Lysogeny

  1. Virulent vs. Temperate Bacteriophages
    1. a virulent phage can only carry out a lytic infection
    2. a temperate phage can do lytic infections or enter a latent or domant state
  2. dormant state of the phage is called a prophage
    1. host cell that contains a prophage is called a lysogen
    2. so, for example, we say that phage lambda is a temperate phage can undergo lysogeny
  3. Overview of lambda infectious programs
    1. lysis
    2. lysogeny
      1. with lambda, this is a temporary state
      2. some prophages are more or less permanently latent
        1. lysogenic conversion can lead to virulence
        2. botulism, cholera, diphtheria  toxins, e.g, are encoded by prophages that convert their bacterial hosts from non-pathogenic to pathogenic
    3. induction is the process that results in the conversion of a prophage into a lytic phageLysog.GIF (23436 bytes)
  4. Requirements of lysogeny
    1. see image
    2. repression of all lytic functions
      1. cI: the lambda repressor, when present and active, will repress lytic functions. The counterpart of cI is cro, a repressor of cI. The initial "decision" that lambda makes is based on the outcome of a battle over cI synthesis
        1. lambda DNA is injected and circularized
        2. initially, cII is made
        3. cII is a positive regulator of cI synthesis. If cII is around long enough, then cI will be made, and cI will repress cro and other lytic functions
        4. if cII is degraded quickly, cro will build up, and cI will not be synthesized
      2. whether cII is degraded or not depends on the health of the host. A healthy cell will degrade cII quickly, and in effect signal to lambda that a lytic cycle would be good. An unhealthy cell will not degrade cII, which is like telling lambda that the cell is too sick to make viral progeny, so lysogeny is the better idea
    3. passive maintenance of phage genome
      1. lambda circle integrates into the host chromosome to form the lambda prophage. The process requires enzymes (lambda integrase (int) and IHF.
      2. requires a special site (hence, we call it site-specific recombination). The site is called att (attachment site), and exists in slightly different forms on phage and chromosome. Phage site is attP, and chromosomal site is attB.
      3. POP and BOB
        1. Campbell recombination produces BOP and POB, with prophage in between
        2. notice different sequence of genes in prophage vs. linear viral genome in particle
    4. The reverse of integration happens upon induction
      1. UV light is a good inducer
      2. induction actually involves RecA. RecA is activated by ssDNA. Activated RecA interacts with cI, and causes cI to proteolyze itself. Without cI, lytic functions are derepressed, and the lytic cycle begins.
      3. induction also results in the synthesis of both Int and Xis
      4. only int is required for integration, but both are required for excision
      5. normal excision (which usually occurs) produces the cicular viral genome, and lysis continues
  5. Specialized Transduction
    1. mistakes can occur during the excision step of induction of lambda
      1. first, look at the sequence around the att site in chromosome
      2. gal att bio in chromosome of host
      3. gal att<prophage>att bio of lysogen

        spec trans.GIF (21074 bytes)

    2. one mistake leads to formation of  ldgal
      1. gal because gal marker is mobilized
      2. d for defective phage
      3. defective because morphology genes are missing in this phage
      4. cannot carry out a lytic infection of another host cell, but it can inject its DNA into another host.  If that happens, that host can be the recipient of foreign DNA.   The gal allele (in this case) can be incorporated, by homologous recombination, into the chromosome of the recipient, in exchange for the host allele.  So, e.g., if the recipient was originally gal-, it might be converted to gal+ by specialized transduction.
    3. other possibility is lbio, which is not defective for lysis because it only lacks regulatory genes.  Thus, it can inject its DNA into a host, and potentially can mount a successful lytic infection, but it cannot lysogenize that host.