It was discovered by Esther Lederberg in Lambda strains, mutated at specific sites, are unable to lysogenize cells; instead, they grow and enter the lytic cycle after superinfecting an already lysogenized cell. The phage particle consists of a head also known as a capsid , a tail, and tail fibers see image of virus below. The head contains the phage's double-strand linear DNA genome. During infection, the phage particle recognizes and binds to its host, E.
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NCBI Bookshelf. An Introduction to Genetic Analysis. New York: W. Freeman; When they proposed the operon model, Jacob and Monod suggested that the genetic activity of temperate phages might be controlled by a system analogous to the lac operon. In the lysogenic state, the prophage genome is inactive repressed. In the lytic phase, the phage genes for reproduction are active induced.
This phage does indeed have an operon-type system controlling its two functional states. By now, you should not be surprised to learn that this system has proved to be more complex than initially suggested. Furthermore, mutations in the N , O , and P genes prevent most of the genome from being expressed after phage infection, with only those loci lying between N and O being active.
We shall soon see the significance of this observation. The positions of nonlethal and conditionally lethal mutations are indicated, and the characteristic clusters of genes with related functions are shown.
From A. Campbell, The Episomes. When wild-type phage particles are placed on a lawn of sensitive bacteria, clearings plaques appear where bacterial cells are infected and lysed, but these plaques are turbid because lysogenized bacteria which are resistant to phage superinfection grow within the plaques. Mutant phages that form clear plaques can be selected as a source of phages that are unable to lysogenize cells.
Such clear c mutants prove to be analogous to I and O mutants in E. For example, conditional mutants for a site called cI are unable to establish a lysogenic state under restrictive conditions , suggesting either a defective repressor or a defective repressor-binding site operator.
However, these mutants also fail to induce lysis after superinfecting a cell that has been lysogenized by a wild-type prophage , so the operator is functional, because repressor from the wild-type phage is clearly able to prevent the mutant from entering the lytic phase. Apparently, the cI mutation produces a defective repressor in the phage control system. Genetic mapping has revealed two operators, designated O L and O R , located on the left and right of cI , respectively.
A race begins to determine whether the phage will express its virulent functions and enter the lytic cycle or whether it will repress these functions and establish lysogeny. P L governs the leftward transcript that encodes the antiterminator protein N, plus a number of lytic functions. P R governs the rightward transcript, including the cro, cII, and Q genes, among others. These transcripts normally terminate at specific critical points, before key genes necessary for lytic development can be transcribed.
However, if the N protein is synthesized, it prevents termination of the transcript see Figure , and the lytic functions are expressed. In the former case, this blocking prevents synthesis of the N protein and thus also blocks the extension of transcripts from P L and P R. The wavy lines represent mRNA and show the origin, direction, and extent of transcription.
A vertical arrow at the beginning of an mRNA marks the site where more The cI gene has two promoters: one, P E , for establishing lysogeny, and the second, P M , for maintaining lysogeny. Transcription initiated at each promoter requires an activator protein. For P M , the activator is the cI -encoded repressor itself.
Therefore, to synthesize repressor in a cell without preexisting repressor molecules, P E must be used. P E is activated by the cII product. Thus, cII must be transcribed to establish lysogeny.
One additional protein is required to establish lysogeny, the integrase, encoded by int. The int gene is transcribed from its own promoter, P I Figure , which also requires activation by the cII gene product. The binding of repressor to O R 1 facilitates the binding of a second molecule of repressor to O R 2.
Self-regulation of cI repressor levels. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed. Turn recording back on. National Center for Biotechnology Information , U.
Freeman ; Search term. Lambda phage: a complex of operons. Figure Self-regulation of cI repressor levels. Freeman and Company. Recent Activity. Clear Turn Off Turn On. Lambda phage: a complex of operons - An Introduction to Genetic Analysis. Support Center Support Center.