Gene Transfer in Bacteria

Transformation – free DNA released from a donor is taken up by a recipient

mostly found in Streptococci, Neisseria, and Haemophilus (relevent in clinical setting)

others can be forced to undergo transformation via electrical stimulation (i.e., E. coli) – electroporation

Griffith/Avery experiments: led to discovery that DNA is genetic material (see fig 1)

streptococci

quorum sensing: streps secrete ‘competence factor’; concentration is sensed and tells the bug if there are any other streptococci around; if high enough concentration, receptor turns on the transformation proteins (10-12 proteins)

once transformation is turned on, bug will take up any DNA; because only turned on in high strep concentration, makes it more likely that DNA will be from other streps Þ specificity

also, if DNA is non-strep, may not be able to find homologous site for insertion Þ specificity

DNA comes from lysed bugs; only one strand enters the cell

heteroduplex is formed; mismatches are repaired Þ variation

haemophilus

no quorum sensing; always competent if in nutrient-rich environment

can only take up haemophilus DNA Þ specificity

based on a signal sequence inserted all over haemophilus DNA

heteroduplex is formed; mismatches are repaired Þ variation

neisseria

no quorum sensing; competent in presence of pili

Transduction – DNA is carried from donor to recipient in a bacteriophage capsid

generalized transduction

donor cell is infected with bacteriophage

error in phage assembly allows bacterial DNA to become packaged in the new phage

1-2% of bacterial DNA on average can fit in a phage

the next cell infected with the altered phage gets the donor DNA and homologous recomb ensues

phage attributes (i.e. lysis) are lost because they cannot fit into phage Þ new host cell lives

specialized transduction

involves a temperate phage (capable of symbiotic relationship with host; DNA incorporated in host genome)

host cell replicates, etc.; few if any viral genes expressed

this state is called lysogeny; symbiont called lysogen; inserted DNA called prophage

some trigger awakens the phage (UV, temp, etc.); phage replicates and lyses cell

errors in phage excision from host genome leads to phage with host DNA Þ transducing particle

when inserts into new host, carries donor DNA

most phages insert into only specific segments of host DNA Þ genes can be transferred

Conjugation – DNA is transferred from donor to recipient via direct cell-cell contact

R-plasmid: resistance; F-plasmid: fertility (contains genes for conjugation)
F is 1-2% size of bacterial genome
exchange events

sex pilus is formed and draws cells into close association
plasmid is nicked at origin of transfer (oriT)
one strand of DNA is threaded to other bacteria
complementary strand sythesized in both cells during transfer
complete transfer takes ~2 min

transfer of chromosomal genes

F can insert into bacterial chromosome
at conjugation, bacterial DNA is threaded though because it is effectively a giant plasmid
complete transfer would take 1.5 hours, but bacteria tend to separate before that
because plamid usually inserts at a few specific sites, some genes more susceptible
strain with inserted F is called Hfr - high frequency of recombination
when F excises along with host DNA it is called F’
so F has three forms: F; Hfr; F’ (fig. 4)

Limits to Transfer Across Species

necessity of homologous recombination

restricted host range of plasmids and viruses

restriction enzymes in many bacteria that degrade foreign DNA