Microbiology Module 8

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

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

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

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

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

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

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

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

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

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

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

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

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