Impaired translesion synthesis in xeroderma pigmentosum variant extracts.
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Xeroderma pigmentosum variant (XPV) cells are characterized by a cellular defect in the ability to synthesize intact daughter DNA strands on damaged templates. Molecular mechanisms that facilitate replication fork progression on damaged DNA in normal cells are not well defined. In this study, we used single-stranded plasmid molecules containing a single N-2-acetylaminofluorene (AAF) adduct to analyze translesion synthesis (TLS) catalyzed by extracts of either normal or XPV primary skin fibroblasts. In one of the substrates, the single AAF adduct was located at the 3' end of a run of three guanines that was previously shown to induce deletion of one G by a slippage mechanism. Primer extension reactions performed by normal cellular extracts from four different individuals produced the same distinct pattern of TLS, with over 80% of the products resulting from the elongation of a slipped intermediate and the remaining 20% resulting from a nonslipped intermediate. In contrast, with cellular extracts from five different XPV patients, the TLS reaction was strongly reduced, yielding only low amounts of TLS via the nonslipped intermediate. With our second substrate, in which the AAF adduct was located at the first G in the run, thus preventing slippage from occurring, we confirmed that normal extracts were able to perform TLS 10-fold more efficiently than XPV extracts. These data demonstrate unequivocally that the defect in XPV cells resides in translesion synthesis independently of the slippage process. (+info)
The DNA binding activity of Translin is mediated by a basic region in the ring-shaped structure conserved in evolution.
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DNA binding proteins, for the most part, function as dimers or tetramers which recognize their target sequences. Here we show that Translin, a novel single-stranded DNA end binding protein, forms a ring-shaped structure conserved throughout evolution and that this structure is responsible for its DNA binding activity. Point mutations at Leu184 and Leu191 in the leucine zipper motif of human Translin resulted in loss of the multimeric structure and abrogation of DNA binding. Point mutations at R86, H88, H90 to T86, N88, N90 in one of the basic regions, however, completely inhibited the DNA binding activity without affecting the multimeric structure. These results support the view that the DNA binding domain of Translin is formed in the ring-shaped structure in combination with its basic region (amino acids 86-97) polypeptides. (+info)
Mutations and allelic deletions of the MEN1 gene are associated with a subset of sporadic endocrine pancreatic and neuroendocrine tumors and not restricted to foregut neoplasms.
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Endocrine pancreatic tumors (EPT) and neuroendocrine tumors (NET) occur sporadically and rarely in association with multiple endocrine neoplasia type 1 (MEN1). We analyzed the frequency of allelic deletions and mutations of the recently identified MEN1 gene in 53 sporadic tumors including 30 EPT and 23 NET (carcinoids) of different locations and types. Allelic deletion of the MEN1 locus was identified in 18/49 (36.7%) tumors (13/30, 43.3% in EPT and 5/19, 26.3% in NET) and mutations of the MEN1 gene were present in 8/52 (15.3%) tumors (4/30 (13.3%) EPT and 4/22 (18.1%) NET). The somatic mutations were clustered in the 5' region of the coding sequence and most frequently encompassed missense mutations. All tumors with mutations exhibited a loss of the other allele and a wild-type sequence of the MEN1 gene in nontumorous DNA. In one additional patient with a NET of the lung and no clinical signs or history of MEN1, a 5178-9G-->A splice donor site mutation in intron 4 was identified in both the tumor and blood DNA, indicating the presence of a thus far unknown MEN1 syndrome. In most tumor groups the frequency of allelic deletions at 11q13 was 2 to 3 times higher than the frequency of identified MEN1 gene mutations. Some tumor types, including rare forms of EPT and NET of the duodenum and small intestine, exhibited mutations more frequently than other types. Furthermore, somatic mutations were not restricted to foregut tumors but were also detectable in a midgut tumor (15.2% versus 16.6%). Our data indicate that somatic MEN1 gene mutations contribute to a subset of sporadic EPT and NET, including midgut tumors. Because the frequency of mutations varies significantly among the investigated tumor subgroups and allelic deletions are 2 to 3 times more frequently observed, factors other than MEN1 gene inactivation, including other tumor-suppressor genes on 11q13, may also be involved in the tumorigenesis of these neoplasms. (+info)
An allosteric synthetic DNA.
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Allosteric DNA oligonucleotides are potentially useful diagnostic reagents. Here we develop a model system for the study of allosteric interactions in DNAs. A DNA that binds either Cibacron blue or cholic acid was isolated and partially characterized. Isolation was performed using a multi-stage SELEX. First, short oligos that bind either Cibacron blue or cholic acid were enriched from random oligonucleotide pools. Then, members of the two pools were fused to form longer oligos, which were then selected for theability to bind Cibacron blue columns and elute with cholic acid. One resulting isolate (A22) was studied. Dye- and cholate-binding functions can be separated on sequences from the 5'- and 3'-regions, respectively. Ligand-column affinity assays indicate that each domain binds only its respective ligand. However, the full-length A22 will bind either dye or cholate columns and elute with the other ligand, as if binding by the ligands is mutually exclusive. Furthermore, S1 nuclease protection assays show that Cibacron blue causes a structural change in A22 and that cholic acid inhibits this change. This system will be useful for elucidating mechanisms of allosteric interactions in synthetic DNAs. (+info)
The binding affinity of Ff gene 5 protein depends on the nearest-neighbor composition of the ssDNA substrate.
