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

- The first T=19l geometry icosahedral bacteriophage structure is reported.
- The T=19l geometry creates a quite large internal volume for packing the dsDNA genome.
- ΦM12's capsid bows at the icosahedral faces when not packed with DNA, similar to other dsDNA phages.

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.

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