Abstract
The Staphylococcus aureus pathogenicity islands (SaPIs), including SaPI1, are a type of mobile genetic elements (MGEs) that are mobilized at high frequency by "helper" bacteriophages, such as 80α, leading to packaging of the SaPI genomes into virions made from helper-encoded structural proteins. 80α and SaPI1 virions consist of an icosahedral head connected via a portal vertex to a long, non-contractile tail. A connector or "neck" forms the interface between the tail and the head. Here, we have determined the high-resolution structure of the neck section of SaPI1 virions, including the dodecameric portal and head-tail-connector proteins, and the hexameric head-tail joining, tail terminator and major tail proteins. We also resolved the DNA, the tail completion protein (TCP), and the tape measure protein (TMP) inside the tail, features that have not previously been observed at high resolution. Our study provides insights into the assembly and infection process in this important group of MGEs.