Temperate Bacteriophages | Genetics

The below mentioned article provides a study note on temperate bacteriophages, these are the phages that can enter into a lysogenic relationship with their host.

Subject-Matter of Temperate Bacteriophage:

There are some bacteria and bacteriophages (bacterial viruses) whose relationships are partially symbiotic. The bacterium may carry such a phage as a part of its chromosome, while in turn, the phage does not lyse the bacterial cell. Such bacterial strains are called lysogenic, and the phenomenon is termed as lysogeny.

The bacteriophage that can enter into a lysogenic relationship with its host is called temperate phage. Examples of temperate bacteriophages are lambda (ʎ), ɸ 80, P1, P2 and Mu etc. A well-studied temperate bacteriophage is the phage lambda (ʎ) which can integrate into the chromosome of E. coli. When integrated within the bacterial chromosome, the phage ʎ is called a prophage or provirus and replicates only with the host chromosome.

Induction of a prophage may occur spontaneously or artificially, e.g., an exposure to U.V. light, X-rays or chemical mutagens, such as, nitrogen mustards and organic peroxide. Induction converts the prophage into a free “virulent” phage through excision of the phage genome from that of its host.

The free phage multiplies vegetatively and causes the lysis of the bacterial cell. High doses of UV may cause the loss of a prophage from the bacterial cell; this process is known as curing. The curing of lysogenic bacteria may also occur spontaneously with a frequency of 1 per 10⁵ cells per generation.

The Lambda (X) Chromosome:

The chromosome of phage X is a double-stranded DNA molecule consisting of 47 kb. Both the 5′-ends are “cohesive” containing 12 nucleotides, i.e., the 5′-ends of both the strands are complementary to each other. The sequence of bases of the 5′-end of one strand is 5′ GGGCGGCGACCT 3′, while it is 3′ CCCGCCGCTGGA 5′ of the other strand. These cohesive ends arc responsible for the circularization of the X DNA (Fig. 18.8).

Before infection, the ʎ chromosome is linear, but once it enters the E. coli cell, it becomes circular (cyclic). The circular ʎ chromosome is not attacked by the host exonucleases. The circular shape is also advantageous for its replication. The ʎ chromosome contains an attachment site that is homologous to a site in the bacterial chromosome.

Integration into and Excision from the Host Chromosome:

When the attachment regions are present in both the viral and the bacterial chromosome, synapsis between them occurs in this region. The X chromosome becomes integrated into the bacterial chromosome through a crossing over in the synapsed region (Fig. 18.9).

The actual synapsis involves a 15 bp long special core sequence called “0” sequence that is present on both the phage and the host chromosomes. A specific phage gene “int” is responsible for the recombination event leading to its integration.

After integration into the bacterial chromosome, the prophage produces a repressor “cl” which prevents the other prophage genes from functioning. This repressor is also responsible for the immunity of lysogenic cells to super-infection by the same phage as well as phages similar to the prophage (homo-immune response).

In vegetative phage, the gene order in the X chromosome is A-J-cl-R. The attachment site 0 of the virus lies between genes J and cl. After recombination with and integration into the bacterial chromosome, the X gene order becomes cI-RA-J (Fig. 18.9). In the bacterial chromosome, the integration site for ʎ lies between the genes governing galactose (gal) and biotin (bio) metabolism.

During “induction”, the prophage chromosome folds out to generate a figure of 8; the folding is precise in that two attachment sites flanking the ʎ genome become paired with each other. A crossing over within this paired attachment site leads to the excision of circular ʎ genome from the host chromosome (Fig. 18.9).

Transduction:

Sometimes, the prophage ʎ coils in such a way that the excised chromosome includes a portion (either containing the gal gene or bio gene) of the host chromosome (Fig. 18.10). The particles ʎ carrying a part of E. coli chromosome are called transducing particles and designated as ʎ gal (ʎ dg) or X bio ʎ db), depending on whether they contain in their genome the host chromosome segment from the gal or the bio region.

Such particles are defective because the ʎ chromosome present in them has lost some phage to accommodate the bacterial genes. For example, the ʎ dg DNA carries the gal locus of E. coli, but upon infection it cannot integrate into the bacterial chromosome as it is deficient in some ʎ wild type functions (as shown in Fig. 18.10 and 18.11, the transducing particle ʎ is deficient for the gene J).

The transducing activity of ʎ dg or ʎ db occurs in association with a “helper” that is an active ʎ phage. After infection, the normal ʎ chromosome (of the helper particle) becomes integrated into the bacterial chromosome; this creates a special attachment site (Fig. 18.11).

The ʎ dg transducing phage chromosome can now pair with the bacterial chromosome at this special attachment site and becomes integrated into it. Such bacterial cells contain two “pro-phages”, one normal and one ʎ dg or ʎ db.

The gal⁺ gene carried by the ʎ dg chromosome produces gal⁺ phenotype in an otherwise gar cell which now has the genotype gar/gat; such cells are known as “partial diploids” or “merozyotes” for the gal region.

In such cases, cells with the gat/gat combination are termed heterozygotes, while those with gal⁺/gar or gal⁺/gal⁺ genotypes are called homozygotes. In the gat/gal” condition, the gat gene present in the ʎdg chromosome is called exogenote, while the gar gene present in the bacterial chromosome is known as endogenote.

Certain mutants of ʎ, such as, ʎ b2, are defective in their attachment region. Therefore, they cannot integrate into bacterial chromosome and, consequently, remain independent in the bacterial cytoplasm.