Bacteriophages exhibit considerable diversity in the nature of their nucleic acid genome, and this diversity is reflected in different modes of replication. Fundamentally different propagation strategies are exhibited by lytic and temperate phages. Lytic phages produce many copies of themselves in a single burst of growth. Temperate phages establish themselves as prophages either by becoming part of an established replicon (chromosome or plasmid) or by forming an independent replicon.

The dsDNA of many lytic phages is linear, and the first stage in their replication is the formation of circular DNA. This process depends upon cohesive ends, complementary single-stranded tails of DNA that hybridize. Ligation, formation of a phosphodiester bond between the 5′ and 3′ DNA ends, gives rise to covalently closed circular DNA that may undergo replication similar to that used for other replicons. Cleavage of the circles produces linear DNA that is packaged inside protein coats to form daughter phages.

The ssDNA of filamentous phages is converted to a circular double-stranded replicative form. One strand of the replicative form is used as a template in a continuous process that produces ssDNA. The template is a rolling circle, and the ssDNA it produces is cleaved and packaged with protein for extracellular extrusion.

The ssRNA phages are among the smallest extracellular particles containing information that allows for their own replication. The RNA of phage MS2, for example, contains (in fewer than 4000 nucleotides) three genes that can act as mRNA following infection. One gene encodes the coat protein, and another encodes an RNA polymerase that forms a dsRNA replicative form. ssRNA produced from the replicative form is the core of new infective particles.

The temperate bacteriophage E. coli phage P1 genome, when undergoing a lysogenic cycle, exists as an autonomous plasmid in the bacterium. The dsDNA of other temperate bacteriophages is established as a prophage by its insertion into the bacterial host chromosome. The site of insertion may be quite specific, as typified by integration of E. coli phage λ at a single int locus on the bacterial chromosome. The specificity of integration is determined by identity of the shared DNA sequence by the int chromosomal locus and a corresponding region of the phage genome. Other temper ate phages, such as E. coli phage Mu, integrate in any of a wide range of chromosomal sites and in this aspect resemble transposons.

Prophages contain genes required for lytic replication (also called vegetative replication), and expression of these genes is repressed during maintenance of the prophage state. A manifestation of repression is that an established prophage frequently confers cellular immunity against lytic infection by similar phage. A cascade of molecular interactions triggers derepression (release from repression) so that a pro phage undergoes vegetative replication, leading to formation of a burst of infectious particles. Stimuli such as ultraviolet (UV) light may cause derepression of the prophage. The switch between lysogeny—propagation of the phage genome with the host—and vegetative phage growth at the expense of the cell may be determined in part by the cell’s physiologic state. A nonreplicating bacterium will not support vegetative growth of phage, but a vigorously growing cell contains enough energy and building blocks to support rapid phage replication.

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مواضيع ذات صلة

Spontaneous Mutations in bacteria

Mechanisms of Gene Transfer in Bacteria

Genotoxicity Through Potential Off-target Nuclease - Induced Double - Strand Breaks

A Broad Range of Genomic Changes by Homologous Recombination–Mediated Genome Editing

Early Transcription of Genes N and Cro

Lambda’s Relatives and Lambda Hybrids

The Genetic Structure of Lambda

The Physical Structure of Lambda

Other Attenuated Systems: Operons, Bacillus subtilis and HIV

Coupling Translation to Termination

trp Multiple Secondary Structures in trp Leader RNA

Early Explorations of the Hypersynthesis