Bacterial Gene Transfer
Bacterial gene exchange differs from eukaryotes:
- Bacteria do
not exchange genes by meiosis. (Why not?) They rarely
exchange two entire genomes.
- Bacteria commonly
exchange small pieces of genome, a few genes at a time, through transformation,
transduction, or conjugation.
- Transfer between
species, even kingdoms, is common; less common in eukaryotes,
though it does occur.
Transformation.
Transformation is the uptake of DNA
from outside the cell. Only a single strand is taken up, through
a special protein complex in the cell membrane. The process
requires calcium ion (Ca2+). Transformation occurs at extremely
low frequency, but with large populations of bacteria, it offers a significant
route for genetic transfer.
Phage
Transduction
There are two types:
- Generalized
transduction (depicted
below). A piece of host DNA gets packaged by mistake, instead
of the phage DNA. This rare event results in a phage delivering
only bacterial DNA to the next host. The DNA then
recombines homologously, replacing the
host allele.
- Specialized
transduction,
in which a lysogenic prophage recombines itself out of the genome
(by site-specific recombination)
and mistakenly includes a piece of bacterial DNA. The resulting
phage progeny can infect cells to produce lysogens with a second
copy of the allele they
had packaged, attached to the phage DNA.
Diagram
of generalized transduction:
Plasmids
Plasmids
are small circles of DNA that contain an origin of replication (ori)
and a small number of genes, some of which may confer a survival advantage
on a host. Some plasmids can transfer between different species;
even between different kingdoms. A
shuttle vector is
a plasmid engineered in the test tube to contain an ori site for bacteria,
and an ori site for animal or plant cells. Shuttle vectors are
enormously useful to clone a gene conveniently in bacteria, then express
it in tissue culture.
Conjugation
Conjugation
is the process by which a plasmid is transferred from an F+ cell into
an F- cell. The F factor in the F+ cell contains genes which express
pili for attachment, and special membrane proteins for the transfer
complex. Some conjugative plasmids carry drug resistant strains--a
big problem for hospitals. If
an F plasmid is integrated into a host genome (an Hfr, for high
frequency recombination)
the F factor can transfer part or all of the genome into the recipient
F- cell.
 I
Electronic
Companion to Genetics, Cogito Learning MediaI
Episomes
and Hfr
The F plasmid can recombine itself into the host
chromosome by site-specific recombination. It can then (a) transfer
part or all of the chromosome into a recipient F- cell, as an Hfr; or
(b) recombine itself out again, and mistakenly pick up a piece of the
host chromosome to carry into the next F- host.
Problem
(5) Explain two different genetic processes in bacteria that can
create a "partial diploid" for a small part of the genome. Explain
why these processes are useful for bacterial genetic analysis.
Mobile
genes.
Some genes, such those encoding resistance to antibiotics,
can move from one genome to another, at a new place in the genetic map.
Some of these mobile genes can even transfer between two distantly related
species of organism.
Transposable
elements
The first transposable elements to
be characterized genetically were controling elements for seed coat
color in maize (corn.) Barbara McClintock
won the Nobel Prize for showing that
DNA is not all "fixed" in the genome, but that some sequences can insert
and excise by intramolecular recombination.
There
are many classes of transposons.
In bacteria, a common structure of a transposon contains:
- An insertion
sequence (IS) at
the right and left ends. The IS contains the gene encoding the
transposase enzyme.
- A
gene encoding antibiotic resistance.
This gene confers a selective advantage
to bacteria containing the transposon, in the presence of the
antibiotic.
Transposons
Some
bacterial transposons can be exchanged
among many different species, usually
carried by plasmids.
Other transposed pieces of DNA can be
inverted at
one place in one species, to turn on or off the regulation of a gene.
An example of such a site-specific transposition event is the flagellar
gene regulation catalyzed by hin
recombinase.
Genomic Islands
| Bacterial
genomes often contain "islands" of DNA transferred relatively
recently from another species. The "genomic
island" may confer special properties to a pathogen,
or two a strain inhabiting a special niche in an ecosystem. |
Herbert Schmidt and Michael Hensel, Clinical Microbiology
Reviews, January 2004, Vol. 17, p. 14-56.
A
pathogenicity island is a genomic
island that converts a "harmless" bacterium to a pathogen.
The pathogenicity
island is a distinct region of DNA present in the genome of
pathogenic bacteria but absent in nonpathogenic strains of the
same species. Transfer may be mediated by an integrase
enzyme (int).
The island
is typically inserted at a tRNA gene
in the core genome.
The pathogenicity
island typically contains virulence genes
(V1 to V4) interspersed with other mobility
elements, such as insertion sequences
(IS).
How
do we recognize a recently inserted pathogenicity island? One
clue typically is a difference in percent
GC content, compared to the core genome. |
Examples
of Pathogenicity Islands
A.
The cag island of H.
pylori (cause of stomach ulcers) harbors genes for a type
IV secretion system that can translocate the toxin CagA into
human cells, causing an inflammatory response.
B. The SP-1
island of Salmonella typhimurium (typhus and food poisoning)
encodes a type III secretion system (grey), secreted proteins
(dark grey), and regulatory proteins. The same island includes
metabolic proteins unrelated to virulence.
C. The HPI
island of Yersinia enterocolitica has genes that encode
a high-affinity iron uptake system (dark grey) needed for extracellular
growth of the pathogen during colonization of the host.
D. The vSAL
island of multiple drug-resistant Staphylococcus
aureus (MRSA) encodes a remarkably high number of
enterotoxins. |
Genomic islands in two strains
of a marine phototroph, Prochlorococcus
The
two strains of Prochlorococcus marinus are cyanobacteria, major
oxygenic producers in the oceans, consuming a large part of atmospheric
CO2. The strains MED4 and MIT96512 differ by only 0.8% of
their genome, yet their distributions throughout the ocean are very
different, for unknown reasons. The reason for the difference in distribution
may have to do with genes encoded within five genomic islands specific
to MED4 (ISL1, ISL4, ISL5) or to MIT9312 (ISL2, ISL3).
|
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