Masumi Hidaka

Fate of DNA replication fork encountering a single DNA lesion during oriC plasmid DNA replication in vitro

Background: The inhibition of DNA replication fork progression by DNA lesions can lead to cell death or genome instability. However, little is known about how such DNA lesions affect the concurrent synthesis of leading‐ and lagging‐strand DNA catalysed by the protein machinery used in chromosomal replication. Using a system of semi‐bidirectional DNA replication of an oriC plasmid that employs purified replicative enzymes and a replication‐terminating protein of Escherichia coli, we examined the dynamics of the replication fork when it encounters a single abasic DNA lesion on the template DNA.

Identification of a site required for DNA replication fork blocking activity in the rRNA gene cluster in Saccharomyces cerevisiae

SummaryThe yeast genome has DNA replication fork blocking sites, that we have named sog sites, in the ribosomal RNA gene (rDNA) cluster. These are located at the 3′ end of the 35S rRNA transcription unit and they block replication fork movement in a direction opposite to that of RNA polymerase I. We cloned this replication blocking site into a YEp-type plasmid and analyzed DNA replication intermediates, using two-dimensional (2D) agarose gel electrophoresis. The blocking activity remained even on a plasmid not involved in 35S rRNA transcription and inhibited fork movement in the same polar fashion as on the yeast chromosome. To define the site further, smaller fragments were subcloned into the YEp-type plasmid. A small 109 by region exhibited sog activity and was located near the enhancer region for 35S rRNA transcription. It overlaps an essential element of the recombinational hot spot HOT1.

Cooperation of the N-terminal helicase and C-terminal endonuclease activities of archaeal Hef protein in processing stalled replication forks

Blockage of replication fork progression often occurs during DNA replication, and repairing and restarting stalled replication forks are essential events in all organisms for the maintenance of genome integrity. The repair system employs processing enzymes to restore the stalled fork. In Archaea Hef is a well conserved protein that specifically cleaves nicked, flapped, and fork-structured DNAs. This enzyme contains two distinct domains that are similar to the DEAH helicase family and XPF nuclease superfamily proteins. Analyses of truncated mutant proteins consisting of each domain revealed that the C-terminal nuclease domain independently recognized and incised fork-structured DNA. The N-terminal helicase domain also specifically unwound fork-structured DNA and Holliday junction DNA in the presence of ATP. Moreover, the endonuclease activity of the whole Hef protein was clearly stimulated by ATP hydrolysis catalyzed by the N-terminal domain. These enzymatic properties suggest that Hef efficiently resolves stalled replication forks by two steps, which are branch point transfer to the 5′-end of the nascent lagging strand by the N-terminal helicase followed by template strand incision for leading strand synthesis by the C-terminal endonuclease.

PCNA–MutSα-mediated binding of MutLα to replicative DNA with mismatched bases to induce apoptosis in human cells

Modified bases, such as O6-methylguanines, are produced in cells exposed to alkylating agents and cause apoptosis. In human cells treated with N-methyl-N-nitrosourea, we detected a protein complex composed of MutSα, MutLα and PCNA on damaged DNA by immunoprecipitation method using chromatin extracts, in which protein–protein interactions were stabilized by chemical crosslinking. Time course experiments revealed that MutSα, consisting of MSH2 and MSH6 proteins, and PCNA bind to DNA to form an initial complex, and MutLα, composed of MLH1 and PMS2, binds to the complex when the DNA is damaged. This sequential mode of binding was further confirmed by the findings that the association of PCNA–MutSα complex on chromatin was observed even in the cells that lack MLH1, whereas in the absence of MSH2 no association of MutLα with the chromatin was achieved. Moreover, reduction in the PCNA content by small-interfering RNA or inhibition of DNA replication by aphidicolin, an inhibitor of DNA polymerase, significantly reduced the levels of the PCNA–MutSα–MutLα complex and also suppressed an increase in the caspase-3 activity, a hallmark for the induction of apoptosis. These observations imply that the induction of apoptosis is coupled with the progression of DNA replication through the action of PCNA.

