In budding yeast, many of the genes that encode enzymes required for DNA precursor synthesis (MCB genes) are expressed under cell cycle control in late G1/S. The relationship between MCB gene expression, dNTP synthesis and DNA synthesis was investigated by using �� factor-synchronized Saccharomyces cerevisiae. The levels of all four dNTPs increased several-fold when cells crossed the G1/S boundary. An even larger increase in the dNTP pools occurred at G1/S when replication initiation was blocked by incubating synchronized dbf4 mutants at the nonpermissive temperature. Thus, dNTP accumulation at G1/S was not dependent on replication initiation. Similarly, MCB gene induction at G1/S was also independent of replication initiation. The accumulation of dNTPs at G1/S was dependent on Swi6, a protein known to be required for normal MCB gene regulation during the cell cycle. Treatment with hydroxyurea, an inhibitor of ribonucleotide reductase, blocked DNA synthesis and prevented the increase in dNTP levels that normally occurred at G1/S, however, it did not exhaust the basal levels of any of the four dNTPs. The mechanism responsible for replication arrest despite the persistence of dNTPs was not dependent on the checkpoint protein Rad53, as rad53 mutants also failed to exhaust basal dNTPs when incubated in HU. The inhibitory effect of HU on DNA synthesis was bypassed when dbf4 cells were allowed to pre-accumulate dNTPs at 37��C before being released to the permissive temperature in the presence of HU. Accumulation of dNTPs at G1/S was not a prerequisite for replication initiation, as dbf4 cells incubated in HU at 25��C were able to initiate replication when cells were switched to the nonpermissive temperature and HU was removed. The results indicate that DNA chain elongation in yeast requires a critical dNTP threshold, below which replication forks are completely arrested. Cells lacking a functional thioredoxin system were deficient in dNTP synthesis. The rate of accumulation was significantly lower in ��trr1 mutants lacking thioredoxin reductase, and dNTP accumulation at G1/S did not occur at all in ��trxl ��trx2 double mutants lacking thioredoxin. The results suggest that thioredoxin serves as the electron donor for ribonucleotide reductase during DNA precursor synthesis in yeast. / Graduation date: 2003

A crucial step in eukaryotic cell proliferation is the initiation of DNA replication, a tightly regulated process mediated by a multitude of protein factors. In Saccharomyces cerevisiae, this occurs as a result of the concerted action of an assembly of proteins acting at origins of replication, known as the pre-replicative complex (pre-RC). While many of the mechanisms pertaining to the functions of these proteins and the associations amongst them have been explored experimentally, mathematical models are needed to effectively explore the network’s dynamic behaviour. An ordinary differential equation (ODE)-based model of the protein-protein interaction network describing DNA replication initiation was constructed. The model was validated against quantified levels of protein factors determined in vivo and from the literature over a range of cell cycle timepoints. The model behaviour conforms to perturbation trials previously reported in the literature and accurately predicts the results of knockdown experiments performed herein. Furthermore, the DNA replication model was successfully incorporated into an established model of the entire yeast cell cycle, thus providing a comprehensive description of these processes. A screen for novel DNA damage response proteins was investigated using a unique proteomics approach that uses chromatin fractionation samples to enrich for factors bound to the DNA. This form of sub-cellular fractionation was combined with differential-in-gel-electrophoresis (DIGE) to detect and quantify low abundance chromatin proteins in the budding yeast proteome. The method was applied to analyze the effect of the DNA damaging agent methyl methanesulfonate (MMS) on levels of chromatin-associated proteins. Up-regulation of several previously characterized DNA damage checkpoint-regulated proteins, such as Rnr4, Rpa1 and Rpa2, was observed. In addition, several novel DNA damage responsive proteins were identified and assessed for genotoxic sensitivity. A strain in which the expression of the Ran-GTPase binding protein Yrb1 was reduced was found to be hypersensitive to genotoxic stress, pointing to a role for this nuclear import-associated protein in DNA damage response. The model presented in this thesis provides a tool for exploring the biochemical network of DNA replication. This is germane to the exploration of new cancer therapeutics considering the link between this disease (and others) and errors in proper cell cycle regulation. The high functional conservation between cell cycle mechanisms in humans and yeast allows predictive analyses of the model to be extrapolated towards understanding aberrant human cell proliferation. Importantly, the model is useful in identifying potential targets for cancer treatment and provides insights into developing highly specific anti-cancer drugs. Finally, the characterization of factors in the proteomic screen opens the door to further investigation of the roles of potential DNA damage response proteins.

