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At least 73 records · Page 4Linked to original sources
PubMed2003

Plant DNA helicases: the long unwinding road.

DNA helicases are molecular motor proteins that use the energy of nucleoside 5'-triphosphate (NTP) hydrolysis to open transiently the energetically stable duplex DNA into single strands and thereby play essential roles in nearly all DNA metabolic transactions. After the discovery of the first prokaryotic DNA helicase from E. coli in 1976 and the first eukaryotic one from the lily plant in 1978, many more have been isolated and characterized including at least eight from plants. All the DNA helicases share some common properties, including nucleic acid binding, NTP binding and hydrolysis and unwinding of duplex DNA in the 3' to 5' or 5' to 3' direction. In plants, DNA helicases are mainly present in nuclei, mitochondria and chloroplasts. The in vivo role of many DNA helicases has not been well investigated in eukaryotic systems including plants. However, through indirect evidence, the involvement of plant DNA helicases has been suggested at least in the following biological processes: DNA recombination, DNA replication, translation initiation, rDNA transcription and in the early stages of pre-rRNA processing, double-strand break repair, maintenance of telomeric length, nucleotide excision repair, cell division/proliferation during flower development, maintenance of genomic methylation patterns, the plant cell cycle, and in the maintenance of the basic activities of cells. A recently discovered Helitron insertion in the maize genome has suggested the possible role of plant DNA helicase(s) in a new class of rolling-circle transposons. All these reflect that plant DNA helicases may play an important role in plant growth and development and thus have important biotechnological applications. In this review, an up-to-date knowledge of plant DNA helicases is summarized. In addition, the historical perspective, biochemical assay and polarity, inhibitors and functions of plant DNA helicases have also been covered.

DNA Helicases↗
PubMed1991

Purification and characterization of two DNA helicases from calf thymus nuclei.

Two DNA helicases from calf thymus nuclei have been purified and characterized. The two proteins, designated as nuclear DNA helicase I and II, were copurified on Bio-Rex 70, DEAE-Sepharose, phosphocellulose and subsequently separated from each other on a heparin-Sepharose column. Final purification of DNA helicase I was achieved on single-stranded DNA-cellulose and that of DNA helicase II on ATP-agarose. On denaturing polyacrylamide gels, nuclear DNA helicase I displayed two polypeptide bands of 170 and 200 kDa; nuclear DNA helicase II also consisted of two bands, in this case of 100 and 130 kDa. Both enzymes catalyzed the unwinding of a DNA primer from a single-stranded DNA template but had different nucleotide requirements for their energy supply. While nuclear DNA helicase I preferred to hydrolyze ATP or dATP to support its unwinding activity, nuclear DNA helicase II could utilize all four rNTPs or dNTPs, though ATP or dATP were still preferred to other nucleotides. ADP, AMP, or adenosine 5'-O-(thiotriphosphate) could not be used by either enzyme in the unwinding reactions. A divalent cation was essential for activity of both enzymes. Nuclear DNA helicase I required 3-5 mM Mg2+ or 1 mM Mn2+ for optimal unwinding. In contrast, nuclear DNA helicase II displayed high activity even at very low concentrations of Mg2+. Nuclear DNA helicase I was stimulated by NaCl, KCl, or potassium acetate up to concentrations of 150 mM; in contrast, nuclear DNA helicase II was strongly inhibited at salt concentrations over 75 mM. Both DNA helicases had an associated ATPase activity. However, while the ATPase activity of nuclear DNA helicase I was stimulated only in presence of single-stranded DNA, the ATPase activity of the nuclear DNA helicase II was stimulated by single-stranded DNA and, even more efficiently, by RNA. Finally, the translocation of both DNA helicases had a polarity from 3' to 5' with respect to the single-stranded DNA template to which the enzymes were bound. A comparison of these DNA helicases with the other reported mammalian DNA helicases will be given. The significance of the association of the two DNA helicases during the process of the purification will be discussed.

