The Lysogenic Life Cycle-Temperate Phages.

The Lysogenic Life Cycle-Temperate Phages.

Lysogeny is an alternative life cycle exhibited by some bacteriophages. A good part of our knowledge on lysogeny comes from studies of Escherichia coli phage lambda (λ). Phage λ is one of a class of phages that can utilize either a lytic or a lysogenic life cycle. Each virion of phage λ consists of a straight molecule of double-stranded DNA packed in a polyhedral capsid with a tail through which the DNA enters the host bacterium.

The lysogenic cycle of phage λ consists of the following four steps:

1. Entry of Phage Genome into Cell. The DNA molecule from the phage passes into the bacterial host cell via the same process described for lytic bacteria and becomes a closed circle.
2. Synthesis of Early Protein. A brief period of mRNA synthesis by the host cell is necessary in order to synthesize a repressor protein coded by the phage DNA. This protein inhibits the synthesis of the specific mRNA that encodes the lytic functions. The crucial regulatory test here is how fast a critical level of the specific repressor can be synthesized. The outcome determines whether the phage will undergo a lytic or a lysogenic cycle. If sufficient repressor is present, it blocks the transcription of all other phage genes. As a consequence, none of the virion proteins is made, and the cell does not lose.
3. Integration of Viral DNA. The phage DNA joins with the DNA in the bacterial chromosomes and is inserted into it by the action of a phage-coded DNA-insertion enzyme (coded by the intgene). Integrated in this way, the viral genome is now called a prophage.

Phage DNA is inserted at a certain position in the E.coli chromosome, between the gal (galactose) and bio (biotin) genes. During the insertion, the phage DNA forms a circle, followed by physical breakage and rejoining of phage and host DNA. This mechanism is shown schematically.

Other temperate phages have their own sites of integration on the bacterial chromosomes. However, some temperate phages, such a phage Mu, have no specific site for insertion and may be able to insert multiple copies of their DNA at various sites in a single bacterial chromosome. Wherever insertion occurs, inactivation of the specific bacterial gene at the location gives rise to a mutant host cell; hence, the phage name Mu.

Attainment of Lysogeny

The bacterial host cell remains alive and continues to grow and multiply, despite having a prophage integrated amoung ts own genes. The phage gene replicate as part of the bacterial chromosome.

Continued production of the repressor maintain the integrated prophage condition in the lysogenic cells. If at all time the repressor is inactivated (e.g., by a protease enzyme induced by exposure to ultraviolet light), the phage operons become derepressed and strat functioning, and the phage enters the lytic cycle and destroys its host cell. Thus a single repressor gene decides the fate of both the bacterial cell and the phage.

Nonintegrative Lysogeny

Most temperate phages enter the lysogenic cycle in the way just described for phage λ-namely, insertion of the prophage at a unique side on the bacterial host chromosome. There is another, less common type of lysogeny in which there is no DNA-insertion system and the phage DNA becomes a plasmid, or independently replicating circular DNA molecule, rather than part of the host chromosome. Escherichia coli phage P1 typically carries out this type of lysogenic life cycle. Following infection, the DNA of P1 circularizes and is repressed, but unlike the DNA of phage λ, it remains as a free DNA molecule in the cytoplasm. During the bacterial life cycle the P1 DNA replicates once, at a time that coincides with the bacterium’s chromosomal replication (the coupling f the two events is controlled by a phage gene). When the bacterial cell divides, each daughter cell receives a P1 plasmid. How this orderly assortment is accomplished is still not known.

Other Chemical Components

Besides nucleic acids, the major chemical component of the virion is protein. In addition to their protein coat, many viruses contain within their capsid one or more enzymes that are released after the virus is uncoated in the host cell. These enzymes function in the replication of the nucleic acid of the virus. The most common viral enzymes are polymerases. Except for the RNA viruses carrying the single (+) strand of mRNA, all RNA viruses contain RNA polymerases. The (+) strand RNA code for their own RNA polymerase, which Is synthesized by the host cell during translation of the viral mRNA. Without a viral polymerase, viral RNA could not be transcribed and the virus could not replicate. The retroviruses contain an enzyme (RNA-dependent DNA polymerase, or Reverse Transcriptase) that synthesizes a DNA strand, using the viral RNA genome as a template. (Remember that in Chapter 14 you learned that this enzyme was very useful in genetic engineering when you need to make cDNA from eucaryotic mRNA). The term Retrovirus is derived from the first two letter of reverse transcriptase.

A wide variety of lipid compounds have also been found in viruses. These include phospholipids, glycolipids, neutral fats, fatty acids, fatty aldehydes, and cholesterol. Phospholipids, found in the viral envelope discussed later in this chapter, are the predominant lipid substances in viruses.

All viruses contain carbohydrate, since the nucleic acid itself contains ribose or deoxyribose. Some enveloped animal viruses , such as the influenza virus ,have spikes made of glycoprotein on the envelope.

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.