Structural basis of bacteriophage Ur-lambda infection initiation

Bacteriophages must recognize host receptors and penetrate the host cell envelope to initiate infection. How the classic phage λ initiates infection is not yet understood. Here, we combine cryo-electron microscopy and tomography to visualize infection initiation by Ur-λ, the original λ isolate that uses side fibers to adsorb rapidly to <i>Escherichia coli</i>. We determine the structure of Ur-λ, resolving the full-length central and side fibers, thus providing a structural basis for host recognition. We show that Ur-λ contains six copies of its tape measure protein. We capture intermediates of the tail tip complex during infection initiation, revealing how extensive conformational changes enable adsorption, and visualize the trans-envelope channel required for genome ejection.