An N-terminal fragment of the gene 4 helicase/primase of bacteriophage T7 retains primase activity in the absence of helicase activity

Proc Natl Acad Sci U S A. 1998 Jul 7;95(14):7957-62. doi: 10.1073/pnas.95.14.7957.


Primase and helicase activities of bacteriophage T7 are present in a single polypeptide coded by gene 4. Because the amino terminal region of the gene 4 protein contributes to primase activity, we constructed a truncated gene 4 encoding the N-terminal 271-aa residues. The truncated protein, purified from cells overexpressing the protein, is a dimer in solution; the full-length protein is a hexamer. Although the fragment is devoid of dTTPase and helicase activities, it catalyzes template-directed synthesis of di-, tri-, and tetranucleotides. The rates for tetraribonucleotide synthesis and for dinucleotide extension on a 20-nucleotide template are similar for the full-length and truncated proteins. However, the activity of the primase fragment is unaffected by dTTP whereas the primase activity of the full-length protein is stimulated >14-fold. The primase fragment is defective in the interaction with T7 DNA polymerase in that primer synthesis cannot be coupled to DNA synthesis.

Publication types

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

MeSH terms

  • Bacteriophage T7 / enzymology*
  • Bacteriophage T7 / genetics*
  • DNA Helicases / genetics*
  • DNA Helicases / metabolism
  • DNA Primase / genetics*
  • DNA Primase / metabolism
  • Enzyme Activation
  • Gene Expression Regulation, Enzymologic*
  • Gene Expression Regulation, Viral*
  • Genes, Viral
  • Viral Proteins / genetics


  • Viral Proteins
  • DNA Primase
  • DNA Helicases