Vaccinia virus infected monkey kidney cells had been previously shown to have an increased ribonucleoside diphosphate reductase (RR) activity. DNA from mutant virus resistant to hydroxyurea were digested with restriction endonucleases and were shown to have substoichiometric amounts of the Hind III F fragment. Additional information from Southern blotting experiments localized the putative small subunit (R2) gene to the left end of the Hind III F fragment of the vaccinia virus genome. The entire open reading frame of the R2 gene and the flanking regions was sequenced and the translated sequence found to be 80% homologous to the mouse R2 polypeptide. A combination of in situ and in vitro experiments addressed the question of macromolecular interactions involving vaccinia ribonucleotide reductase (FIR). Replication of double stranded viral DNA occurs in very discrete loci in infected cells and these DNA factories can be isolated from gently lysed cell in sucrose gradients. RR was detected at low levels (less than 5% of the total R2) with the rapidly sedimenting DNA by using antibodies against FIR. In situ crosslinking experiments were attempted with no specific interaction determined at this time. Immunolocalization experiments have given evidence for localization of large subunit (R1) polypeptide to the viral inclusion bodies. The most conclusive results utilized anti-idiotypic antibodies against the antibodies to R2 protein. lmmunolocalization experiments have shown the putative R2 binding protein to be localized at the sites of viral DNA synthesis. lmmunoprecipitations show a single predominant viral polypeptide which also has proven to be a DNA binding (phospho)protein. Screening a lambda phale expression library of vaccinia with the anti-idiotypic antibody localized the binding site to the carboxy terminal 81 amino acids in open reading frame 1-3 of the vaccinia genome. The open reading frame was cloned into a pET11c expression vector and the partially purified recombinant protein was shown to have specificity for single-stranded DNA as well as stimulate vaccinia RR activity. / Graduation date: 1993

Bacteriophage ë has been used as a model replicon system for forty years. While the basic ë replication initiation scheme has been elucidated for several decades, many aspects of the mechanisms are unclear. I wished to study two unanswered issues in ë replication initiation. </p><p>Replication initiation of E. coli and ë each depend upon a protein generally called a licensing factor, which brings the DnaB helicase protein to the origin site to begin DNA synthesis. The licensing factors are the products of host gene dnaC and ë gene P. The synthesis of P from ë DNA in an E. coli cell can competitively interfere with DnaC activity needed for E. coli replication initiation. I wished to learn more about what happens to a host cell when exposed to extended P expression. Previous studies in this laboratory suggested that i) the continuous expression of P was tolerated by a subset of exposed cells and that ii) host defects mapping to dnaB could suppress the effect of extended P expression (P-lethality). I used DNA sequencing to determine if these suppressor mutations were within dnaB. I screened known host mutations for their influence on P-lethality. In summary: E. coli strains with GrpD55 and GrpA80 defects were found to each have two point mutations within their dnaB genes. I was unable to isolate mutations within P that suppressed P-lethality and instead obtained regulatory mutations preventing wild type P expression. Two of these sequenced mutations showed that a cI[Ts] lambda repressor was reverted to cI wild type, blocking P expression at all assay temperatures. P-lethality was reversible in cells exposed to P for up to five hours, causing me to suggest that P-Interference be used in place of the term P-lethality. A non-inducible allele of lexA prevented P-mediated cellular filamentation and enhanced P-Interference. This suggests that induction of the SOS response helps cells to tolerate extended P expression. A host strain containing a defective ClpXP protease significantly enhanced cellular sensitivity to P-Interference. This suggests an important role for the ClpXP chaperone-protease complex in degradation of P and cellular resistance to P expression. I present models to explain the P-Interference Phenotype.</p><p>Recent reports have re-opened the possibility that the tO-oop-pO element influences ë DNA replication initiation. I have also been investigating this possibility. I found that a plasmid with tO-oop-pO (the terminator, nucleotide sequence and promoter for OOP RNA) and orië DNA sequence was inhibitory to the development of repë phages, and designated this the Inhibition Phenotype (IP). In pursuing the mechanism for this inhibition, I mutated the tO-oop-pO and orië elements. I found that the expression of the 77nt OOP RNA transcript and the presence of four 18 base pair repeats (iterons) within orië were required for the IP. I isolated spontaneous phage mutants, resistant to the IP. I determined that singly infected cells were sensitive to the IP but that multiply infected cells escaped the IP. I propose that the IP to repë phage development is directed to the initial or theta mode of ë replication initiation. I found that the theta-mode of ë replication initiation can be bypassed, likely via recombination between multiple phage genomes within a singe cell. I propose models to explain the IP and also suggest a role for OOP RNA in the regulation of ë DNA replication.

