Microbiology Module 8

  1. Creating Genetic Diversity via Horizontal Gene Transfer (Slide 2)
    Conjugation: Plasmid transferred and replicates autonomously

    Transformation: DNA is taken up and incorporated via homologous recombination

    Transduction: Phage carries DNA that is incorporated into genome via homologous recombination or as prophage

    In all cases DNA may be degraded if it is recognized as foreign
  2. Bacterial Conjugation (Slide 3)
    Gram (-) Negative Cells: Special conjugative plasmids contain genes that cause the transfer of plasmid from one cell to another (often unrelated genetically)

    Some plasmids contain genes that result in transfer of themselves from bacteria to eukaryotic cells

    Ti (Tumor-Inducing) plasmid is transferred from Agrobacteria to plant cells
  3. Bacterial Conjugation Cont. (Slide 4)
    R Plasmids: Special conjugative plasmids also transfer antibiotic resistance genes

    Many conjugative plasmids carry virulence genes that confer the ability to cause disease

    Some Gram (+) Positive cells also employ conjugation as a mechanism to create genetic diversity
  4. F Factor (Plasmid) mediated conjugation in Gram (-) Negative Cells (Slide 5)
    Sex pilus (F Pilus) from F+ cell attaches to F- (without plasmid) and reels it in

    SS-DNA is synthesized from oriT on F plasmid and moves into F- cell via Type IV secretion system

    SS-DNA is replicated and circularizes

    Result is 2 F+ cells
  5. Type IV Secretion System (Slide 6)
    Proteins for system are encoded in the tra operon (28 genes), all of which are on the F plasmid

    Uses some components from sex pilus (proteins for pilus also on F plasmid)

    ATP provides energy for transfer of ssDNA plus some proteins to F- cell
  6. Bacterial Transformation (Slide 7)
    Uptake and integration of exogenous DNA

    Natural transformation is rare. Observed in a few species of gram positive and negative bacteria

    In most species involves 8-30 gene products and is inducible.

    Non-Plasmid DNA is stably incorporated via recombination. i.e. must have sequences homologous to host chromosome
  7. Bacterial Transformation Cont. (Slide 8)
    Only competent cells within a population can take up exogenous DNA

    Competence rates between 0.1-100%, depending on species

    In lab, competence (1%) induced by electrical current or via cold CaCl2

    In lab, transformation typically is done with plasmids that autonomously replicate within cells
  8. DNA uptake systems used in natural transformation (Slide 9)
    DS-DNA binds to receptor; SS-DNA enters cell, hybridizes with chromosome and is incorporated via recombination (must be genetically related to recipient)

    In some species, only a few specific DNA sequences bind to receptor
  9. Transduction (Slide 10)
    Mediated by bacteriophage (either virulent/lytic or temperate/lysogenic)

    Temperate phage may go through several lytic cycles before they lysogenize into chromosome of host cells to become prophage and only go in FEW spaces in the chromosome

    Induction of prophage excision occurs in response to environmental factors or DNA damage. Lytic stage reappears
  10. Generalized Transduction (Slide 11)
    Happens during lytic cycle when phage construct heads and tails as separate structures and then reel their DNA up into the heads

    Rarely (1-2%) microbial host DNA gets packaged instead of phage DNA --> transducing phage

    Transducing phage DNA can be introduced into another cell and incorporated via homologous recombination

    Transduced DNA is random
  11. Specialized Transduction (Slide 12)
    Occurs only with temperate bacteriophage during the end of the lysogenic cycle

    Very infrequently (1 in a million) prophage excision carries part of adjacent host DNA and this DNA is packaged and injected into new host

    Transducing phage may be infectious

    Sequences transduced are only those adjacent to a few phage insertion sequences therefore specialized
  12. Transposable Elements (Transposons) (Slide 13)
    Jumping Genes. Occur in bacteria and archaea. Found on chromosomes, phage or plasmids and are able to jump between segments of DNA. Are autoregulated

    Insertion Sequence (IS) contain inverted Repeats (IR) flanking transposase genses

    Composite Transposons (Tn) contain auxiliary genes (often toxins or antibiotic resistance) flanked by IS
  13. Transposons (Slide 14)
    Some transposons complete excise when they jump--Cut and Paste mechanism of transposition

    Some composite transposons contain resolvase genes that encode proteins that replicate transposon that moves --> replicative mechanism of transposition, resulting in many copies of transposon in the host DNA
Card Set
Microbiology Module 8
Module 8