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J. Biol. Chem., Vol. 268, Issue 27, 20046-20054, 09, 1993

DNA binding properties of the deoxyguanosine triphosphate triphosphohydrolase of Escherichia coli

SM Wurgler and CC Richardson
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115.

The dgt gene of Escherichia coli encodes a deoxyguanosine triphosphate triphosphohydrolase (dGTPase) that hydrolyzes dGTP to deoxyguanosine and tripolyphosphate. The enzyme is highly specific for dGTP which is hydrolyzed with a Km of 2-5 microM. Nitrocellulose filter binding assays demonstrate that, under physiological salt conditions, dGTPase binds with apparent cooperativity to single-stranded DNA with an association constant of 7.7 x 10(6) M-1. In the presence of NaCl, dGTPase binds weakly to double-stranded DNA. In the absence of NaCl, dGTPase binds both single- and double-stranded DNA with an association constant of 1 x 10(7) M-1. The dGTPase-double-stranded DNA complex, however, is readily dissociated with NaCl. Divalent cations such as Mg2+ or Mn2+ enhance, but are not required for DNA binding. The presence of dGTP or GTP does not effect the ability of dGTPase to bind DNA. dGTPase binds to oligonucleotides of length 17-35, but with lower affinities. The homopolymers poly(dT) and poly(rU) act as effective competitors of single-stranded DNA for binding to dGTPase. The bacteriophage T7 gene 1.2 protein, which specifically inhibits the enzymatic activity of dGTPase, also prevents dGTPase from binding to single-stranded DNA. dGTPase inhibits the activity of T7 DNA polymerase on a poly(dA)-oligo(dT) template. This inhibition is reversed by prior incubation of dGTPase with the T7 gene 1.2 protein.
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