Thesis (Ph. D.)--University of Chicago, Dept. of Pathology, March 2001. / Includes bibliographical references. Also available on the Internet.

In this study, DNA methylation was examined during the eukaryotic cell cycle, and shown to occur throughout the S phase as well as during the "early" G₂ phase. However, DNA synthesis and methylation of newly synthesized DNA did not occur simultaneously, but the latter lagged behind DNA synthesis by about two hours. Once added during the S phase, the methyl groups were stably maintained in the DNA. Various compounds which are known to affect DNA synthesis in tissue cultured cells, were tested for their ability to alter the methylation status of DNA. The effects of three DNA synthesis inhibitors, viz. hydroxyurea (HU), 1-S-D-arabinofuranosyl cytosine (ara-C) and aphidicolin were examined on a normal embryonic lung fibroblast cell line (WI-38) and its two transformed counterparts, a simian virus 40 (SV 40) transformed line (SVWI-38) and a y-irradiation transformed cell line (CT-1). HU was shown to enhance hypermethylation of pre-existing DNA strands in the normal cells, while ara-C and aphidicolin caused hypermethylation of newly synthesized DNA strands. The effects of various concentrations of a known inducer of gene expression, sodium butyrate, were examined on these three cell lines as well. During a 16-20 hour treatment period, at butyrate concentrations of between 5 and 20 mM, no adverse effect on cell morphology was observed. Cell growth, in the presence of butyrate for 14 hours, showed that butyrate was more toxic on the transformed cells than on the normal cells. However, at 5 mM butyrate, DNA synthesis was inhibited by 75% in the normal cells, and was unaffected in the transformed lines. RNA synthesis was not affected in the transformed cells, whilst in the normal cell line, RNA synthesis was decreased to 76% of the control value, at sodium butyrate concentrations as low as 5 mM. Protein synthesis also was unaffected in the transformed cells and only slightly (+ 10%) inhibited in the normal cells at 20 mM butyrate. SDS polyacrylamide gel electrophoresis of proteins synthesized in the presence of 10 mM sodium butyrate, showed that most proteins were unaffected. Two high molecular weight proteins in the WI-38 cells appeared to be modified during butyrate. treatment, while one protein was induced by butyrate treatment in the CT-1 cells. More importantly though, butyrate treatment also resulted in hypermethylation of DNA, as shown by MSP 1 and Hpa II restriction endonuclease digestion and high-pressure liquid chromatography analysis. Butyrate appeared to specifically cause hypermethylation of pre-existing DNA strands in the WI-38 cells, while the SVWI-38 and CT-1 cells showed preferential hypermethylation of newly synthesized DNA strands. However, the hyper-methylated state was only heritable if the methylation event occurred in newly synthesized DNA. Hypermethylation on pre-existing DNA was rapidly lost in the subsequent generation. It would therefore appear that methylcytosines are only maintained in the DNA if they are generated on newly synthesized DNA. This study has clearly shown that the heritability of DNA methylation patterns is closely linked to DNA replication.

DNA replication initiates at a precise location on the bacterial chromosome, the origin of replication (oriC). This work has localized the origin of DNA replication on the Sinorhizobium meliloti chromosome to a region spanning the hemE gene. A genetic dissection of the locus revealed that a much larger fragment of DNA (1802 bp) is required for a functional oriC than that of the other characterized alpha-proteobacterial chromosome origin from Caulobacter crescentus. Site-directed mutations of predicted DnaA binding sites has identified several essential elements for replication of the plasmid borne oriC. Mutations in these DnaA boxes also reduce transcription of hemE and thus it is likely that transcription of hemE and replication of the S. meliloti chromosome are coupled. The ColEl plasmid pUCP30T can autonomously replicate when the S. meliloti oriC is cloned into the suicide vector (pTH838) and can be efficiently mobilized out of S. meliloti into E. coli. The pTH838 oriC plasmid when transferred into S. meliloti results in both small and large colonies and both of these transconjugant classes take longer to form than the S. meliloti recA::Tn5 recipient. We attributed this phenotype to the very low copy number of the pTH838 plasmid which was determined to be 0.053 - 0.135 copies per chromosome. The DnaA protein responsible for replication initiation in many bacteria has been purified and used in electrophoretic mobility shift assays. The DnaA protein interacts specifically with sequences in the hemE - Y02793 intergenic region and upstream of the repA2 gene on the pSymA megaplasmid. The DnaA protein has also been implicated as a link between DNA replication and cell division in S. meliloti as overexpression of DnaA in both E. coll and S. meliloti results in filamentation. / Thesis / Master of Science (MSc)

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In eukaryotes, chromosomes are non-randomly positioned within the nucleus.  The perinuclear localization of <i>S. cerevisiae </i>telomeres provides a useful model for studying mechanisms that control chromosome positioning.  In budding yeast, telomeres tend to be localized at the nuclear periphery during early interphase, but following S phase they delocalize and remain randomly positioned within the nucleus.  In this thesis, I investigate whether DNA replication causes telomere dislodgment from the nuclear periphery. First, using live-cell fluorescence microscopy I show that delaying DNA replication causes a corresponding delay in the dislodgement of telomeres from the nuclear envelope, demonstrating that replication of individual telomeres causes their delocalization.  Second, I show that telomere dislodgment is not simply the result of recruitment of telomeres to a replication factory that is formed in the nuclear interior, since I found that telomeric DNA replication can occur either at the nuclear periphery or in the nuclear interior.  The telomere binding complex Ku is one of the factors that establishes telomere localization to the nuclear envelope.  Using a gene locus tethering assay,  I show that the Ku-mediated telomere localization pathway is inactivated after DNA replication. Based on these findings, I propose that DNA replication causes telomere delocalization by triggering stable repression of the Ku-mediated anchoring pathway.  In addition to maintaining genetic information, DNA replication may therefore regulate subnuclear organization of chromatin.