Adenosine Triphosphatases↗
PubMed1994

Involvement of a replicative DNA helicase of bacteriophage T4 in DNA recombination.

Bacteriophage T4 gene 41 encodes a replicative DNA helicase that is a subunit of the primosome which is essential for lagging-strand DNA synthesis. A mutation, rrh, was generated and selected in the helicase gene on the basis of limited DNA replication that ceases early. The survival of ultraviolet-irradiated phage and the frequency of genetic recombination are reduced by rrh. In addition, rrh diminishes the production of concatemeric DNA. These results strongly suggest that the gene 41 replicative helicase participates in DNA recombination.

Amino Acid Sequence↗
PubMed1997

Inhibition of DNA unwinding and ATPase activities of human DNA helicase II by chemotherapeutic agents.

DNA helicases catalyze the unwinding of duplex DNA and thus play important roles in the processing of DNA, little is known about the effects of various cytotoxic or antitumor chemotherapeutic agents on purified human DNA helicases. We have determined the effect of actinomycin C1, VP-16, camptothecin, ethidium bromide, ellipticine, nogalamycin, novobiocin, genistein, m-AMSA, aphidicolin and daunorubicin on the enzymatic activities of purified human DNA helicase II which was identified as Ku autoantigen. Ku contains DNA helicase, ATPase and DNA end binding activities. Our data have shown that out of several chemotherapeutic agents tested ethidium bromide, actinomycin C1, daunorubicin and nogalamycin were inhibitors of DNA unwinding activity of human DNA helicase II with ID50 values of 8.44 microM, 11.68 microM, 6.23 microM and 0.42 microM respectively. These inhibitors also inhibited the ATPase activity but not the DNA binding activity of this helicase. This inhibition could be due to binding of these drugs to DNA, thereby impeding the movement of the helicase for unwinding action which may be their most important pharmacological function against cancer cells.

Adenosine Triphosphatases↗
PubMed1984

Identification of the uvrD gene product of Escherichia coli as DNA helicase II and its induction by DNA-damaging agents.

Taking advantage of overproduction of the uvrD protein in cells that harbor multicopy plasmids carrying the uvrD gene, we have purified protein to physical homogeneity. The purified protein possesses single-stranded DNA-dependent ATPase and ATP-dependent DNA unwinding activities, and both activities are equally inactivated by antibodies raised against DNA helicase II. Molecular weight (75,000) and chromatographic properties of the uvrD protein are also similar to those of DNA helicase II and DNA-dependent ATPase I (Richet, E., and Kohiyama, M. (1976) J. Biol. Chem. 251, 808-812; Abdel-Monem, M., Dürwald, H., and Hoffmann-Berling, H. (1977) Eur. J. Biochem. 79, 39-45). Thus, it is concluded that the uvrD protein is identical with DNA helicase II and DNA-dependent ATPase I. Expression of the uvrD gene, as assayed by DNA-dependent ATPase activity, was stimulated by exposure of bacteria to nalidixic acid or mitomycin C. No increase in ATPase activity was observed with recA mutant cells, although basic levels of DNA-dependent ATPase activity in recA+ and recA- strains were almost the same. Thus, the uvrD gene is constitutively expressed but also regulated in a recA-dependent fashion.

Antibiotics, Antineoplastic↗
PubMed2002

A dominant-negative mutant of human DNA helicase B blocks the onset of chromosomal DNA replication.

A cDNA encoding a human ortholog of mouse DNA helicase B, which may play a role in DNA replication, has been cloned and expressed as a recombinant protein. The predicted human DNA helicase B (HDHB) protein contains conserved helicase motifs (superfamily 1) that are strikingly similar to those of bacterial recD and T4 dda proteins. The HDHB gene is expressed at low levels in liver, spleen, kidney, and brain and at higher levels in testis and thymus. Purified recombinant HDHB hydrolyzed ATP and dATP in the presence of single-stranded DNA, displayed robust 5'-3' DNA helicase activity, and interacted physically and functionally with DNA polymerase alpha-primase. HDHB proteins with mutations in the Walker A or B motif lacked ATPase and helicase activity but retained the ability to interact with DNA polymerase alpha-primase, suggesting that the mutants might be dominant over endogenous HDHB in human cells. When purified HDHB protein was microinjected into the nucleus of cells in early G(1), the mutant proteins inhibited DNA synthesis, whereas the wild type protein had no effect. Injection of wild type or mutant protein into cells at G(1)/S did not prevent DNA synthesis. The results suggest that HDHB function is required for S phase entry.

