of the cancer. Here, we report a novel Cdk1-interacting protein, YPL014W, which we name Cip1 (for Cdk1-interacting protein 1). Our results show that Cip1 specifically interacts with G1 /S-phase Cln2-Cdk1 complex but not with S-phase Clb5-Cdk1 or M-phase Clb2-Cdk1 complexes. Also Cip1 phosphorylation is cell cycle regulated in a S-phase Cdk1-dependent manner. Over-expression of Cip1 blocks cell cycle progression in G1 and stabilizes the S-phase Cdk1 inhibitor Sic1 invivo. In addition, disruption of CIP1 (cip1Δ) leads to faster G1 /S-phase transition compared to wild-type cells. Moreover, Cip1 inhibits Cln2-CDK activity both invivo and invitro. Our finding proves Cip1 as a novel negative regulator of cyclin-dependent kinase in S.cerevisiae.

Transient receptor potential ion channel Trpm7 regulates exocrine pancreatic epithelial proliferation by Mg2+-sensitive Socs3a signaling in development and cancer

Genetic analysis of pancreatic development has provided new insights into the mechanisms underlying the formation of exocrine pancreatic neoplasia. Zebrafish sweetbread (swd) mutants develop hypoplastic acini and dysmorphic ducts in the exocrine pancreas, with impeded progression of cell division cycle and of epithelial growth. Positional cloning and allelic complementation have revealed that the swd mutations affect the transient receptor potential melastatin-subfamily member 7 (trpm7) gene, which encodes a divalent cation-permeable channel with kinase activity. Supplementary Mg2+ partially rescued the exocrine pancreatic defects of the trpm7 mutants by improving cell-cycle progression and growth and repressing the suppressor of cytokine signaling 3a (socs3a) gene. The role of Socs3a in Trpm7-mediated signaling is supported by the findings that socs3a mRNA level is elevated in the trpm7 mutants, and antisense inhibition of socs3a expression improved their exocrine pancreatic growth. TRPM7 is generally overexpressed in human pancreatic adenocarcinoma. TRPM7-deficient cells are impaired in proliferation and arrested in the G0-G1 phases of the cell division cycle. Supplementary Mg2+ rescued the proliferative defect of the TRPM7-deficient cells. Results of this study indicate that Trpm7 regulates exocrine pancreatic development via the Mg2+-sensitive Socs3a pathway, and suggest that aberrant TRPM7-mediated signaling contributes to pancreatic carcinogenesis.

Targeting p27Kip1 protein: its relevance in the therapy of human cancer

Introduction: Cell division cycle progression is achieved by a sequential and stringently concerted activation of a family of serine–threonine kinases, namely the cyclin-dependent kinases (CDKs). p27Kip1 is a pivotal CDK inhibitor and a tight modulator of CDK-dependent phenotypes. Thus, p27Kip1 plays a fundamental role in key cellular processes such as proliferation, differentiation, apoptosis, substrate adhesion and motility. Intriguingly, when p27Kip1 is localized in the nucleus, it acts as an antiproliferative protein, while, in the cytosol, p27Kip1 promotes cytoskeleton remodeling and might positively influence metastatization. Downregulation of p27Kip1 nuclear level or its cytosolic mislocalization are consistently correlated with poor prognosis of numerous types of human epithelial and non-epithelial cancers.Areas covered: This review illustrates the basic structural features of p27Kip1 protein, its metabolism and alterations in human malignancies, along with describing anticancer strategies based on targeting p27Kip1.Expert opinion: Given the role of p27Kip1 in the control of cell proliferation and its decreased level observed in malignancies with poor outcome, drugs able to handle the protein levels and localization might represent an important goal for novel specific and effective anticancer strategies. Although no convincing proofs have been reported, putative negative consequences of p27Kip1 targeting might be also conceivable.

Transient receptor potential ion channel Trpm7 regulates exocrine pancreatic epithelial proliferation by Mg2+-sensitive Socs3a signaling in development and cancer

SUMMARYGenetic analysis of pancreatic development has provided new insights into the mechanisms underlying the formation of exocrine pancreatic neoplasia. Zebrafish sweetbread (swd) mutants develop hypoplastic acini and dysmorphic ducts in the exocrine pancreas, with impeded progression of cell division cycle and of epithelial growth. Positional cloning and allelic complementation have revealed that the swd mutations affect the transient receptor potential melastatin-subfamily member 7 (trpm7) gene, which encodes a divalent cation-permeable channel with kinase activity. Supplementary Mg2+ partially rescued the exocrine pancreatic defects of the trpm7 mutants by improving cell-cycle progression and growth and repressing the suppressor of cytokine signaling 3a (socs3a) gene. The role of Socs3a in Trpm7-mediated signaling is supported by the findings that socs3a mRNA level is elevated in the trpm7 mutants, and antisense inhibition of socs3a expression improved their exocrine pancreatic growth. TRPM7 is generally overexpressed in human pancreatic adenocarcinoma. TRPM7-deficient cells are impaired in proliferation and arrested in the G0-G1 phases of the cell division cycle. Supplementary Mg2+ rescued the proliferative defect of the TRPM7-deficient cells. Results of this study indicate that Trpm7 regulates exocrine pancreatic development via the Mg2+-sensitive Socs3a pathway, and suggest that aberrant TRPM7-mediated signaling contributes to pancreatic carcinogenesis.

Redox rhythmicity: clocks at the core of temporal coherence

Ultradian rhythms are those that cycle many times in a day and are therefore measured in hours, minutes, seconds or even fractions of a second. In yeasts and protists, a temperature-compensated clock with a period of about an hour (30-90 minutes) provides the time base upon which all central processes are synchronized. A 40-minute clock in yeast times metabolic, respiratory and transcriptional processes, and controls cell division cycle progression. This system has at its core a redox cycle involving NAD(P)H and dithiol-disulfide interconversions. It provides an archetype for biological time keeping on longer time scales (e.g. the daily cycles driven by circadian clocks) and underpins these rhythms, which cannot be understood in isolation. Ultradian rhythms are the foundation upon which the coherent functioning of the organism depends.

Disrupted G 1 to S phase clearance via cyclin signaling impairs liver tissue repair in thioacetamide-treated type 1 diabetic rats

Previously we reported that a nonlethal dose of thioacetamide (TA, 300 mg/kg) causes 90% mortality in type 1 diabetic (DB) rats because of irreversible acute liver injury owing to inhibited hepatic tissue repair, primarily due to blockage of G 0 to S phase progression of cell division cycle. On the other hand, DB rats receiving 30 mg TA/kg exhibited equal initial liver injury and delayed tissue repair compared to nondiabetic (NDB) rats receiving 300 mg TA/kg, resulting in a delay in recovery from liver injury and survival. The objective of the present study was to test the hypothesis that impaired cyclin-regulated progression of G 1 to S phase of the cell cycle may explain inhibited liver tissue repair, hepatic failure, and death, contrasted with delayed liver tissue repair but survival observed in the DB rats receiving 300 in contrast to 30 mg TA/kg. In the TA-treated NDB rats sustained MAPKs and cyclin expression resulted in higher phosphorylation of retinoblastoma (pRb), explaining prompt tissue repair and survival. In contrast, DB rats receiving the same dose of TA (300 mg/kg) exhibited suppressed MAPKs and cyclin expression that led to inhibition of pRb, inhibited tissue repair, and death. On the other hand, DB rats receiving 30 mg TA/kg exhibited delayed up regulation of MAPK signaling that delayed the expression of CD1 and pRb, explaining delayed stimulation of tissue repair observed in this group. In conclusion, the hepatotoxicant TA has a dose-dependent adverse effect on cyclin-regulated pRb signaling: the lower dose causes a recoverable delay, whereas the higher dose inhibits it with corresponding effect on the ultimate outcomes on hepatic tissue repair; this dose-dependent adverse effect is substantially shifted to the left of the dose response curve in diabetes.

Rubisco of Dunaliella tertiolecta is redistributed between the pyrenoid and the stroma as a light?/shade response

We used an immunofluorescence technique to investigate the effects of varying light regimes on ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in individual cells of the chlorophyte Dunaliella tertiolecta Butcher. These studies were carried out between September 1995 and February 1996. The population was heterogeneous with respect to Rubisco localization in the pyrenoid, i.e. in some cells Rubisco staining was highly concentrated in the pyrenoid while in others it was evenly distributed throughout the chloroplast stroma. When light intensity was varied sevenfold, the fraction of the cell population that displayed distinct Rubisco staining in the pyrenoid (PR-index) was correlated with light intensity, although the average Rubisco abundance per cell or per total cellular protein appeared fairly constant. In darkness, or when treated with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU), the PR-index decreased markedly during the first 4 and 3 h, respectively, and then remained at low levels, while the cell division cycle progression remained unaffected. We conclude that D. tertiolecta probably possesses an adaptive mechanism, i.e. the redistribution of Rubisco between the pyrenoid (probably the active site of Rubisco activation and CO2 fixation) and the stroma (probably a reservoir of deactivated Rubisco that is readily available for transport to the pyrenoid and activation), to respond to variations in irradiance or photosynthetic inhibition. Our results also suggest that this mechanism is insensitive to slight variations in growth irradiance and to seasonal changes in photoperiod and temperature.

Magnesium-limited growth of Saccharomyces cerevisiae

The growth response of Saccharomyces cerevisiae to magnesium limitation when propagated aerobically in batch culture under conditions of glucose repression was hyperbolic, thus indicating a Monod relationship. The saturation coefficient, K s , was 36 μ m magnesium, and exogenous magnesium requirements for optimal yeast growth rate were established in minimal medium. An investigation into growth and metabolism of S. cerevisiae over a range of growth rates using a magnesium-limited chemostat revealed that at growth rates less than μ max , cells exhibited a physiological state characteristic of non-carbon-limited growth. This was exemplified by an increase in the biomass yield from increased magnesium availability and alterations in the relative contributions of fermentation and respiration to the overall respirofermentative activity as the growth rate increased toward μ max . Studies on the growth behaviour of S. cerevisiae when cells were released from magnesium limitation within a chemostat suggested that magnesium primarily exerts its influence on cell division cycle progression in this yeast.

