Inhibition of the Bloom's and Werner's syndrome helicases by G-quadruplex interacting ligands

G-Quadruplex DNAs are folded, non-Watson-Crick structures that can form within guanine-rich DNA sequences such as telomeric repeats. Previous studies have identified a series of trisubstituted acridine derivatives that are potent and selective ligands for G-quadruplex DNA. These ligands have been shown previously to inhibit the activity of telomerase, the specialized reverse transcriptase that regulates telomere length. The RecQ family of DNA helicases, which includes the Bloom's (BLM) and Werner's (WRN) syndrome gene products, are apparently unique among cellular helicases in their ability to efficiently disrupt G-quadruplex DNA. This property may be relevant to telomere maintenance, since it is known that the sole budding yeast RecQ helicase, Sgs1p, is required for a telomerase-independent telomere lengthening pathway reminiscent of the "ALT" pathway in human cells. Here, we show that trisubstituted acridine ligands are potent inhibitors of the helicase activity of the BLM and WRN proteins on both G-quadruplex and B-form DNA substrates. Inhibition of helicase activity is associated with both a reduction in the level of binding of the helicase to G-quadruplex DNA and a reduction in the degree to which the G-quadruplex DNA can support DNA-dependent ATPase activity. We discuss these results in the context of the possible utility of trisubstituted acridines as antitumor agents for the disruption of both telomerase-dependent and telomerase-independent telomere maintenance.

The Cockayne Syndrome group B gene product is involved in general genome base excision repair of 8-hydroxyguanine in DNA

Cockayne Syndrome (CS) is a human genetic disorder with two complementation groups, CS-A and CS-B. The CSB gene product is involved in transcription-coupled repair of DNA damage but may participate in other pathways of DNA metabolism. The present study investigated the role of different conserved helicase motifs of CSB in base excision repair. Stably transformed human cell lines with site-directed CSB mutations in different motifs within its putative helicase domain were established. We find that CSB null and helicase motif V and VI mutants had greater sensitivity than wild type cells to gamma-radiation. Whole cell extracts from CSB null and motif V/VI mutants had lower activity of 8-hydroxyguanine incision in DNA than wild type cells. Also, 8-hydroxyguanine accumulated more in CSB null and motif VI mutant cells than in wild type cells after exposure to gamma-radiation. We conclude that a deficiency in general genome base excision repair of selective modified DNA base(s) might contribute to CS pathogenesis. Furthermore, whereas the disruption of helicase motifs V or VI results in a CSB phenotype, mutations in other helicase motifs do not cause this effect. The biological functions of CSB in different DNA repair pathways may be mediated by distinct functional motifs of the protein.

Coordinate action of the helicase and 3' to 5' exonuclease of Werner syndrome protein

Werner syndrome is a human disorder characterized by premature aging, genomic instability, and abnormal telomere metabolism. The Werner syndrome protein (WRN) is the only known member of the RecQ DNA helicase family that contains a 3' --> 5'-exonuclease. However, it is not known whether both activities coordinate in a biological pathway. Here, we describe DNA structures, forked duplexes containing telomeric repeats, that are substrates for the simultaneous action of both WRN activities. We used these substrates to study the interactions between the WRN helicase and exonuclease on a single DNA molecule. WRN helicase unwinds at the forked end of the substrate, whereas the WRN exonuclease acts at the blunt end. Progression of the WRN exonuclease is inhibited by the action of WRN helicase converting duplex DNA to single strand DNA on forks of various duplex lengths. The WRN helicase and exonuclease act in concert to remove a DNA strand from a long forked duplex that is not completely unwound by the helicase. We analyzed the simultaneous action of WRN activities on the long forked duplex in the presence of the WRN protein partners, replication protein A (RPA), and the Ku70/80 heterodimer. RPA stimulated the WRN helicase, whereas Ku stimulated the WRN exonuclease. In the presence of both RPA and Ku, the WRN helicase activity dominated the exonuclease activity.

