The primase active site is on the outside of the hexameric bacteriophage T7 gene 4 helicase-primase ring

Margaret S. VanLoock, Yen Ju Chen, Xiong Yu, Smita S. Patel, Edward H. Egelman

Research output: Contribution to journalArticle

28 Scopus citations


Gene 4 of bacteriophage T7 encodes a protein (gp4) that can translocate along single-stranded DNA, couple the unwinding of duplex DNA with the hydrolysis of dTTP, and catalyze the synthesis of short RNA oligo-ribonucleotides for use as primers by T7 DNA polymerase. Electron microscopic studies have shown that gp4 forms hexameric rings, and X-ray crystal structures of the gp4 helicase domain and of the highly homologous RNA polymerase domain of Escherichia coli DnaG have been determined. Earlier biochemical studies have shown that when single-stranded DNA is bound to the hexameric ring, the primase domain remains accessible to free DNA. Given these results, a model was suggested in which the primase active site in the gp4 hexamer is located on the outside of the hexameric ring. We have used electron microscopy and single-particle image analysis to examine T7 gp4, and have determined that the primase active site is located on the outside of the hexameric ring, and therefore provide direct structural support for this model.

Original languageEnglish (US)
Pages (from-to)951-956
Number of pages6
JournalJournal of molecular biology
Issue number5
StatePublished - Aug 31 2001


All Science Journal Classification (ASJC) codes

  • Structural Biology
  • Molecular Biology


  • Electron microscopy
  • Image analysis
  • Molecular modeling
  • Protein-DNA complexes
  • Ring proteins

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