Adenosine Triphosphatases↗
PubMed1993

The E1 protein of bovine papilloma virus 1 is an ATP-dependent DNA helicase.

For efficient DNA replication of papillomaviruses, only two viral-encoded proteins, E1 and E2, are required. Other proteins and factors are provided by the host cell. E2 is an enhancer of both transcription and replication and is known to help E1 bind cooperatively to the origin of DNA replication. E1 is sufficient for replication in extracts prepared from permissive cells, but the activity is enhanced by E2. Here we show that purified E1 can act as an ATP-dependent DNA helicase. To measure this activity, we have used strand displacement, unwinding of topologically constrained DNA, denaturation of duplex fragments, and electron microscopy. The ability of E1 to unwind circular DNA is found to be independent of origin-specific viral DNA sequences under a variety of experimental conditions. In unfractionated cellular extracts, E1-dependent viral DNA replication is origin-dependent, but at elevated E1 concentrations, replication can occur on non-origin-containing DNA templates. This conversion from an origin-dependent replication system to a nonspecific initiator system is discussed in the context of the current understanding of the initiation of chromosomal DNA replication.

Adenosine Triphosphate↗
PubMed1997

Dimerization of simian virus 40 T-antigen hexamers activates T-antigen DNA helicase activity.

Chromosomal DNA replication in higher eukaryotes takes place in DNA synthesis factories containing numerous replication forks. We explored the role of replication fork aggregation in vitro, using as a model the simian virus 40 (SV40) large tumor antigen (T antigen), essential for its DNA helicase and origin-binding activities. Previous studies have shown that T antigen binds model DNA replication forks primarily as a hexamer (TAgH) and to a lesser extent as a double hexamer (TAgDH). We find that DNA unwinding in the presence of ATP or other nucleotides strongly correlates with the formation of TAgDH-DNA fork complexes. TAgH- and TAgDH-fork complexes were isolated, and the TAgDH-bound fork was denatured at a 15-fold-higher rate during the initial times of unwinding. TAgDH bound preferentially to a DNA substrate containing a 50-nucleotide bubble, indicating the bridging of each single-stranded DNA/duplex DNA junction, and this DNA molecule was also unwound at a high rate. Both the TAgH- and TAgDH-fork complexes were relatively stable, with the half-life of the TAgDH-fork complex greater than 40 min. Our data therefore indicate that the linking of two viral replication forks serves to activate DNA replication.

Antigens, Polyomavirus Transforming↗
PubMed1989

The bacteriophage T4 DNA replication fork. Only DNA helicase is required for leading strand DNA synthesis by the DNA polymerase holoenzyme.

Seven bacteriophage T4-encoded proteins reconstitute a DNA replication apparatus that catalyzes coupled leading and lagging strand DNA synthesis at a replication fork in vitro. The proteins involved are the T4 DNA polymerase holoenzyme (the products of T4 genes 43, 44/62, and 45), a helix-destabilizing (SSB) protein (gene 32 protein), and the T4 primosome which is composed of a DNA helicase (gene 41 protein) and a primase (gene 61 protein). We show here that the presence of 41 protein on the lagging strand of the fork enables the polymerase holoenzyme to catalyze leading strand DNA synthesis at a maximum rate and with high processivity. This leading strand synthesis is unaffected by the addition of either the gene 32 or the gene 61 protein; the 41 protein cannot be replaced by the dda protein, a second T4-encoded DNA helicase. When the 61 protein is added to the 41 protein to complete the primosome, Okazaki fragment synthesis on the lagging strand accompanies leading strand DNA synthesis in this system even in the absence of the 32 protein. However, the addition of 32 protein decreases the size of the Okazaki fragments made, as expected for an increase in the lagging strand polymerization rate at a fork that has coupled leading and lagging strand DNA polymerase molecules.

