The structure of Sinorhizobium meliloti phage ΦM12, which has a novel T=19l triangulation number and is the founder of a new group of T4-superfamily phages

Virology. 2014 Feb:450-451:205-12. doi: 10.1016/j.virol.2013.11.019. Epub 2014 Jan 6.


ΦM12 is the first example of a T=19l geometry capsid, encapsulating the recently sequenced genome. Here, we present structures determined by cryo-EM of full and empty capsids. The structure reveals the pattern for assembly of 1140 HK97-like capsid proteins, pointing to interactions at the pseudo 3-fold symmetry axes that hold together the asymmetric unit. The particular smooth surface of the capsid, along with a lack of accessory coat proteins encoded by the genome, suggest that this interface is the primary mechanism for capsid assembly. Two-dimensional averages of the tail, including the neck and baseplate, reveal that ΦM12 has a relatively narrow neck that attaches the tail to the capsid, as well as a three-layer baseplate. When free from DNA, the icosahedral edges expand by about 5nm, while the vertices stay at the same position, forming a similarly smooth, but bowed, T=19l icosahedral capsid.

Keywords: Bacteriophage; Icosahedral; Myovirus; Sinorhizobium meliloti; T=19; ΦM12.

Publication types

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

MeSH terms

  • Amino Acid Sequence
  • Bacteriophage T4 / genetics
  • Bacteriophage T4 / isolation & purification*
  • Bacteriophage T4 / ultrastructure*
  • Capsid / metabolism
  • Capsid / ultrastructure*
  • Capsid Proteins / chemistry
  • Capsid Proteins / genetics
  • Cryoelectron Microscopy
  • Models, Molecular
  • Molecular Sequence Data
  • Sequence Alignment
  • Sinorhizobium meliloti / virology*


  • Capsid Proteins