Structure and infection dynamics of Bxb1, a phage infecting Mycobacterium smegmatis

Bacteriophages are viruses that infect bacteria and can be used therapeutically to treat antibiotic-resistant bacterial infections. Mycobacteriophage Bxb1 is a well-characterized virus of Mycobacterium smegmatis with double-stranded DNA and a long flexible tail. Mycobacteriophages show considerable potential as therapies for Mycobacterium infections, but little is known of the structural details of these phages or how they bind to and traverse the complex Mycobacterium cell wall. Here we report the complete structure and atomic model of phage Bxb1, including the arrangement of immunodominant domains of both the capsid and tail tube subunits, and the assembly of the protein subunits in the tail tip complex. The structure contains protein assemblies with three-fold, five-fold, six-fold, and 12-fold symmetries, which interact to satisfy several symmetry mismatches. Cryo-electron tomography of phage particles bound to M. smegmatis reveals the structural transitions that occur for free phage particles to bind to the cell surface and navigate through the cell wall to enable DNA injection into the cytoplasm.