55.2, a phage T4 ORFan gene, encodes an inhibitor of Escherichia coli topoisomerase I and increases phage fitness

PLoS One. 2015 Apr 14;10(4):e0124309. doi: 10.1371/journal.pone.0124309. eCollection 2015.


Topoisomerases are enzymes that alter the topological properties of DNA. Phage T4 encodes its own topoisomerase but it can also utilize host-encoded topoisomerases. Here we characterized 55.2, a phage T4 predicted ORF of unknown function. High levels of expression of the cloned 55.2 gene are toxic in E. coli. This toxicity is suppressed either by increased topoisomerase I expression or by partial inactivation of the ATPase subunit of the DNA gyrase. Interestingly, very low-level expression of 55.2, which is non-lethal to wild type E. coli, prevents the growth of a deletion mutant of the topoisomerase I (topA) gene. In vitro, gp55.2 binds DNA and blocks specifically the relaxation of negatively supercoiled DNA by topoisomerase I. In vivo, expression of gp55.2 at low non-toxic levels alters the steady state DNA supercoiling of a reporter plasmid. Although 55.2 is not an essential gene, competition experiments indicate that it is required for optimal phage growth. We propose that the role of gp55.2 is to subtly modulate host topoisomerase I activity during infection to insure optimal T4 phage yield.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacteriophage T4 / genetics*
  • Bacteriophage T4 / growth & development
  • Bacteriophage T4 / physiology
  • Cloning, Molecular
  • DNA Topoisomerases, Type I / chemistry
  • DNA Topoisomerases, Type I / genetics
  • DNA Topoisomerases, Type I / metabolism*
  • DNA, Superhelical / chemistry
  • DNA, Superhelical / metabolism
  • Electrophoretic Mobility Shift Assay
  • Escherichia coli / enzymology*
  • Escherichia coli / growth & development
  • Escherichia coli / virology
  • Open Reading Frames / genetics*
  • Plasmids / genetics
  • Plasmids / metabolism
  • Protein Binding
  • Recombinant Fusion Proteins / biosynthesis
  • Recombinant Fusion Proteins / genetics
  • Topoisomerase I Inhibitors / chemistry
  • Topoisomerase I Inhibitors / metabolism
  • Viral Proteins / chemistry
  • Viral Proteins / genetics
  • Viral Proteins / metabolism


  • DNA, Superhelical
  • Recombinant Fusion Proteins
  • Topoisomerase I Inhibitors
  • Viral Proteins
  • DNA Topoisomerases, Type I

Grants and funding

Fondation Schmidheiny DB Geneva Department of Education The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript