Deoxyribonucleic acid polymerase of bacteriophage T7. Characterization of the exonuclease activities of the gene 5 protein and the reconstituted polymerase

J Biol Chem. 1979 Nov 25;254(22):11598-604.


Homogeneous gene 5 protein of bacteriophage T7, a subunit of T7 DNA polymerase, catalyzes the stepwise hydrolysis of single-stranded DNA in a 3' leads to 5' direction to yield nucleoside 5'-monophosphates. The gene 5 protein itself does not hydrolyze duplex DNA. However, in the presence of Escherichia coli thioredoxin, the host-specified subunit of T7 DNA polymerase, duplex DNA is hydrolyzed in a 3' leads to 5' direction to yield nucleoside 5'-monophosphates. The apparent Km for thioredoxin in the reaction is 4.8 x 10(-8) M, a value similar to that for the apparent Km of thioredoxin in the complementation assay with gene 5 protein to restore T7 DNA polymerase activity. Both exonuclease activities require Mg2+ and a sulfhydryl reagent for optimal activity, and both activities are sensitive to salt concentration. Deoxyribonucleoside 5'-triphosphates inhibit hydrolysis by both exonuclease activities; hydrolysis of single-stranded DNA by the gene 5 protein is inhibited even in the absence of thioredoxin where there is less than 2% active T7 DNA polymerase. E. coli DNA binding protein (helix destabilizing protein) stimulates the hydrolysis of duplex DNA up to 9-fold under conditions where the hydrolysis of the single-stranded DNA is inhibited 4-fold.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • DNA Helicases / metabolism
  • DNA, Single-Stranded
  • DNA-Directed DNA Polymerase / metabolism*
  • Escherichia coli / enzymology*
  • Exonucleases / metabolism*
  • Genes, Viral*
  • Kinetics
  • T-Phages / enzymology*
  • Thioredoxins / pharmacology
  • Viral Proteins / isolation & purification
  • Viral Proteins / metabolism*


  • DNA, Single-Stranded
  • Viral Proteins
  • Thioredoxins
  • DNA-Directed DNA Polymerase
  • Exonucleases
  • DNA Helicases