DNA Helicases↗
PubMed1994

A role for two DNA helicases in the replication of T4 bacteriophage DNA.

The T4 bacteriophage gene 41 protein is the highly processive DNA helicase of the T4 primosome, a central part of the protein machinery that moves the T4 DNA replication fork. The T4 gene 59 protein accelerates the loading of 41 protein onto DNA covered with 32 protein (the T4 single strand binding protein), and it makes the 41 protein DNA helicase activity rapidly available to catalyze replication fork movement through a DNA double helix (Barry, J., and Alberts, B.M. (1994) J. Biol. Chem. 269, 33049-33062). With the aid of the 59 protein, we show that the T4 primosome (the T4 gene 41 and 61 proteins) can move rapidly through a promoter-bound RNA polymerase molecule that would otherwise stop replication fork movement. A second, very different DNA helicase, the T4 dda protein, provides an alternative pathway for replication past this DNA-bound RNA polymerase (Bedinger, P., Hochstrasser, M., Jongeneel, C. V., and Alberts, B.M. (1983) Cell 34, 115-123). Combined with other data, these in vitro experiments allow us to propose a model that explains why either the 59 protein or the dda protein, but not both, are required to begin efficient DNA replication inside the T4 bacteriophage-infected cell.

Bacteriophage T4↗
PubMed2002

Physical and functional interaction between the mini-chromosome maintenance-like DNA helicase and the single-stranded DNA binding protein from the crenarchaeon Sulfolobus solfataricus.

Mini-chromosome Maintenance (MCM) proteins play an essential role in both initiation and elongation phases of DNA replication in Eukarya. Genes encoding MCM homologs are present also in the genomic sequence of Archaea and the MCM-like protein from the euryarchaeon Methanobacterium thermoautotrophicum (Mth MCM) was shown to possess a robust ATP-dependent 3'-5' DNA helicase activity in vitro. Herein, we report the first biochemical characterization of a MCM homolog from a crenarchaeon, the thermoacidophile Sulfolobus solfataricus (Sso MCM). Gel filtration and glycerol gradient centrifugation experiments indicate that the Sso MCM forms single hexamers (470 kDa) in solution, whereas the Mth MCM assembles into double hexamers. The Sso MCM has NTPase and DNA helicase activity, which preferentially acts on DNA duplexes containing a 5'-tail and is stimulated by the single-stranded DNA binding protein from S. solfataricus (Sso SSB). In support of this functional interaction, we demonstrated by immunological methods that the Sso MCM and SSB form protein.protein complexes. These findings provide the first in vitro biochemical evidence of a physical/functional interaction between a MCM complex and another replication factor and suggest that the two proteins may function together in vivo in important DNA metabolic pathways.

Adenosine Triphosphatases↗
PubMed1992

Three new DNA helicases from Saccharomyces cerevisiae.

At least six DNA helicases have been identified during fractionation of extracts from the yeast Saccharomyces cerevisiae. Three of those, DNA helicases B, C, and D, have been further purified and characterized. DNA helicases B and C co-purified with DNA polymerase delta through several chromatographic steps, but were separated from the polymerase by hydrophobic chromatography. DNA helicase D co-purified with Replication Factor C over seven chromatographic steps, and was only separated from it by glycerol gradient centrifugation in the presence of 0.2 M NaCl. All three helicases are DNA dependent ATPases with Km values for ATP of 190 microM, 325 microM, and 60 microM for DNA helicases B, C, and D, respectively. Their DNA helicase activities are comparable. They are 5'-3' helicases and have pH optima of 6.5-7 and Mg2+ optima of 1-2 mM. However, they differ in the nucleotide requirement for helicase action. Whereas all three helicases preferred ATP, dATP, UTP, CTP, and dCTP as cofactors, DNA helicase C also used GTP, but not dTTP. On the other hand, DNA helicase D used dTTP, but not GTP, and DNA helicase B used neither nucleotide as cofactor. These studies allowed us to conclude that DNA helicases B, C, and D are not only distinct enzymes, but also different from two previously identified yeast DNA helicases, the RAD3 protein and ATPase III.

