The T4 bacteriophage is a type of virus that specifically infects bacteria, with Escherichia coli (E. coli) being its primary host. Its distinct structural elements work together to facilitate its life cycle, which involves infecting a bacterial cell and replicating within it.
General Body Plan
The T4 bacteriophage exhibits a distinctive “tadpole-like” shape, characterized by three main structural divisions: a head, a tail, and tail fibers. This large virus measures approximately 90 nanometers (nm) in width and 200 nm in length. These components are initially synthesized independently within the host cell before they are precisely assembled to form the mature, infectious virion.
The head, also known as the capsid, serves as a protective shell for the phage’s genetic material. Extending from the head is the tail, a hollow structure designed to facilitate the transfer of nucleic acid during infection. At the base of the tail, intricate tail fibers extend outwards, playing a role in recognizing and attaching to the bacterial host.
The Icosahedral Head
The head of the T4 bacteriophage is an elongated icosahedron. This structure measures approximately 120 nm in length and 86 nm in width. Its outer shell, the capsid, is primarily constructed from multiple copies of two main proteins: gp23 and gp24.
The major capsid protein, gp23, forms the hexagonal lattice that makes up the bulk of the capsid shell. At 11 of the 12 vertices of this elongated icosahedron, pentamers of the minor capsid protein, gp24, are found. The twelfth vertex is unique, featuring a dodecameric portal vertex composed of gp20, which acts as the gateway for the viral DNA to enter during packaging and exit during infection. Within this protein shell, the T4 bacteriophage tightly packs its approximately 171-172 kilobase pair (kbp) linear double-stranded DNA genome.
The Contractile Tail
The T4 bacteriophage’s tail is approximately 92.5 nm long and 24 nm in diameter. It is composed of several distinct parts, beginning with a neck or collar region that connects the tail to the head. Beneath this collar lies a central hollow core, which is a non-contractile, rigid structure made of gp19 protein subunits.
Surrounding the central tail tube is a contractile sheath, composed of gp18 protein. At the very end of the tail is a complex, hexagonal baseplate, measuring about 27 nm high and 52 nm in diameter. This multiprotein baseplate plays a role in regulating the tail’s function.
Fibers for Host Recognition
Extending from the hexagonal baseplate of the T4 bacteriophage are two types of fibers: six long tail fibers and six short tail pins, also known as short tail fibers. The long tail fibers, measuring approximately 145 nm in length, are attached to the periphery of the baseplate. These long fibers are composed of multiple proteins and feature a tip region responsible for host recognition.
The short tail pins are positioned beneath the baseplate and are initially folded. The long tail fibers are the primary sensors, reversibly recognizing specific receptor molecules on the surface of the bacterial host. This initial recognition event then signals the baseplate, causing the short tail fibers to unfold and bind irreversibly to the host cell surface, increasing the efficiency of the infection.