Werner syndrome protein interacts with human flap endonuclease 1 and stimulates its cleavage activity

Werner syndrome (WS) is a human premature aging disorder characterized by chromosomal instability. The cellular defects of WS presumably reflect compromised or aberrant function of a DNA metabolic pathway that under normal circumstances confers stability to the genome. We report a novel interaction of the WRN gene product with the human 5' flap endonuclease/5'-3' exonuclease (FEN-1), a DNA structure-specific nuclease implicated in DNA replication, recombination and repair. WS protein (WRN) dramatically stimulates the rate of FEN-1 cleavage of a 5' flap DNA substrate. The WRN-FEN-1 functional interaction is independent of WRN catalytic function and mediated by a 144 amino acid domain of WRN that shares homology with RecQ DNA helicases. A physical interaction between WRN and FEN-1 is demonstrated by their co-immunoprecipitation from HeLa cell lysate and affinity pull-down experiments using a recombinant C-terminal fragment of WRN. The underlying defect of WS is discussed in light of the evidence for the interaction between WRN and FEN-1.

Escherichia coli SecA helicase activity is not required in vivo for efficient protein translocation or autogenous regulation

SecA is an essential ATP-driven motor protein that binds to preproteins and the translocon to promote protein translocation across the eubacterial plasma membrane. Escherichia coli SecA contains seven conserved motifs characteristic of superfamily II of DNA and RNA helicases, and it has been shown previously to possess RNA helicase activity. SecA has also been shown to be an autogenous repressor that binds to its translation initiation region on secM-secA mRNA, thereby blocking and dissociating 30 S ribosomal subunits. Here we show that SecA is an ATP-dependent helicase that unwinds a mimic of the repressor helix of secM-secA mRNA. Mutational analysis of the seven conserved helicase motifs in SecA allowed us to identify mutants that uncouple SecA-dependent protein translocation activity from its helicase activity. Helicase-defective secA mutants displayed normal protein translocation activity and autogenous repression of secA in vivo. Our studies indicate that SecA helicase activity is nonessential and does not appear to be necessary for efficient protein secretion and secA autoregulation.

p53 Modulates the exonuclease activity of Werner syndrome protein

Werner syndrome (WS) is characterized by the early onset of symptoms of premature aging, cancer, and genomic instability. The molecular basis of the defects is not understood but presumably relates to the DNA helicase and exonuclease activities of the protein encoded by the WRN gene that is mutated in the disease. The attenuation of p53-mediated apoptosis in WS cells and reported physical interaction between WRN and the tumor suppressor p53 suggest that p53 and WRN functionally interact in a pathway necessary for the normal cellular response. In this study, we have demonstrated that p53 inhibits the exonuclease activity of the purified full-length recombinant WRN protein. p53 did not have an effect on a truncated amino-terminal WRN fragment that retains exonuclease activity but lacks the physical interaction domain for p53 located in the carboxyl terminus. Two naturally occurring p53 mutants found in human cancer displayed a reduced ability to inhibit WRN exonuclease activity. In cells arrested in S phase with hydroxyurea, WRN exits the nucleolus and colocalizes with p53 in the nucleoplasm. The regulation of WRN function by p53 is likely to play an important role in the maintenance of genomic integrity and prevention of cancer and other clinical symptoms associated with WS.

The Bloom's and Werner's syndrome proteins are DNA structure-specific helicases

BLM and WRN, the products of the Bloom's and Werner's syndrome genes, are members of the RecQ family of DNA helicases. Although both have been shown previously to unwind simple, partial duplex DNA substrates with 3'-->5' polarity, little is known about the structural features of DNA that determine the substrate specificities of these enzymes. We have compared the substrate specificities of the BLM and WRN proteins using a variety of partial duplex DNA molecules, which are based upon a common core nucleotide sequence. We show that neither BLM nor WRN is capable of unwinding duplex DNA from a blunt-ended terminus or from an internal nick. However, both enzymes efficiently unwind the same blunt-ended duplex containing a centrally located 12 nt single-stranded 'bubble', as well as a synthetic X-structure (a model for the Holliday junction recombination intermediate) in which each 'arm' of the 4-way junction is blunt-ended. Surprisingly, a 3'-tailed duplex, a standard substrate for 3'-->5' helicases, is unwound much less efficiently by BLM and WRN than are the bubble and X-structure substrates. These data show conclusively that a single-stranded 3'-tail is not a structural requirement for unwinding of standard B-form DNA by these helicases. BLM and WRN also both unwind a variety of different forms of G-quadruplex DNA, a structure that can form at guanine-rich sequences present at several genomic loci. Our data indicate that BLM and WRN are atypical helicases that are highly DNA structure specific and have similar substrate specificities. We interpret these data in the light of the genomic instability and hyper-recombination characteristics of cells from individuals with Bloom's or Werner's syndrome.