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The Ff gene 5 protein (g5p) is considered to be a nonspecific single-stranded DNA binding protein, because it binds cooperatively to and saturates the Ff bacteriophage single-stranded DNA genome and other single-stranded polynucleotides. However, the binding affinity Komega (the intrinsic binding constant times a cooperativity factor) differs by over an order of magnitude for binding to single-stranded polynucleotides such as poly[d(A)] and poly[d(C)]. A polynucleotide that is more stacked, like poly[d(A)], binds more weakly than one that is less stacked, like poly[d(C)]. To test the hypothesis that DNA base stacking, a nearest-neighbor property, is involved in the binding affinity of the Ff g5p for different DNA sequences, Komega values were determined as a function of NaCl concentration for binding to six synthetic sequences 48 nucleotides in length: dA48, dC48, d(AAC)16, d(ACC)16, d(AACC)12, and d(AAACC)9A3. The binding affinities of the protein for these sequences were indeed found to be related to the nearest-neighbor compositions of the sequences, rather than to simple base compositions. That is, the g5p binding site, which is spanned by four nucleotides, discriminates among these sequences on the basis of the relative numbers of nearest neighbors (AA, CC, and AC plus CA) in the sequence. The results support the hypothesis that the extent of base stacking/unstacking of the free, nonbound ssDNA plays an important role in the binding affinity of the Ff gene 5 protein. (+info)
Identification of RNase T as a high-copy suppressor of the UV sensitivity associated with single-strand DNA exonuclease deficiency in Escherichia coli.
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There are three known single-strand DNA-specific exonucleases in Escherichia coli: RecJ, exonuclease I (ExoI), and exonuclease VII (ExoVII). E. coli that are deficient in all three exonucleases are abnormally sensitive to UV irradiation, most likely because of their inability to repair lesions that block replication. We have performed an iterative screen to uncover genes capable of ameliorating the UV repair defect of xonA (ExoI-) xseA (ExoVII-) recJ triple mutants. In this screen, exonuclease-deficient cells were transformed with a high-copy E. coli genomic library and then irradiated; plasmids harvested from surviving cells were used to seed subsequent rounds of transformation and selection. After several rounds of selection, multiple plasmids containing the rnt gene, which encodes RNase T, were found. An rnt plasmid increased the UV resistance of a xonA xseA recJ mutant and uvrA and uvrC mutants; however, it did not alter the survival of xseA recJ or recA mutants. RNase T also has amino acid sequence similarity to other 3' DNA exonucleases, including ExoI. These results suggest that RNase T may possess a 3' DNase activity capable of substituting for ExoI in the recombinational repair of UV-induced lesions. (+info)
Nuclear foci of mammalian recombination proteins are located at single-stranded DNA regions formed after DNA damage.
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A sensitive and rapid in situ method was developed to visualize sites of single-stranded (ss) DNA in cultured cells and in experimental test animals. Anti-bromodeoxyuridine antibody recognizes the halogenated base analog incorporated into chromosomal DNA only when substituted DNA is in the single strand form. After treatment of cells with DNA-damaging agents or gamma irradiation, ssDNA molecules form nuclear foci in a dose-dependent manner within 60 min. The mammalian recombination protein Rad51 and the replication protein A then accumulate at sites of ssDNA and form foci, suggesting that these are sites of recombinational DNA repair. (+info)
Structure of DNA-dependent protein kinase: implications for its regulation by DNA.
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DNA double-strand breaks are created by ionizing radiation or during V(D)J recombination, the process that generates immunological diversity. Breaks are repaired by an end-joining reaction that requires DNA-PKCS, the catalytic subunit of DNA-dependent protein kinase. DNA-PKCS is a 460 kDa serine-threonine kinase that is activated by direct interaction with DNA. Here we report its structure at 22 A resolution, as determined by electron crystallography. The structure contains an open channel, similar to those seen in other double-stranded DNA-binding proteins, and an enclosed cavity with three openings large enough to accommodate single-stranded DNA, with one opening adjacent to the open channel. Based on these structural features, we performed biochemical experiments to examine the interactions of DNA-PKCS with different DNA molecules. Efficient kinase activation required DNA longer than 12 bp, the minimal length of the open channel. Competition experiments demonstrated that DNA-PKCS binds to double- and single-stranded DNA via separate but interacting sites. Addition of unpaired single strands to a double-stranded DNA fragment stimulated kinase activation. These results suggest that activation of the kinase involves interactions with both double- and single-stranded DNA, as suggested by the structure. A model for how the kinase is regulated by DNA is described. (+info)