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

• Published in: Virology (New York, N.Y.) Vol. 450; pp. 205 - 212
• Main Authors: Stroupe, M. Elizabeth, Brewer, Tess E, Sousa, Duncan R, Jones, Kathryn M
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.02.2014
• Subjects: Amino Acid Sequence; Bacteriophage; Bacteriophage T4 - genetics; Bacteriophage T4 - isolation & purification; Bacteriophage T4 - ultrastructure; Capsid - metabolism; Capsid - ultrastructure; Capsid Proteins - chemistry; Capsid Proteins - genetics; Cryoelectron Microscopy; Icosahedral; Infectious Disease; Models, Molecular; Molecular Sequence Data; Myovirus; Sequence Alignment; Sinorhizobium meliloti; Sinorhizobium meliloti - virology; T=19; ΦM12; ΦM12; Icosahedral; Bacteriophage; Myovirus; T=19; Sinorhizobium meliloti
• ISSN: 0042-6822; 1096-0341
• DOI: 10.1016/j.virol.2013.11.019

Abstract 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 5 nm, while the vertices stay at the same position, forming a similarly smooth, but bowed, T =19l icosahedral capsid.