Unwinding of a DNA triple helix by the Werner and Bloom syndrome helicases

Bloom syndrome and Werner syndrome are genome instability disorders, which result from mutations in two different genes encoding helicases. Both enzymes are members of the RecQ family of helicases, have a 3' --> 5' polarity, and require a 3' single strand tail. In addition to their activity in unwinding duplex substrates, recent studies show that the two enzymes are able to unwind G2 and G4 tetraplexes, prompting speculation that failure to resolve these structures in Bloom syndrome and Werner syndrome cells may contribute to genome instability. The triple helix is another alternate DNA structure that can be formed by sequences that are widely distributed throughout the human genome. Here we show that purified Bloom and Werner helicases can unwind a DNA triple helix. The reactions are dependent on nucleoside triphosphate hydrolysis and require a free 3' tail attached to the third strand. The two enzymes unwound triplexes without requirement for a duplex extension that would form a fork at the junction of the tail and the triplex. In contrast, a duplex formed by the third strand and a complement to the triplex region was a poor substrate for both enzymes. However, the same duplex was readily unwound when a noncomplementary 5' tail was added to form a forked structure. It seems likely that structural features of the triplex mimic those of a fork and thus support efficient unwinding by the two helicases.

DNA repair and mutagenesis in Werner syndrome

Werner syndrome (WS) is the hallmark premature aging syndrome in which the patients appear much older than their actual chronological age. The disorder is associated with significantly increased genome instability and with transcriptional deficiencies. There has been some uncertainty about whether WS cells are defective in DNA repair. We thus examined repair in vitro in nuclear and mitochondrial DNA. Whereas cellular studies so far do not show significant DNA repair deficiencies, biochemical studies with the Werner protein clearly indicate that it plays a role in DNA repair.

Molecular characterization of an acidic region deletion mutant of Cockayne syndrome group B protein

Cockayne syndrome (CS) is a human genetic disorder characterized by post-natal growth failure, neurological abnormalities and premature aging. CS cells exhibit high sensitivity to UV light, delayed RNA synthesis recovery after UV irradiation and defective transcription-coupled repair (TCR). Two genetic complementation groups of CS have been identified, designated CS-A and CS-B. The CSB gene encodes a helicase domain and a highly acidic region N-terminal to the helicase domain. This study describes the genetic characterization of a CSB mutant allele encoding a full deletion of the acidic region. We have tested its ability to complement the sensitivity of UV61, the hamster homolog of human CS-B cells, to UV and the genotoxic agent N-acetoxy-2-acetylaminofluorene (NA-AAF). Deleting 39 consecutive amino acids, of which approximately 60% are negatively charged, did not impact on the ability of the protein to complement the sensitive phenotype of UV61 cells to either UV or NA-AAF. Our data indicate that the highly acidic region of CSB is not essential for the TCR and general genome repair pathways of UV- and NA-AAF-induced DNA lesions.

Replication protein A physically interacts with the Bloom's syndrome protein and stimulates its helicase activity

Bloom's syndrome is a rare autosomal recessive disorder characterized by genomic instability and predisposition to cancer. BLM, the gene defective in Bloom's syndrome, encodes a 159-kDa protein possessing DNA-stimulated ATPase and ATP-dependent DNA helicase activities. We have examined mechanistic aspects of the catalytic functions of purified recombinant BLM protein. Through analyzing the effects of different lengths of DNA cofactor on ATPase activity, we provide evidence to suggest that BLM translocates along single-stranded DNA in a processive manner. The helicase reaction catalyzed by BLM protein was examined as a function of duplex DNA length. We show that BLM catalyzes unwinding of short DNA duplexes (</=71 base pairs (bp)) but is severely compromised on longer DNA duplexes (>/=259-bp). The presence of the human single-stranded DNA-binding protein (human replication protein A (hRPA)) stimulates the BLM unwinding reaction on the 259-bp partial duplex DNA substrate. Heterologous single-stranded DNA-binding proteins fail to stimulate similarly the helicase activity of BLM protein. This is the first demonstration of a functional interaction between BLM and another protein. Consistent with a functional interaction between hRPA and the BLM helicase, we demonstrate a direct physical interaction between the two proteins mediated by the 70-kDa subunit of RPA. The interactions between BLM and hRPA suggest that the two proteins function together in vivo to unwind DNA duplexes during replication, recombination, or repair.

