Download Mechanisms of horizontal gene transfer (HGT) in bacteria DNA can

Zakład Biotechnologii i Inżynierii Genetycznej SUM
Gene & genome manipulation 2016/2017
Genetic transformation of prokaryotic cells
Mechanisms of horizontal gene transfer (HGT) in bacteria
DNA can be exchanged among bacteria by three methods: transformation, transduction and conjugation.
http://textbookofbacteriology.net/resantimicrobial_3.html
In transformation, pieces of DNA released from donor bacteria are taken up directly from the
extracellular environment by recipient bacteria. Recombination occurs between single molecules of
transforming DNA and the chromosomes of recipient bacteria. Molecules of transforming DNA correspond
to very small fragments of the bacterial chromosome.
Transformation was first discovered in bacteria. The initial experiment on transformation was performed by
Frederick Griffith in England in 1928 when he was working with two strains of Streptococcus pneumoniae.
Griffith’s experiments were based on the fact that S. pneumoniae makes two types of different appearing
colonies, one type made by pathogenic bacteria and the other type made by bacteria that are incapable of
causing infections. The colonies made by the pathogenic strains appear smooth on agar plates, because the
bacteria excrete a polysaccharide capsule. Bacterial transformation has been widely investigated in
Pneumococcus, Haemophilus influenzae, Bacillus subtilis and certain other bacteria. During the process of
transformation, genes are transferred from one bacterium to another as ‘naked’ DNA solution. In nature,
some bacteria, perhaps after death and cell lysis, release their DNA into the environment. Other bacteria can
then encounter the DNA and, depending on the particular species and growth conditions, take up fragments
of DNA and integrate them into their own chromosomes by recombination. Transformation works best when
the donor and recipient cells are very closely related.
http://www.ncbi.nlm.nih.gov/books/NBK7908/
Zakład Biotechnologii i Inżynierii Genetycznej SUM
Gene & genome manipulation 2016/2017
Genetic transformation of prokaryotic cells
Two elements are required in a transformation system. The first element is a suitable host bacterium. For
this, commonly we use E.coli as host organism. The strain of E.coli has been cultured in the laboratory and
it has been selected for characteristics that make it especially useful in the molecular biology laboratory.
Plasmid is the other important element in the transformation system. Plasmid encodes some enzymes and
antibiotic resistant markers which are expressed in the bacterium after transformation.
There are two forms of transformation: natural and artificial, each process depends on the ability of the
organism to transform the DNA into the host cells. In natural transformation, bacteria are capable of DNA
naturally which means they can take up DNA from their environment directly. That kind of bacteria is called
as naturally transformable. In artificial transformation, bacterial cells are not naturally transformable and
they have been exposed to particular chemical or electrical treatments to make them more permeable and
then only the cells can take up DNA efficiently.
Transduction is a process whereby DNA is introduced into a host cell with the help of a phage
particle or a bacterial virus. These bacteriophage are infectious particles and are referred to as transducing
phage. Phage infection ordinarily involves attachment to specific receptors at the surface of host cell. The
phage genome is then injected like a needle into the host.
Phage infection can proceed either by the lysogenic or the lytic cycle. The lytic cycle involves infection,
replication and production of phage particles, and lyses of the host bring about the release of phage. The
lysogenic cycle involves infection, and integration resulting in the silencing of the phage genome.
Generalized transduction results from infection with a transducing phage that has randomly picked up a
fragment of the host genome usually during a lytic infection. Specialized transduction follows in the specific
case when the phage genome after it enters the host, is silenced by repressors and the integrates into the host
genome.
In conjugation, transfer of genetic material between bacteria through direct cell-to-cell contact is
needed. It requires a pilus or conjugation bridge which is composed often of a single protein. This is
mediated by a plasmid, a small extrachromosomal, circular DNA molecule which is autonomous, that is
capable of its own replication. Plasmid genes that confer resistance to antibiotics, or which encode some
other useful metabolic function can be transferred to sensitive strains making them resistant.
During conjugation the plasmid replicates in such a way that only one of the two strands is transferred via
the pilus. The other strand remains behind and is replicated. Cells that contain the plasmids are referred to as
male or + strains. + strains are only capable of conjugating with female or - cells that do not have a copy of
the plasmid. In this way the male cell remains male, and the female becomes male.
http://www.ncbi.nlm.nih.gov/books/NBK7908/