DNA replication is a key process required by organisms during cell division, with a concomitant requirement for genome synthesis by DNA polymerases. Biotechnological exploitation of thermostable DNA polymerases for DNA amplification by the Polymerase Chain Reaction (PCR), provides a significant market for novel enzymes or those with improved properties. An approach was taken to isolate alternative thermostable DNA polymerases, by enriching thermophilic bacteria from a novel thermal environment, aerobically spoiling silage. In addition, a novel DNA polymerase (Abr polBl) was cloned from the thermoacidophilic archaeon, Acidianus brierleyi, with the intention of characterising its in vivo role and application to PCR. Protein sequence analysis suggested a proofreading (high fidelity) DNA synthesis activity most related to polBl DNA polymerases from Crenarchaeota. Abr polBl was heterologously expressed in bacteria and protein purified to homogeneity. Biochemical assays confirmed high-temperature DNA polymerase and 3'-5'exonuclease activities of Abr polBl, with an accompanying proofreading ability. Sequence analysis, processivity, strand displacement and lesion bypass activities indicated potential roles in genome replication and DNA repair. Abr polBl could not amplify DNA under a range of PCR conditions, presumably following its low intrinsic thermostability. Biophysical analyses confirmed irreversible unfolding of Abr polBl at temperatures required for PCR. Supplementation with organic compounds and ionic salts stabilised Abr polBl, promoting retention of conformational stability and DNA synthesis activity following thermal incubation, but could not promote DNA amplification with Abr polB 1.

<p>The complete and faithful duplication of the genome is essential to ensure normal cell division and organismal development. Eukaryotic DNA replication is initiated at multiple sites termed origins of replication that are activated at different time through S phase. The replication timing program is regulated by the S-phase checkpoint, which signals and repairs replicative stress. Eukaryotic DNA is packaged with histones into chromatin, thus DNA-templated processes including replication are modulated by the local chromatin environment such as post-translational modifications (PTMs) of histones.</p><p>One such epigenetic mark, methylation of lysine 20 on histone H4 (H4K20), has been linked to chromatin compaction, transcription, DNA repair and DNA replication. H4K20 can be mono-, di- and tri-methylated. Monomethylation of H4K20 (H4K20me1) is mediated by the cell cycle-regulated histone methyltransferase PR-Set7 and subsequent di-/tri- methylation is catalyzed by Suv4-20. Prior studies have shown that PR-Set7 depletion in mammalian cells results in defective S phase progression and the accumulation of DNA damage, which may be partially attributed to defects in origin selection and activation. Meanwhile, overexpression of mammalian PR-Set7 recruits components of pre-Replication Complex (pre-RC) onto chromatin and licenses replication origins for re-replication. However, these studies were limited to only a handful of mammalian origins, and it remains unclear how PR-Set7 impacts the replication program on a genomic scale. Finally, the methylation substrates of PR-Set7 include both histone (H4K20) and non-histone targets, therefore it is necessary to directly test the role of H4K20 methylation in PR-Set7 regulated phenotypes. </p><p>I employed genetic, cytological, and genomic approaches to better understand the role of H4K20 methylation in regulating DNA replication and genome stability in Drosophila melanogaster cells. Depletion of Drosophila PR-Set7 by RNAi in cultured Kc167 cells led to an ATR-dependent cell cycle arrest with near 4N DNA content and the accumulation of DNA damage, indicating a defect in completing S phase. The cells were arrested at the second S phase following PR-Set7 downregulation, suggesting that it was an epigenetic effect that coupled to the dilution of histone modification over multiple cell cycles. To directly test the role of H4K20 methylation in regulating genome integrity, I collaborated with the Duronio Lab and observed spontaneous DNA damage on the imaginal wing discs of third instar mutant larvae that had an alanine substitution on H4K20 (H4K20A) thus unable to be methylated, confirming that H4K20 is a bona fide target of PR-Set7 in maintaining genome integrity. </p><p>One possible source of DNA damage due to loss of PR-Set7 is reduced origin activity. I used BrdU-seq to profile the genome-wide origin activation pattern. However, I found that deregulation of H4K20 methylation states by manipulating the H4K20 methyltransferases PR-Set7 and Suv4-20 had no impact on origin activation throughout the genome. I then mapped the genomic distribution of DNA damage upon PR-Set7 depletion. Surprisingly, ChIP-seq of the DNA damage marker γ-H2A.v located the DNA damage to late replicating euchromatic regions of the Drosophila genome, and the strength of γ-H2A.v signal was uniformly distributed and spanned the entire late replication domain, implying stochastic replication fork collapse within late replicating regions. Together these data suggest that PR-Set7-mediated monomethylation of H4K20 is critical for maintaining the genomic integrity of late replicating domains, presumably via stabilization of late replicating forks.</p><p>In addition to investigating the function of H4K20me, I also used immunofluorescence to characterize the cell cycle regulated chromatin loading of Mcm2-7 complex, the DNA helicase that licenses replication origins, using H4K20me1 level as a proxy for cell cycle stages. In parallel with chromatin spindown data by Powell et al. (Powell et al. 2015), we showed a continuous loading of Mcm2-7 during G1 and a progressive removal from chromatin through S phase.</p> / Dissertation