Werner's syndrome protein (WRN) migrates Holliday junctions and co-localizes with RPA upon replication arrest

Individuals affected by the autosomal recessive disorder Werner's syndrome (WS) develop many of the symptoms characteristic of premature ageing. Primary fibroblasts cultured from WS patients exhibit karyotypic abnormalities and a reduced replicative life span. The WRN gene encodes a 3'-5' DNA helicase, and is a member of the RecQ family, which also includes the product of the Bloom's syndrome gene (BLM). In this work, we show that WRN promotes the ATP-dependent translocation of Holliday junctions, an activity that is also exhibited by BLM. In cells arrested in S-phase with hydroxyurea, WRN localizes to discrete nuclear foci that coincide with those formed by the single-stranded DNA binding protein replication protein A. These results are consistent with a model in which WRN prevents aberrant recombination events at sites of stalled replication forks by dissociating recombination intermediates.

Potent inhibition of werner and bloom helicases by DNA minor groove binding drugs

Maintenance of genomic integrity is vital to all organisms. A number of human genetic disorders, including Werner Syndrome, Bloom Syndrome and Rothmund-Thomson Syndrome, exhibit genomic instability with some phenotypic characteristics of premature aging and cancer predisposition. Presumably the aberrant cellular and clinical phenotypes in these disorders arise from defects in important DNA metabolic pathways such as replication, recombination or repair. These syndromes are all characterized by defects in a member of the RecQ family of DNA helicases. To obtain a better understanding of how these enzymes function in DNA metabolic pathways that directly influence chromosomal integrity, we have examined the effects of non-covalent DNA modifications on the catalytic activities of purified Werner (WRN) and Bloom (BLM) DNA helicases. A panel of DNA-binding ligands displaying unique properties for interacting with double helical DNA was tested for their effects on the unwinding activity of WRN and BLM helicases on a partial duplex DNA substrate. The levels of inhibition by a number of these compounds were distinct from previously reported values for viral, prokaryotic and eukaryotic helicases. The results demonstrate that BLM and WRN proteins exhibit similar sensitivity profiles to these DNA-binding ligands and are most potently inhibited by the structurally related minor groove binders distamycin A and netropsin (K(i) Collapse

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MESH Headings

• Adenosine Triphosphatases/antagonists & inhibitors
• Adenosine Triphosphatases/chemistry
• Bloom Syndrome/enzymology
• DNA Helicases/antagonists & inhibitors
• DNA Helicases/chemistry
• Distamycins/pharmacology
• Enzyme Inhibitors/chemistry
• Enzyme Inhibitors/pharmacology
• Exodeoxyribonucleases
• Humans
• Intercalating Agents/chemistry
• Intercalating Agents/pharmacology
• Kinetics
• Ligands
• Netropsin/pharmacology
• RecQ Helicases
• Recombinant Proteins/antagonists & inhibitors
• Topoisomerase I Inhibitors
• Werner Syndrome/enzymology
• Werner Syndrome Helicase

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Grants Collapse

Ku complex interacts with and stimulates the Werner protein

Werner syndrome (WS) is the hallmark premature aging disorder in which affected humans appear older than their chronological age. The protein WRNp, defective in WS, has helicase function, DNA-dependent ATPase, and exonuclease activity. Although WRNp functions in nucleic acid metabolism, there is little or no information about the pathways or protein interactions in which it participates. Here we identify Ku70 and Ku86 as proteins that interact with WRNp. Although Ku proteins had no effect on ATPase or helicase activity, they strongly stimulated specific exonuclease activity. These results suggest that WRNp and the Ku complex participate in a common DNA metabolic pathway.