Adenosine Triphosphatases↗
PubMed2002

The hepatitis C viral NS3 protein is a processive DNA helicase with cofactor enhanced RNA unwinding.

The RNA helicase/protease NS3 plays a central role in the RNA replication of hepatitis C virus (HCV), a cytoplasmic RNA virus that represents a major worldwide health problem. NS3 is, therefore, an important drug target in the effort to combat HCV. Most work has focused on the protease, rather than the helicase, activities of the enzyme. In order to further characterize NS3 helicase activity, we evaluated individual stages of duplex unwinding by NS3 alone and in complex with cofactor NS4A. Despite a putative replicative role in RNA unwinding, we found that NS3 alone is a surprisingly poor helicase on RNA, but that RNA activity is promoted by cofactor NS4A. In contrast, NS3 alone is a highly processive helicase on DNA. Phylogenetic analysis suggests that this robust DNA helicase activity is not vestigial and may have specifically evolved in HCV. Given that HCV has no replicative DNA intermediate, these findings suggest that NS3 may have the capacity to affect host DNA.

Base Sequence↗
PubMed1994

Disruption of a topoisomerase-DNA cleavage complex by a DNA helicase.

The type II DNA topoisomerases are targets for a variety of chemotherapeutic agents, including the antibacterial quinolones and several families of antitumor drugs. These agents stabilize an enzyme-DNA cleavage complex that consists of the topoisomerase covalently linked to the 5' phosphates of a double-stranded DNA break. Although the drug-stabilized cleavage complex is readily reversible, it can result in cell death by a mechanism that remains uncertain. Here we demonstrate that the action of a DNA helicase can convert the cleavage complex into a nonreversible DNA break by displacing DNA strands from the complex. Formation of a nonreversible DNA break, induced by a DNA helicase, could explain the cytotoxicity of these topoisomerase poisons.

Adenosine Triphosphatases↗
PubMed2002

[Functional analysis of yeast homologue gene associated with human DNA helicase causative syndromes].

Proteins having DNA helicase activity play very important roles in many processes involving DNA workings such as replication, repair, and recombination. In this decade, many DNA helicase genes have been cloned as the causative genes of human recessive heredity diseases. These are the causative genes for Xeroderma pigmentosum (XPB and XPD), Cockayne syndrome (CSB), diffuse collagen disease (Ku80), alpha-thalassmia (ATR-X), Bloom syndrome (BLM), Werner syndrome (WRN) and Rothmund-Thomson syndrome (RTS). The yeast homologue genes of these human DNA helicase genes exist. S. cerevisiae RAD25/SSL2, RAD3, RAD26, YKU80/HDF2 and RAD54 are the homologue for XPB/ERCC3, XPD/ERCC2, CSB/ERCC6, Ku80/XRCC5 and ATR-X/HX2, respectively. E coli. recQ gene and S. cerevisiae SGS1 are the homologue for all BLM, WRN and RTS. A search of whole genome of S. cerevisiae revealed that SGS1 is the sole homologue of recQ in S. cerevisiae. Thus it seems likely that SGS1 is a functional homologue of one or several human RecQ family genes. Many basic or essential functions are well conserved in the cells from lower eukaryotic to higher mammalian. The functional analysis in yeast could make an useful insight for the human homologue. To clarify the functions of S. cerevisiae Sgs1 and to get an insight into the functions of Blm, Wrn and Rts, in this study, we analyzed the phenotype of sgs1 disruptant and in detail the cause of the poor sporulation phenotype of sgs1 disruptants in relation to meiotic processes including meiotic recombination. The poor sporulation of sgs1 disruptants was complemented with a mutated SGS1 gene encoding a protein lacking DNA helicase activity; however, the mutated gene could suppress neither the sensitivity of sgs1 disruptants to methyl methanesulfonate (MMS) and hydroxyurea nor the mitotic hyperrecombination phenotype of sgs1 disruptants. The N-terminal 1-45 amino acid region and 698-1195 amino acid region of Sgs1, which including helicase domain and C-terminal RecQ conserved region with helicase activity, were required for complementation of MMS sensitivity and suppression of hyperrecombination of sgs1 disruptants in mitotic growth. The 126-400 and 596-1195 amino acid regions of Sgs1 were required for complementation of poor sporulation and of reduced meiotic functions. These regions required for the mitotic or meiotic functions of Sgs1 were well overlapped with the interaction regions of Top3 and Top2. Some of these results might explain the mechanism of the symptom of RecQ-related syndromes.

