The Lytic Life Cycle-Virulent Phages

The Lytic Life Cycle-Virulent Phages

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Fig:The lytic cycle of phage T4, a virulent phage Attachment.

The basic lytic cycle is , using the T-even coliphages (types with even-numbered names) with double-stranded DNA as a model. It consists of the following six steps:

1. Adsorption (Attachment). The first step in infection of a host bacterial cell by a phages is attachment of the phage to none specific type of bacterial cell. The tip of the phage tail becomes absorbed on the cell at specific receptor sites on the bacterial cell surface. This attachment can take place only when the phage tail and the bacterial receptor have molecular configurations that are complementary to each other, like the key that fits a specific lock, the adsorption of phage T4 on its host by means of its tail. Infection of a host bacterial cell cannot occur without adsorption. Some bacterial mutants, having lost the ability to synthesize specific receptor, become resistant to infection by specific phages.

Initial adsorption of the phage on the receptor is reversible when only the tail fibers are attached to the cell surface. Adsorption soon becomes irreversible when the tail pins attach7.

Most filamentous phages (with single-stranded DNA) are adsorbed on the tips of the special hollow pili known as the F pili.Exactly how they enter the cell is still unknown, but it may be that the entire virion is pulled into the cytoplasm via the F pili.

2. Phage Genome Enters Cell. After the phage has attached by to the host cell, the DNA in the phage head passes through the cell wall and into the cytoplasm of the bacterium.This is accomplished in one of several ways, depending on the phage. In the T-even phages, passage takes place through the following steps:
3. The phage sheath contracts, forcing the tail core tube into the cell through the cell wall and membrane.
4. The DNA in the phage head passes through the tube and into the cytoplasm of the bacterial host cell.

The protein coat, which forms the phage head, and the tail structure remain outside the cell. In this process, the nucleic acid injected into the cell is never exposed to the medium outside the host cell.

Phages such as T1 and T5 that do not have a contractile sheath also inject their nucleic acid through the cell envelope, possibly at adhesion sites between the inner and outer membranes. Thus sheath contraction is not a prerequisite for phage infection. With tailless phages, the protein coat may break open and first release its nucleic acid onto the cell wall

The filamentous rod-shaped single-stranded DNA phages (such as fd and M13) enter the bacterial cell as discrete virions, without leaving part of their structure outside the cell. As the DNA penetrates into the cell, the capsid protein becomes incorporated into the cell’s cytoplasmic membrane and is later reutilized during virus release.

3. Conversion of Host Cell to Phage-Producing Cell. The synthesis of viral components inside the host cell can be divided into early and late function. Early functions are those events that involve the take-over of the cell and the synthesis of early viral mRNA (as discussed in the section). Late functions include the subsequent synthesis of other proteins and assembly of the nucleocapsid. Enzymes made early in the life cycle (such as nucleases and DNA- dependent RNA polymerase) are called early proteins. The protein made late in life cycle (the late protein) are different from the early proteins and include both enzymes and structural proteins (such as those in phage heads, tails, and tail fibers). The early proteins are encoded in early mRNA, and the structural proteins are encoded in late mRNA.

Within a few minutes of early of phage DNA, the bacterial host loses the ability either to replicate or to transcribe its own DNA; sometimes it loses both abilities. This shutdown of bacterial DNA or RNA synthesis is carried out in different ways, depending on the phage-species. For example, host DNA may be quickly degraded to small fragments, dispersing and thus becoming inactivated.

The phage directs the bacterial synthesis of copies of phage nucleic acid, using several mechanisms and the bacterium’s replication proteins. Transcription of phage mRNA from the phage DNA is almost initiated by the host-cell RNA polymerase, which recognize the viral promoter that governs the transcription of the genes coding for viral protein synthesis. For the most part, the phage mRNA codes for nucleases that break down host DNA. This makes the nucleotides of the host DNA available for phage DNA synthesis. After the first phage mRNA is made, either the host-cell polymerase is modified to recognize the other viral promoters, or a special phage-specific RNA polymerase is synthesized.

In the filamentous phages, the entering single-stranded DNA (the plus DNA strand) serves as a template for synthesis f its complementary strand (the minus DNA strand). The cell’s DNA polymerase is used, since this process takes place before transcription of mRNA begins. The resulting double-stranded DNA is the replicative from multiple copies of the plus strand are synthesized, using the minus strand as a template.

RNA-containing phages differ from DNA-containing phages with respect to use of host replication enzymes. RNA phages must code their own replication enzymes, because the host cell does not have enzymes that replicate RNA. A phage that contains single-stranded RNA uses the RNA as a plus strand (as a mRNA for the synthesis of RNA polymerase and other proteins). The RNA polymerase makes a minus RNA strand (replicates strand) using the viral genome (the plus RNA strand) as a template. The replicative strand (the minus RNA strand) is used as a template to make numerous viral plus, RNA strands, which combine with coat protein to from many infectious viruses.

Thus viral transcription, which produces viral mRNA form viral nucleic acid, represents the key event in viral infection- when control of biochemical synthesis by cellular genes is switched to control by viral genes.

4. Production of Phage Nucleic Acid and Proteins. Nucleic acid replication, which follows early protein synthesis, serves as a demarcation line between early and late functions in a viral replication process. Since late mRNA is not synthesized until after viral nucleic acid replication has begun, it is transcribed from viral progeny genomes. This means that the late protein synthesis takes place after nucleic acid replication. Translation of mRNA into protein takes place in the host-cell cytoplasm and uses ribosomes, transfer RNAs, and enzymes found in the cytoplasm. (If the mRNA is synthesized in the nucleus of the host cell, it first passes to the cell cytoplasm before translaction).

Toward the end of the replication cycle, the phages synthesize late proteins, including the structural proteins necessary for virion self-assembly, enzymes involved in the maturation process, and enzymes used in the release of phages from the bacterial cell.

5. Assembly of Phage Particles. Two kinds of late proteins are required for phages assembly: structural proteins of phage particle and enzymes that catalyze reaction of the assembly process but do not become integral parts of the bacteriophages. Assembly of icosahedral phages takes place in several steps:
6. Aggregation of phage structural proteins to from a head and a tail (if needed). At this time, the tail is not attached to the head.
7. Condensation of the nucleic acid and entry into a preformed head.
8. Attachment of the tail to a packed head.

About 25 min after initial infection of a host cell, usually 50 to 1000 phage particle have been assembled, the number depending on the particular species and the growth conditions of the culture.

6. Release of Assembled Phages. One of the late proteins synthesized in the lytic cycle of infection is an enzyme called an endolysin. This enzyme lyses the bacterial cell and releases the mature phages.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.