Identification of phage antibodies toward the Werner protein by selection on Western blots

A procedure was established for selecting phage antibodies (phage-abs) from phage-displayed antibody repertoires by panning against proteins, separated by sodium dodecyl phosphate-polyacrylamide gel electrophoresis (SDS-PAGE) and electroblotted onto nitrocellulose membranes (Western blots). This immobilization strategy is applicable for secondary rounds of panning in selections against semipurified proteins, and directs the selection toward antibodies suitable as immunochemical reagents in Western blots. In model experiments, enrichment factors as high as 1.9x10(5) were obtained in a single round of panning. Furthermore, we demonstrate the application of this approach by selection of phage-abs recognizing the human Werner protein, which is defective in a premature aging syndrome.

Role of the ATPase domain of the Cockayne syndrome group B protein in UV induced apoptosis

Cockayne syndrome (CS) is a human autosomal recessive disorder characterized by many neurological and developmental abnormalities. CS cells are defective in the transcription coupled repair (TCR) pathway that removes DNA damage from the transcribed strand of active genes. The individuals suffering from CS do not generally develop cancer but show increased neurodegeneration. Two genetic complementation groups (CS-A and CS-B) have been identified. The lack of cancer formation in CS may be due to selective elimination of cells containing DNA damage by a suicidal pathway. In this study, we have evaluated the role of the CSB gene in UV induced apoptosis in human and hamster cells. The hamster cell line UV61 carries a mutation in the homolog of the human CSB gene. We show that both human CS-B and hamster UV61 cells display increased apoptotic response following UV exposure compared with normal cells. The increased sensitivity of UV61 cells to apoptosis is complemented by the transfection of the wild type human CSB gene. In order to determine which functional domain of the CSB gene participates in the apoptotic pathway, we constructed stable cell lines with different CSB domain disruptions. UV61 cells were stably transfected with the human CSB cDNA containing a point mutation in the highly conserved glutamic acid residue in ATPase motif II. This cell line (UV61/ pc3.1-CSBE646Q) showed the same increased apoptosis as the UV61 cells. In contrast, cells containing a deletion in the acidic domain at the N-terminal end of the CSB protein had no effect on apoptosis. This indicates that the integrity of the ATPase domain of CSB protein is critical for preventing the UV induced apoptotic pathway. In primary human CS-B cells, the induction and stabilization of the p53 protein seems to correlate with their increased apoptotic potential. In contrast, no change in the level of either p53 or activation of mdm2 protein by p53 was observed in hamster UV61 cells after UV exposure. This suggests that the CSB dependent apoptotic pathway can occur independently of the transactivation potential of p53 in hamster cells.

The ATPase domain but not the acidic region of Cockayne syndrome group B gene product is essential for DNA repair

Cockayne syndrome (CS) is a human genetic disorder characterized by UV sensitivity, developmental abnormalities, and premature aging. Two of the genes involved, CSA and CSB, are required for transcription-coupled repair (TCR), a subpathway of nucleotide excision repair that removes certain lesions rapidly and efficiently from the transcribed strand of active genes. CS proteins have also been implicated in the recovery of transcription after certain types of DNA damage such as those lesions induced by UV light. In this study, site-directed mutations have been introduced to the human CSB gene to investigate the functional significance of the conserved ATPase domain and of a highly acidic region of the protein. The CSB mutant alleles were tested for genetic complementation of UV-sensitive phenotypes in the human CS-B homologue of hamster UV61. In addition, the CSB mutant alleles were tested for their ability to complement the sensitivity of UV61 cells to the carcinogen 4-nitroquinoline-1-oxide (4-NQO), which introduces bulky DNA adducts repaired by global genome repair. Point mutation of a highly conserved glutamic acid residue in ATPase motif II abolished the ability of CSB protein to complement the UV-sensitive phenotypes of survival, RNA synthesis recovery, and gene-specific repair. These data indicate that the integrity of the ATPase domain is critical for CSB function in vivo. Likewise, the CSB ATPase point mutant failed to confer cellular resistance to 4-NQO, suggesting that ATP hydrolysis is required for CSB function in a TCR-independent pathway. On the contrary, a large deletion of the acidic region of CSB protein did not impair the genetic function in the processing of either UV- or 4-NQO-induced DNA damage. Thus the acidic region of CSB is likely to be dispensable for DNA repair, whereas the ATPase domain is essential for CSB function in both TCR-dependent and -independent pathways.

