The infection of Escherichia coli by T2 and T4 bacteriophages as seen in the electron microscope II. Structure and function of the baseplate

Abstract

The baseplate of the T-even bacteriophage seems to play a central role in the infection of the host. In the native particle it is attached both to the extended sheath and to the tip of the central hollow needle. It has the shape of a flat hexagon with a short pin at each corner and a trigonal plug in its center. In addition, six long tail fibers are attached to its vertices. These fibers are responsible for the initial attachment of the particle to the susceptible host cell. Subsequently, the short pins become attached to the cell surface. A series of structural changes in the baseplate and the sheath follow. The baseplate becomes a six-pointed star with the tips pointing diagonally away from the bacterial surface and with the six long tail fibers attached to them. The baseplate's attachment to the needle is dissolved and an 80 A hole is found instead of the baseplate's central plug. These transformations in the baseplate may initiate the contraction of the sheath. Since the sheath is connected to the needle near the neck of the particle, its contraction thus causes the needle to slide through the baseplate and press against the bacterial wall. This forces the baseplate away from the cell wall and creates tension in the pins which are drawn out into short tail fibers. It seems that the needle is thus forced to penetrate the cell wall (which may be weakened by phage enzymes) and to serve as a channel through which the phage's DNA may pass into the cell.