States of phage T3/T7 capsids: buoyant density centrifugation and cryo-EM

Philip Serwer, Elena T. Wright, Borries Demeler, Wen Jiang

Research output: Contribution to journalReview articlepeer-review

7 Scopus citations


Mature double-stranded DNA bacteriophages have capsids with symmetrical shells that typically resist disruption, as they must to survive in the wild. However, flexibility and associated dynamism assist function. We describe biochemistry-oriented procedures used to find previously obscure flexibility for capsids of the related phages, T3 and T7. The primary procedures are hydration-based buoyant density ultracentrifugation and purified particle-based cryo-electron microscopy (cryo-EM). We review the buoyant density centrifugation in detail. The mature, stable T3/T7 capsid is a shell flexibility-derived conversion product of an initially assembled procapsid (capsid I). During DNA packaging, capsid I expands and loses a scaffolding protein to form capsid II. The following are observations made with capsid II. (1) The in vivo DNA packaging of wild type T3 generates capsid II that has a slight (1.4%), cryo-EM-detected hyper-expansion relative to the mature phage capsid. (2) DNA packaging in some altered conditions generates more extensive hyper-expansion of capsid II, initially detected by hydration-based preparative buoyant density centrifugation in Nycodenz density gradients. (3) Capsid contraction sometimes occurs, e.g., during quantized leakage of DNA from mature T3 capsids without a tail.

Original languageEnglish (US)
Pages (from-to)583-596
Number of pages14
JournalBiophysical Reviews
Issue number2
StatePublished - Apr 1 2018


  • Bacteriophage assembly
  • Capsid flexibility
  • DNA injection
  • DNA packaging
  • Nycodenz

ASJC Scopus subject areas

  • Biophysics
  • Structural Biology
  • Molecular Biology


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