Initiation of DNA replication requires the action of the Dbf4/Cdc7 kinase complex (DDK) which is also a phosphorylation target of Rad53 kinase in the S-phase checkpoint. DDK is thought to trigger DNA replication by phosphorylating members of the Mcm2-7 complex present at origins of replication. While DDK phosphorylation sites have been identified on Mcm2-7, the contributions made by Dbf4 and Cdc7 to the targeting of the complex have not been established. DDK has also been implicated in the S-phase checkpoint response since it is removed from chromatin in a Rad53-dependent manner. The interaction of Dbf4 and Cdc7 with each of the Mcm2-7 subunits was assessed and showed an interaction between Dbf4 and Mcm2 and Mcm6, while interactions between Cdc7 and Mcm4 and Mcm5 were observed. Mutations in Mcm2 and Mcm4 that disrupt the interactions with Dbf4 or Cdc7 showed modest growth impairment and compromised DNA replication, while simultaneous abrogation of both interactions resulted in lethality. Strains overexpressing Mcm2 or Mcm4 were sensitive to genotoxic agents, while overexpression of Mcm2 in a Mcm4Δ175-333 strain background resulted in a severe growth impairment as well as sensitivity to genotoxic stress. ChIP analysis revealed the possibility of Dbf4/Cdc7 localization to origin flanking regions through most of S-phase, which may redistribute to origins at the time of firing. Fluorescence microscopy of Mcm2 and Dbf4 in S-phase seem to show a punctate pattern of staining, consistent with these factors’ localization to ‘replication factories.’ By using a Dbf4ΔN mutant, the N-motif was shown to be required for the Rad53-mediated removal of Dbf4 from chromatin under checkpoint conditions. Initial optimization of a DNA combing protocol was also performed, which along with Dbf4ΔN mutant and the fluorescently-epitope tagged strains, will be useful tools for evaluating a role for DDK in the S-phase checkpoint response. Altered levels of DNA replication factors have been implicated in many human cancers. The data presented in this study provide novel insight into the normal process of the initiation of DNA replication which can be applied to research involving higher eukaryotes, including humans, and can serve as a benchmark for comparison with the cancer phenotype.

The heterohexameric origin recognition complex (ORC) acts as a scaffold for the G1 phase assembly of pre-replicative complexes. Only the Orc1-5 subunits are required for origin binding in budding yeast, yet Orc6 is an essential protein for cell proliferation. In comparison to other eukaryotic Orc6 proteins, budding yeast Orc6 appears to be quite divergent. Two-hybrid analysis revealed that Orc6 only weakly interacts with other ORC subunits. In this assay Orc6 showed a strong ability to self-associate, although the significance of this dimerization or multimerization remains unclear. Imaging of Orc6-eYFP revealed a punctate sub-nuclear localization pattern throughout the cell cycle, representing the first visualization of replication foci in live budding yeast cells. Orc6 was not detected at the site of division between mother and daughter cells, in contrast to observations from metazoans. An essential role for Orc6 in DNA replication was identified by depleting the protein before and during G1 phase. Surprisingly, Orc6 was required for entry into S phase after pre-replicative complex formation, in contrast to what has been observed for other ORC subunits. When Orc6 was depleted in late G1, Mcm2 and Mcm10 were displaced from chromatin, the efficiency of replication origin firing was severely compromised, and cells failed to progress through S phase. Depletion of Orc6 late in the cell cycle indicated that it was not required for mitosis or cytokinesis. However, Orc6 was shown to be associated with proteins involved in regulating these processes, suggesting that it may act as a signal to mark the completion of DNA replication and allow mitosis to commence.

