2. DNA exchange among bacteriaDNA exchange among bacteria
• DNA can be exchanged among bacteria in three
The transfer of genetic material between
bacterial cells by direct cell-to-cell contact or by a
bridge-like connection between two cells.
A phage carries DNA from one bacterium to
Cells take up free DNA directly from their
• In 1928, Fred Griffith found that one form of the
pathogenic pneumococci (now called
Streptococcus pneumoniae) could be
mysteriously “transformed” into another form.
• Griffith made a conclusion that the dead
pathogenic bacteria gave off a “transforming
principle” that changed the live nonpathogenic
rough-colony-forming bacteria into the
pathogenic smooth-colony form.
• The ability of some bacteria to take up naked
DNA from their environment.
• It is genetically programmed. Generally, more
than a dozen genes are involved, encoding both
regulatory and structural components.
• The general steps that occur in natural
transformation differ in Gram-negative
and positive bacteria.
8. Natural TransformationNatural Transformation
• In this case DNA take-up occurs without outside
• Naturally competent bacterium – They can
take up DNA from the environment without
requiring special treatment.
• About 40 species have been found to be
naturally competent or transformable.
• Examples: Bacillus subtilis, Streptococcus
pneumoniae, Haemophilus influenzae, Neisseria
gonorrhoeae, Helicobacter pylori, Acinetobacter
baylyi, and some species of marine
9. Steps involved in naturalSteps involved in natural
Following proteins are involved in this process:
•ComEA: binds directly extracellular double-stranded DNA.
•The comF genes encode proteins that translocate the
DNA into the cell.
• ComFA provides the energy for translocation of DNA
through the membrane.
•ComEA, ComEC, and ComFA form a sort of ATP-
DNA into the cell.
•The genes in the comG operon encode proteins that
might form a “pseudopilus” which helps move DNA
through the ComEC channel.
They might bind to extracellular DNA, perhaps acting
through the ComEA DNA-binding protein, and then retract,
drawing the DNA into the cell.
• The comE, comF and comG operons are all under the
transcriptional control of ComK.
• Some of genes involved in the transformation process
are not designated as com, because such genes were
first discovered on the basis of their involvement in other
1. The nucA gene product makes double-strand breaks
in extracellular DNA.
2. Other examples are single-stranded-DNA binding
protein (SSB), and RecA functions in the
recombination of transforming DNA with chromosome
• The lengths of single-stranded DNA
incorporated into the recipient chromosome are
about 8.5 to 12 kb on cotransformation of
• incorporation takes only few minutes to be
15. Artificially induced competenceArtificially induced competence
• Bacteria can be sometimes be made competent by
certain chemical treatments or DNA can be forced into
bacteria by a strong electric field in a process called
1. Chemical Treatment (with calcium ions).
• Chemically induced transformation is usually
inefficient, and only a small percentage of the cells are
• The cells must be plated under conditions, selective
for the transformed cells.
• Therefore, the DNA used for the transformation should
contain a selectable gene such as encoding resistance
to an antibiotic.
16. Artificially induced competenceArtificially induced competence
• The bacteria are mixed with DNA and briefly
exposed to a strong electric field.
• The bacteria first be washed extensively in
buffer with very low ionic strength such as
• The brief electric field across the cellular
membranes might create artificial pore of H2O
lined by phospholipid head groups. DNA can
pass through these temporary hydrophilic
• Electroporation requires specialized equipment.
18. Applications of TransformationApplications of Transformation
Bacterial transformation is used:
•To make multiple copies of DNA, called DNA
•To make large amounts of specific human
proteins, for example, human insulin, which can
be used to treat people with Type I diabetes.
•To genetically modify a bacterium or other cell.
Notes de l'éditeur
ComEA encoded by the first gene of the comE operon,
ComFA is an ATPase
ComK, a transcriptional factor that is itself regulated by ComA.
The free DNA ends become the substrates for the competence proteins.
ComEA binds directly extracellular double- stranded DNA.
The comF genes encode proteins that translocate the DNA into
ComEA, ComEC, and ComFA form a sort of ATP-binding
cassette (ABC) transporter.
The genes in the comG operon encode proteins that might form
a “pseudopilus” which helps move DNA through the ComEC
channel, and the ComECs retract,drawing the DNA into the cell.
PilQ (secretion proteins): 12 ~ 14 copies making the pore through the outer membrane.
. ComA protein of Neisseria is an ortholog of ComEC of B. subtilis.
2. The DNA is shown running through the cell wall alonside
the pseudopilus (ComG in B. subtilis; PilE in G – systems).
3. In most G – bacteria specific sequences are required for the
binding of DNA, so that these species usually take up DNA only
of the same species.
. the buffer usually also contains a nonionic solute such glycerol to prevent osmotic shock.