Enzymatic and DNA binding properties of purified WRN protein: high affinity binding to single-stranded DNA but not to DNA damage induced by 4NQO

Mutations in the WRN gene result in Werner syndrome, an autosomal recessive disease in which many characteristics of aging are accelerated. A probable role in some aspect of DNA metabolism is suggested by the primary sequence of the WRN gene product. A recombinant His-tagged WRN protein (WRNp) was overproduced in insect cells using the baculovirus system and purified to near homogeneity by several chromatographic steps. This purification scheme removes both nuclease and topoisomerase contaminants that persist following a single Ni(2+)affinity chromatography step and allows for unambiguous interpretation of WRNp enzymatic activities on DNA substrates. Purified WRNp has DNA-dependent ATPase and helicase activities consistent with its homology to the RecQ subfamily of proteins. The protein also binds with higher affinity to single-stranded DNA than to double-stranded DNA. However, WRNp has no higher affinity for various types of DNA damage, including adducts formed during 4NQO treatment, than for undamaged DNA. Our results confirm that WRNp has a role in DNA metabolism, although this role does not appear to be the specific recognition of damage in DNA.

Functional and physical interaction between WRN helicase and human replication protein A

The human premature aging disorder Werner syndrome (WS) is associated with a large number of symptoms displayed in normal aging. The WRN gene product, a DNA helicase, has been previously shown to unwind short DNA duplexes (</=53 base pairs) in a reaction stimulated by single-stranded DNA-binding proteins. We have studied the helicase activity of purified WRN protein on a variety of DNA duplex substrates to characterize the unwinding properties of the enzyme in greater detail. WRN helicase can catalyze unwinding of long duplex DNA substrates up to 849 base pairs in a reaction dependent on human replication protein A (hRPA). Escherichia coli SSB and bacteriophage T4 gene 32 protein (gp32) completely failed to stimulate WRN helicase to unwind long DNA duplexes indicating a specific functional interaction between WRN and hRPA. So far, there have been no reports of any physical interactions between WRN helicase and other proteins. In support of the functional interaction, we demonstrate a direct interaction between WRN and hRPA by coimmunoprecipitation of purified proteins. The physical and functional interaction between WRN and hRPA suggests that the two proteins may function together in vivo in a pathway of DNA metabolism such as replication, recombination, or repair.

A point mutation in Escherichia coli DNA helicase II renders the enzyme nonfunctional in two DNA repair pathways. Evidence for initiation of unwinding from a nick in vivo

Biosynthetic errors and DNA damage introduce mismatches and lesions in DNA that can lead to mutations. These abnormalities are susceptible to correction by a number of DNA repair mechanisms, each of which requires a distinct set of proteins. Escherichia coli DNA helicase II has been demonstrated to function in two DNA repair pathways, methyl-directed mismatch repair and UvrABC-mediated nucleotide excision repair. To define further the role of UvrD in DNA repair a site-specific mutant was characterized. The mutation, uvrDQ251E, resides within helicase motif III, a conserved segment of amino acid homology found in a superfamily of prokaryotic and eukaryotic DNA helicases. The UvrD-Q251E protein failed to complement the mutator and ultraviolet light-sensitive phenotypes of a uvrD deletion strain indicating that the mutant protein is inactive in both mismatch repair and excision repair. Biochemical characterization revealed a significant defect in the ability of the mutant enzyme to initiate unwinding at a nick. The elongation phase of the unwinding reaction was nearly normal. Together, the biochemical and genetic data provide evidence that UvrD-Q251E is dysfunctional because the mutant protein fails to initiate unwinding at the nick(s) used to initiate excision and subsequent repair synthesis. These results provide direct evidence to support the notion that helicase II initiates unwinding from a nick in vivo in mismatch repair and excision repair.

