Injection of bacteriophage DNA into Escherichia coli

Bacteriophages must interact with a receptor on the bacterial cell surface to successfully initiate replication. The outer surface of a prokaryotic cell presents a set of features to the external environment that includes structural materials (glycoproteins and lipopolysaccharides), transport machinery (amino acid or sugar transport complexes), and cell-to-cell interaction apparatus — the F or sex pilus. Sex pili are used by the bacteria in conjugation and exchange of genetic material with other bacteria of the opposite "sex." Attachment of the phage to host cells may employ any one of these structures, depending on the particular virus. Some features utilized by bacteriophages replicating in Escherichia coli are shown in Table 6.2.

In some cases, attachment of phage to the host cell involves a physical rearrangement of the virus particle. For example, attachment of bacteriophage T4 to the surface of susceptible E. coli cells occurs in two steps, which are shown in Fig. 6.4. First, there is a relatively weak interaction between the tips of the phage tail fibers and lipopolysaccharide residues on the surface of the cell's outer membrane. This triggers a second, stronger, and irreversible interaction. In this, tail pins on the base plate of the virion interact with structures in the outer membrane itself,

Table 6.2 Some E. coli bacteriophage receptors.

Vi rus

Structure

Normal function

T2 T4 T6

T1 and T5 MS2

OmpF

Li popolysacch aride OmpC

Li popolysacch aride

TonA

LamB

F pilus

Porin protein

Outer membrane structure Porin protein

Outer membrane structure Nucleoside transport protein Ferrichrome transport Maltose transport protein Conjugation

Fig. 6.4 Entry of T4 bacteriophage DNA into an E. coli cell. Initial attachment is between the fibers to the ompC lipopolysaccharide receptor on the bacterial cell wall (a). The binding of protein pins on the base plate to the cell wall leads to contraction of the tail fibers and sheath proteins, leading to insertion of the tail tube through the cell wall. As shown in the electron micrograph (b), phage pilot protein allows the highly charged viral DNA genome to penetrate the bacterial plasma membrane and enter the cell. Phage DNA can be seen as shadowy lines emanating from the tail tube. (From Dimmock NJ, Primrose SB. Introduction to modern virology, 4th ed. Boston: Blackwell Science, 1994.)

Fig. 6.4 Entry of T4 bacteriophage DNA into an E. coli cell. Initial attachment is between the fibers to the ompC lipopolysaccharide receptor on the bacterial cell wall (a). The binding of protein pins on the base plate to the cell wall leads to contraction of the tail fibers and sheath proteins, leading to insertion of the tail tube through the cell wall. As shown in the electron micrograph (b), phage pilot protein allows the highly charged viral DNA genome to penetrate the bacterial plasma membrane and enter the cell. Phage DNA can be seen as shadowy lines emanating from the tail tube. (From Dimmock NJ, Primrose SB. Introduction to modern virology, 4th ed. Boston: Blackwell Science, 1994.)

requiring a change in conformation of the tail fibers. This ultimately results in compression of the phage tail's contractile sheath and injection of phage DNA into the host cell. In this process, the phage tail tube penetrates the cell wall, but phage DNA must still cross the inner cell membrane. This last step is carried out with the help of a viral gene product called a pilot protein.

With some other phages, the interaction between virion and cell results in no immediate alterations to the phage structure, for instance, in attachment of bacteriophage X to its receptor

LamB. Again, the attachment of MS2 bacteriophage to the F pilus does not result in changes to the virus structure. Since cells with a pilus structure (the product of an F-plasmid) are called male, MS2 and similar phages are sometimes termed male-specific phage.

The actual amount of bacteriophage that enters the host cell is quite variable. In the case of tailed phage, only phage DNA and certain accessory proteins enter the host cell. For a nontailed phage such as MS2, however, the entire phage particle enters the cell and is uncoated in the cytoplasm.

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