Adenosine Triphosphatases↗
PubMed2006

[DNA helicases and human diseases].

DNA helicases are molecular motors that catalyse the unwinding of energetically unstable structures into single strands and have therefore an essential role in nearly all metabolism transactions. Defects in helicase function can result in human syndromes in which predisposition to cancer and genomic instability are common features. So far different helicase genes have been found associated in 8 such disorders. RecQ helicases are a family of conserved enzymes required for maintaining the genome integrity that function as suppressors of inappropriate recombination. Mutations in RecQ4, BLM and WRN give rise to various disorders: Bloom syndrome, Rothmund-Thomson syndrome, and Werner syndrome characterized by genomic instability and increased cancer susceptibility. The DNA helicase BRIP1/BACH1 is involved in double-strand break repair and is defective in Fanconi anemia complementation group J. Mutations in XPD and XPB genes can result in xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy, three genetic disorders with different clinical features but with association of transcription and NER defects. This review summarizes our current knowledge on the diverse biological functions of these helicases and the molecular basis of the associated diseases.

Abnormalities, Multiple↗
PubMed2000

Interaction of different oligomeric states of hexameric DNA-helicase RepA with single-stranded DNA studied by analytical ultracentrifugation.

Analytical ultracentrifugation was used to determine the molecular mass, M, of hexameric DNA-helicase RepA at pH 5.8 and 7.6. At pH 7.6, a molecular mass of 179.5+/-2.6 kDa was found, consistent with the known hexameric state of RepA, (RepA)(6). At pH 5.8, (RepA)(6) associates to form a dimer with a molecular mass of 366.2+/-4.1 kDa. Analytical ultracentrifugation was also applied to characterize the interaction of single-stranded DNA (ssDNA) with the two different oligomeric states of (RepA)(6) at pH 5.8 and 7.6. The dissociation constants, K(d), for the equilibrium binding of (dA)(30) to the (RepA)(6) dimer at pH 5.8 and to (RepA)(6) at pH 7.6 were determined at 10 degrees C in the presence of 0.5 mM ATPgammaS, 10 mM MgCl(2) and 60 mM NaCl as K(d5.8)=0.94+/-0.13 microM at pH 5.8 and K(d7. 6)=25.4+/-6.4 microM at pH 7.6. The stoichiometries, n, for the two complexes (dA)(30)/(RepA)(6) dimer and (dA)(30)/(RepA)(6) at pH 5.8 and 7.6 were calculated from the corresponding binding curves. At pH 5.8 one (dA)(30) molecule was bound per (RepA)(6) dimer, while at pH 7.6 one (dA)(30) molecule was bound to one (RepA)(6). Binding curves were compatible with a single ssDNA binding site present on the (RepA)(6) dimer and on (RepA)(6), respectively, with no indication of cooperativity. (RepA)(6) tends to form larger aggregates under acidic conditions (pH<6.0) which are optimal for ssDNA binding. In contrast, at pH 5.8 in the presence of 60 mM NaCl, only the (RepA)(6) dimer was observed both in the absence and presence of (dA)(30).

Biopolymers↗