Regulation of cell division cycle progression by bcl-2 expression: a potential mechanism for inhibition of programmed cell death.

Expression of the bcl-2 gene has been shown to effectively confer resistance to programmed cell death under a variety of circumstances. However, despite a wealth of literature describing this phenomenon, very little is known about the mechanism of resistance. In the experiments described here, we show that bcl-2 gene expression can result in an inhibition of cell division cycle progression. These findings are based upon the analysis of cell cycle distribution, cell cycle kinetics, and relative phosphorylation of the retinoblastoma tumor suppressor protein, using primary tissues in vivo, ex vivo, and in vitro, as well as continuous cell lines. The effects of bcl-2 expression on cell cycle progression appear to be focused at the G1 to S phase transition, which is a critical control point in the decision between continued cell cycle progression or the induction programmed cell death. In all systems tested, bcl-2 expression resulted in a substantial 30-60% increase in the length of G1 phase; such an increase is very substantial in the context of other regulators of cell cycle progression. Based upon our findings, and the related findings of others, we propose a mechanism by which bcl-2 expression might exert its well known inhibition of programmed cell death by regulating the kinetics of cell cycle progression at a critical control point.

A new axis for cell division

Despite the considerable contribution of Japanese and British research to our understanding of the complexities of how eukaryotic cells coordinate and physically achieve cell division, there has been no forum for direct discussions between these geographically distinct groups. Thus, the recent Japan/UK cell cycle meeting*, arranged to catalyse the strengthening of ties for future research and collaboration, was both timely and appropriate. Timing the meeting to coincide with the blooming of the cherry trees was equally fitting, as the attendees could celebrate one of the great annual cycles, held in awe by the Samurai, whilst contemplating the similarly ephemeral cycle they hold in equal regard. Cell division cycle progression is the controlled execution of four events: growth, the decision to enter the cycle, DNA synthesis, and coordinated chromosome segregation and cell cleavage. Different organisms approach this task in different ways, so that cycles are 'tailor made' according to need, be it cell division without growth in many examples of embryogenesis, or chromosome segregation without either growth or cell division as in Drosophila's syncitial cleavages. Thus, different organisms have their strengths for assessing different problems, and the Kyoto meeting brought together experts on the cell cycle of yeast, fish, frogs, man and flies.

Cell cycle-dependent change of proteasome distribution during embryonic development of the ascidian Halocynthia roretzi

The proteasome is a multicatalytic proteinase complex composed of nonidentical subunits. By immunocytochemical analysis using monoclonal antibody raised against the egg proteasome, we demonstrate that the proteasome undergoes changes in its subcellular distribution, depending on the cell division cycle during embryonic development of the ascidian Halocynthia roretzi. During interphase, the proteasome is localized in the nucleus, i.e., in the nucleoplasm and along the nuclear membrane. The proteasome disappears from the nucleoplasm in prophase and from the nuclear envelope in prometaphase. During early metaphase, the proteasome is detectable in the chromosomes and, at late stages of metaphase, the immunoreactivity also occurs in the peripheral region of each spindle pole and at the mitotic spindle. In anaphase, however, the staining disappears in the mitotic apparatus. In telophase, the proteasome is again localized in the newly formed nucleus. In addition to the localization in the nucleus and around the mitotic apparatus, the proteasome shows cytoplasmic localization throughout the cell division cycle. Such a change of subcellular distribution of the proteasome is clearly demonstrated in the synchronously dividing blastomeres and also is believed to occur in the postcleavage embryos. These observations suggest that the proteasome may play a key role in the progression of cell division cycle.

Characteristic alteration in the nuclear DNA polymerase activity during the cell division cycle of Saccharomyces cerevisiae.

Specific activity of the intranuclear DNA polymerase in cdc-mutant cells of Saccharomyces cerevisiae was found to be characteristically changed by arrest in their specific stage of cell division cycle without a notable alteration in the total cellular activity. The activities were low in the nuclei of cdc 25, cdc 28 and cdc 4, which were arrested in early to mid G1 phase by temperature shift-up, and in the nuclei of wild-type cells (A364A), which were arrested in early G1 phase by alpha-factor treatment, while high level of the activity was found in the nuclei of cdc 7 and cdc 8, which were arrested at late G1 and S phase, respectively. Activity-gel analysis of DNA polymerase in the nuclear extracts revealed the presence of two active peptides (120K and 72K), and the characteristic decrease in both active peptides was induced by arrest in early to mid G1 phase. Consequently, it is strongly suggested that intranuclear DNA polymerase activity alters in a dependent fashion on progression of cell division cycle. Subunit analysis indicated that the purified DNA polymerase I is constructed from two subunit peptides of 120K and 62K, and the large subunit possesses catalytic activity.