Short Noncontractile Tail Machines: Adsorption and DNA Delivery by Podoviruses

Abstract

Tailed dsDNA bacteriophage virions bind to susceptible cells with the tips of their tails and then deliver their DNA through the tail into the cells to initiate infection. This chapter discusses what is known about this process in the short-tailed phages (Podoviridae). Their short tails require that many of these virions adsorb to the outer layers of the cell and work their way down to the outer membrane surface before releasing their DNA. Interestingly, the receptor-binding protein of many short-tailed phages (and some with long tails) has an enzymatic activity that cleaves their polysaccharide receptors. Reversible adsorption and irreversible adsorption to primary and secondary receptors are discussed, including how sequence divergence in tail fiber and tailspike proteins leads to different host specificities. Upon reaching the outer membrane of Gram-negative cells, some podoviral tail machines release virion proteins into the cell that help the DNA efficiently traverse the outer layers of the cell and/or prepare the cell cytoplasm for phage genome arrival. Podoviruses utilize several rather different variations on this theme. The virion DNA is then released into the cell; the energetics of this process is discussed. Phages like T7 and N4 deliver their DNA relatively slowly, using enzymes to pull the genome into the cell. At least in part this mechanism ensures that genes in late-entering DNA are not expressed at early times. On the other hand, phages like P22 probably deliver their DNA more rapidly so that it can be circularized before the cascade of gene expression begins.

Conformational Changes Leading to T7 DNA Delivery upon Interaction with the Bacterial Receptor

“…The Gram-negative bacterial envelope is composed of a peptidoglycan layer surrounded by two membranes (9,19). During infection, the phage is thought to interact first with a surface molecule that allows correct tail orientation relative to the bacterial envelope, followed by an irreversible interaction with the same or a different receptor; this second interaction is needed to trigger opening of the tail channel (20). Viruses that infect Gram-negative bacteria, such as Escherichia, Salmonella, Shigella, or Yersinia genus, often use lipopolysaccharide (LPS) 3 as a receptor, assisted by porins or outer membrane proteins (21)(22)(23).…”

“…A few years later, Lindberg (21) claimed that T7 phages only infect rough strains of E. coli and Shigella, corroborating the importance of LPS in the infection mechanism. The tendency in the early 2000s was to consider LPS only a primary phage T7 receptor that mediates reversible binding to the bacterial surface, whereas another secondary receptor was suggested to be necessary for irreversible binding and infection (20,25).…”

Structural Characterization of the Bacteriophage T7 Tail Machinery

“…The Podoviridae family constitutes the group with the simplest tail morphology, composed of a small number of proteins (6). Structural analysis of different viruses belonging to this group (29 (7), P22 (8), P-SPP7 (9), N4 (10), ⑀15 (11), and K1E and K1-5 (12)) has shown that these structures are formed by a central knob or nozzle contoured by 6 -12 trimeric appendages named fibers or spikes.…”

“…The tail is attached to the head through the tail adaptor protein, a dodecameric ring with similarities to the gatekeeper proteins described for long-tailed phages (13). The short length of the Podoviridae requires additional components to drill through the whole bacterial envelope, and it has been proposed to be enlarged by internal capsid proteins, which would be also ejected during infection (6,14). This hypothesis has been recently supported by a cryo-electron tomography study, which has defined for the first time a detailed description of the sequential steps involved in the interaction of the bacteriophage T7 with Escherichia coli (15).…”

“…Finally, 32 subunits of the protein gp7.3 have been predicted as a component of the tail, but its location and function are controversial. This protein might be involved either in helping the tail protein assembly process or as a part of the tail channel or tip, thus contributing to the host cell interaction (6,14), and it has also been proposed to be injected inside the bacteria during infection (14).…”