A partially functional DNA helicase II mutant defective in forming stable binary complexes with ATP and DNA. A role for helicase motif III

To address the functional significance of motif III in Escherichia coli DNA helicase II, the conserved aspartic acid at position 248 was changed to asparagine. UvrDD248N failed to form stable binary complexes with either DNA or ATP. However, UvrDD248N was capable of forming an active ternary complex when both ATP and single-stranded DNA were present. The DNA-stimulated ATPase activity of UvrDD248N was reduced relative to that of wild-type UvrD with no significant change in the apparent Km for ATP. The mutant protein also demonstrated a reduced DNA unwinding activity. The requirement for high concentrations of UvrDD248N to achieve unwinding of long duplex substrates likely reflects the reduced stability of various binary and ternary complexes that must exist in the catalytic cycle of a helicase. The data suggest that motif III may act as an interface between the ATP binding and DNA binding domains of a helicase. The uvrDD248N allele was also characterized in genetic assays. The D248N protein complemented the UV-sensitive phenotype of a uvrD deletion strain to levels nearly equivalent to wild-type helicase II. In contrast, the mutant protein only partially complemented the mutator phenotype. A correlation between the level of genetic complementation and the helicase activity of UvrDD248N is discussed.

Mutations in motif II of Escherichia coli DNA helicase II render the enzyme nonfunctional in both mismatch repair and excision repair with differential effects on the unwinding reaction

Site-directed mutagenesis has been employed to address the functional significance of the highly conserved aspartic and glutamic acid residues present in the Walker B (also called motif II) sequence in Escherichia coli DNA helicase II. Two mutant proteins, UvrDE221Q and UvrDD220NE221Q, were expressed and purified to apparent homogeneity. Biochemical characterization of the DNA-dependent ATPase activity of each mutant protein demonstrated a kcat that was < 0.5% of that of the wild-type protein, with no significant change in the apparent Km for ATP. The E221Q mutant protein exhibited no detectable unwinding of either partial duplex or blunt duplex DNA substrates. The D220NE221Q mutant, however, catalyzed unwinding of both partial duplex and blunt duplex substrates, but at a greatly reduced rate compared with that of the wild-type enzyme. Both mutants were able to bind DNA. Thus, the motif II mutants E221Q and D220NE221Q were able to bind ATP and DNA to the same extent as wild-type helicase II but demonstrate a significant reduction in ATP hydrolysis and helicase functions. The mutant uvrD alleles were also characterized by examining their abilities to complement the mutator and UV light-sensitive phenotypes of a uvrD deletion mutant. Neither the uvrDE221Q nor the uvrDD220NE221Q allele, supplied on a plasmid, was able to complement either phenotype. Further genetic characterization of the mutant uvrD alleles demonstrated that uvrDE221Q confers a dominant negative growth phenotype; the uvrDD220NE221Q allele does not exhibit this effect. The observed difference in effect on viability may reflect the gene products' dissimilar kinetics for unwinding duplex DNA substrates in vitro.

A dominant negative allele of the Escherichia coli uvrD gene encoding DNA helicase II. A biochemical and genetic characterization

A site-specific lysine to methionine mutation has been engineered at the invariant Lys35 residue in the ATPase A binding site of the Escherichia coli uvrD gene encoding DNA helicase II. The mutant protein (UvrDK35M) has been purified to apparent homogeneity and characterized. The kcat for DNA-dependent ATP hydrolysis was less than 0.5% that of the wild-type enzyme with no change in the apparent Km for ATP. No unwinding of partial duplex DNA substrates could be detected using the mutant protein. Moreover, the mutant protein inhibited the unwinding reaction catalyzed by the wild-type protein at ratios of mutant enzyme to wild-type enzyme < 1. We conclude that the K35M mutation renders helicase II catalytically inactive as a DNA helicase with little or no effect on the ability of the enzyme to bind ATP, DNA, or other proteins. In vivo complementation assays indicate that the mutant protein cannot substitute for the wild-type protein in methyl-directed mismatch repair, suggesting that the ATPase and/or helicase activity of helicase II is required in this repair pathway. Additional genetic characterization of the uvrDK35M allele, supplied on a plasmid, suggests that expression of the mutant protein, at levels equivalent to that of the wild-type protein, results in a dominant negative phenotype. Expression of lower levels of the mutant protein, both in the presence and absence of wild-type helicase II, results in a constitutive induction of the cellular SOS response and extensive filamentation of cells. This induction of the SOS response is not due to a defect in methyl-directed mismatch repair. Taken together, these data are consistent with the notion that E. coli helicase II may have a role in DNA replication.