The heterohexameric origin recognition complex (ORC) acts as a scaffold for the G1 phase assembly of pre-replicative complexes. Only the Orc1-5 subunits are required for origin binding in budding yeast, yet Orc6 is an essential protein for cell proliferation. In comparison to other eukaryotic Orc6 proteins, budding yeast Orc6 appears to be quite divergent. Two-hybrid analysis revealed that Orc6 only weakly interacts with other ORC subunits. In this assay Orc6 showed a strong ability to self-associate, although the significance of this dimerization or multimerization remains unclear. Imaging of Orc6-eYFP revealed a punctate sub-nuclear localization pattern throughout the cell cycle, representing the first visualization of replication foci in live budding yeast cells. Orc6 was not detected at the site of division between mother and daughter cells, in contrast to observations from metazoans. An essential role for Orc6 in DNA replication was identified by depleting the protein before and during G1 phase. Surprisingly, Orc6 was required for entry into S phase after pre-replicative complex formation, in contrast to what has been observed for other ORC subunits. When Orc6 was depleted in late G1, Mcm2 and Mcm10 were displaced from chromatin, the efficiency of replication origin firing was severely compromised, and cells failed to progress through S phase. Depletion of Orc6 late in the cell cycle indicated that it was not required for mitosis or cytokinesis. However, Orc6 was shown to be associated with proteins involved in regulating these processes, suggesting that it may act as a signal to mark the completion of DNA replication and allow mitosis to commence.

Bacteriophage ë has been used as a model replicon system for forty years. While the basic ë replication initiation scheme has been elucidated for several decades, many aspects of the mechanisms are unclear. I wished to study two unanswered issues in ë replication initiation. </p><p>Replication initiation of E. coli and ë each depend upon a protein generally called a licensing factor, which brings the DnaB helicase protein to the origin site to begin DNA synthesis. The licensing factors are the products of host gene dnaC and ë gene P. The synthesis of P from ë DNA in an E. coli cell can competitively interfere with DnaC activity needed for E. coli replication initiation. I wished to learn more about what happens to a host cell when exposed to extended P expression. Previous studies in this laboratory suggested that i) the continuous expression of P was tolerated by a subset of exposed cells and that ii) host defects mapping to dnaB could suppress the effect of extended P expression (P-lethality). I used DNA sequencing to determine if these suppressor mutations were within dnaB. I screened known host mutations for their influence on P-lethality. In summary: E. coli strains with GrpD55 and GrpA80 defects were found to each have two point mutations within their dnaB genes. I was unable to isolate mutations within P that suppressed P-lethality and instead obtained regulatory mutations preventing wild type P expression. Two of these sequenced mutations showed that a cI[Ts] lambda repressor was reverted to cI wild type, blocking P expression at all assay temperatures. P-lethality was reversible in cells exposed to P for up to five hours, causing me to suggest that P-Interference be used in place of the term P-lethality. A non-inducible allele of lexA prevented P-mediated cellular filamentation and enhanced P-Interference. This suggests that induction of the SOS response helps cells to tolerate extended P expression. A host strain containing a defective ClpXP protease significantly enhanced cellular sensitivity to P-Interference. This suggests an important role for the ClpXP chaperone-protease complex in degradation of P and cellular resistance to P expression. I present models to explain the P-Interference Phenotype.</p><p>Recent reports have re-opened the possibility that the tO-oop-pO element influences ë DNA replication initiation. I have also been investigating this possibility. I found that a plasmid with tO-oop-pO (the terminator, nucleotide sequence and promoter for OOP RNA) and orië DNA sequence was inhibitory to the development of repë phages, and designated this the Inhibition Phenotype (IP). In pursuing the mechanism for this inhibition, I mutated the tO-oop-pO and orië elements. I found that the expression of the 77nt OOP RNA transcript and the presence of four 18 base pair repeats (iterons) within orië were required for the IP. I isolated spontaneous phage mutants, resistant to the IP. I determined that singly infected cells were sensitive to the IP but that multiply infected cells escaped the IP. I propose that the IP to repë phage development is directed to the initial or theta mode of ë replication initiation. I found that the theta-mode of ë replication initiation can be bypassed, likely via recombination between multiple phage genomes within a singe cell. I propose models to explain the IP and also suggest a role for OOP RNA in the regulation of ë DNA replication.

Thesis (Ph. D.)--University of Toledo, 2008. / Typescript. "Submitted as partial fulfillment of the requirements for the Doctor of Philosophy in Chemistry." Includes bibliographical references (leaves 305-309).

Thesis (Ph. D.)--University of Chicago, Department of Molecular Genetics and Cell Biology, 2001. / Includes bibliographical references. Also available on the Internet.