Specific chromosomal sites enhancing homologous recombination in Escherichia coli mutants defective in RNase H

To clone new replication origin(s) activated under RNase H-defective (rnh−) conditions in Escherichia coli cells, whole chromosomal DNA digested with EcoRI was to with a Kmr DNA fragment and transformed into an rnh− derivative host. From the Kmr transformants, we obtained eight kinds of plasmid-like DNA, each of which contained a specific DNA fragment, termed “Hot”, derived from the E. coli genome. Seven of the Hot DNAs (HotA-G) mapped to various sites within a narrow DNA replication termination region (about 280 kb), without any particular selection. Because Hot DNA could not be transformed into a mutant strain in which the corresponding Hot region had been deleted from the chromosome, the Hot DNA, though obtained as covalently closed circular (ccc) DNA, must have arisen by excision from the host chromosome into which it had initially integrated, rather than by autonomous replication of the transformed species. While Hot DNA does not have a weak replication origin it does have a strong recombinational hotspot active in the absence of RNase H. This notion is supported by the finding that Chi activity was present on all Hot DNAs tested and no Hot-positive clone without Chi activity was obtained, with the exception of a DNA clone carrying the dif site.

The WD40-repeat protein Pwp1p associates in vivo with 25S ribosomal chromatin in a histone H4 tail-dependent manner

The tails of core histones (H2A, H2B, H3 and H4) are critical for the regulation of chromatin dynamics. Each core histone tail is specifically recognized by various tail binding proteins. Here we screened for budding yeast histone H4-tail binding proteins in a protein differential display approach by two-dimensional gel electrophoresis (2DGE). To obtain highly enriched chromatin proteins, we used a Mg2+-dependent chromatin oligomerization technique. The Mg2+-dependent oligomerized chromatin from H4-tail deleted cells was compared with that from wild-type cells. We used mass spectrometry to identify 22 candidate proteins whose amounts were reduced in the oligomerized chromatin from the H4-tail deleted cells. A Saccharomyces Genome Database search revealed 10 protein complexes, each of which contained more than two candidate proteins. Interestingly, 7 out of the 10 complexes have the potential to associate with the H4-tail. We obtained in vivo evidence, by a chromatin immunoprecipitation assay, that one of the candidate proteins, Pwp1p, associates with the 25S ribosomal DNA (rDNA) chromatin in an H4-tail-dependent manner. We propose that the complex containing Pwp1p regulates the transcription of rDNA. Our results demonstrate that the protein differential display approach by 2DGE, using a histone-tail mutant, is a powerful method to identify histone-tail binding proteins.

Function of high-mobility group A proteins in the DNA damage signaling for the induction of apoptosis

O6-Methylguanine produced in DNA can pair with thymine during DNA replication, thus leading to a G-to-A transition mutation. To prevent such outcomes, cells harboring O6-methylguanine-containing mispair undergo apoptosis that requires the function of mismatch repair (MMR) protein complex. To identify the genes involved in the induction of apoptosis, we performed gene-trap mutagenesis and isolated a clone of mouse cells exhibiting an increased resistance to the killing effect of an alkylating agent, N-methyl-N-nitrosourea (MNU). The mutant carries an insertion in the Hmga2 gene, which belongs to a gene family encoding the high-mobility group A non-histone chromatin proteins. To elucidate the function of HMGA proteins in the apoptosis pathway, we introduced siRNAs for HMGA1 and/or HMGA2 into human HeLa MR cells defective in O6-methylguanine-DNA methyltransferase. HMGA1- and HMGA2-single knockdown cells showed an increased resistance to MNU, and HMGA1/HMGA2-double knockdown cells exhibited further increased tolerance compared to the control. The phosphorylation of ATR and CHK1, the appearance of a sub-G1 population, and caspase-9 activation were suppressed in the knockdown cells, although the formation of mismatch recognition complex was unaffected. These results suggest that HMGA family proteins function at the step following the damage recognition in the process of apoptosis triggered by O6-methylguanine.