Abstract
The P22-like bacteriophages have short tails. Their virions bind to their polysaccharide receptors through six trimeric tailspike proteins that surround the tail tip. These short tails also have a trimeric needle protein that extends beyond the tailspikes from the center of the tail tip, in a position that suggests that it should make first contact with the host’s outer membrane during the infection process. The base of the needle serves as a plug that keeps the DNA in the virion, but role of the needle during adsorption and DNA injection is not well understood. Among the P22-like phages are needle types with two completely different C-terminal distal tip domains. In the phage Sf6-type needle, unlike the other P22-type needle, the distal tip folds into a “knob” with a TNF-like fold, similar to the fiber knobs of bacteriophage PRD1 and Adenovirus. The phage HS1 knob is very similar to that of Sf6, and we report here its crystal structure which, like the Sf6 knob, contains three bound L-glutamate molecules. A chimeric P22 phage with a tail needle that contains the HS1 terminal knob efficiently infects the P22 host, Salmonella enterica, suggesting the knob does not confer host specificity. Likewise, mutations that should abrogate the binding of L-glutamate to the needle do not appear to affect virion function, but several different other genetic changes to the tip of the needle slow down potassium release from the host during infection. These findings suggest that the needle plays a role in phage P22 DNA delivery by controlling the kinetics of DNA ejection into the host.
Highlights
Tailed bacteriophage virions deliver DNA to susceptible cells after adsorbing to specific receptors on the surface of bacteria
Considerable information is known about phage virion proteins that bind various bacterial receptors, but much less is known of the detailed mechanism of DNA release from the virion into the cell
There are currently 164 genome sequences available for P22like phages and prophages that infect 12 different species of Enterobacteriaceae bacteria
Summary
Tailed bacteriophage virions deliver DNA to susceptible cells after adsorbing to specific receptors on the surface of bacteria. The Podoviridae in particular have been shown to release some virion protein molecules, called ejection proteins, along with the DNA. These proteins are required for successful DNA injection, but they have no pre-existing structure that could deliver the DNA through the membranes and periplasm [4,6,7]. The short-tailed phage T7 has recently been shown to rearrange its virion proteins during injection to build a structure that could serve as such a conduit, but the details of this structure and exactly how it might function remain mysterious [8,9]. Little is known about the trigger mechanism that signals the virion to release its DNA for any phage
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