From Wikipedia,
the free encyclopedia.
Bacterial conjugation is
the transfer of genetic material
between bacteria through
cell-to-cell contact. (as opposed
to
transformation or
transfection)
Bacterial conjugation is often
regarded as the
bacterial equivalent of
sexual reproduction or
mating. However, it is not
actually sexual, as it does not
involve the fusing of
gametes and the creation of a
zygote. It is merely the
transfer of genetic information
from a donor cell to a recipient.
In order to perform conjugation,
one of the bacteria, the donor,
must play host to a conjugative or
mobilizable genetic element, most
often a conjugative
plasmid. Most conjugative
plasmids have systems ensuring
that the recipient cell
does not already contain a similar
element.
These elements are best viewed
as genetic parasites on the
bacterium, and conjugation as a
mechanism evolved by the element
to spread itself into new hosts.
The
prototype for conjugative
plasmids is the F-plasmid,
also called the F-factor. The
F-plasmid is an
episome (a plasmid that can
integrate itself into the
bacterial
chromosome by
genetic recombination) of
about 100 kb length. (One kb is
one thousand
base pairs) It carries its own
origin of replication, called
oriV. There can only be one
copy of the F-plasmid in a
bacterium (which is then called
F-positive), either free or
integrated.
Among other genetic
information, the F-plasmid carries
a tra and a trb
locus, which together are
about 33 kb long and consist of
about 40
genes. The tra locus
includes the pilin gene and
regulatory genes, which together
form
pili on the cell surface,
polymeric
proteins that can attach
themselves to the surface of
F-negative bacteria and
initiate the mating. Though there
is some debate on the issue, the
pili themselves do not seem to be
the structures through which the
actual exchange of DNA takes
place; rather, some proteins coded
in the tra or trb
loci seem to open a channel
between the bacteria.
When conjugation is initiated,
via a mating signal, a complex of
proteins called the relaxosome
creates a
nick in one plasmid DNA strand
at the origin of transfer, or
oriT. In the F-plasmid system,
the relaxosome consists of
proteins TraI, TraY, TraM, and the
integrated host factor, IHF. The
transferred, or T-strand,
is unwound from the duplex and
transferred into the recipient
bacterium in a 5'-terminus to
3'-terminus direction. The
remaining strand is replicated,
either independent of conjugative
action (vegetative replication,
beginning at the oriV) or
in concert with conjugation
(conjugative replication similar
to the
rolling circle replication of
lambda phage).
If the F-plasmid becomes
integrated into the host genome,
donor chromosomal DNA may be
transferred along with plasmid
DNA. The amount of chromosomal DNA
that is transferred depends on how
long the bacteria remain in
contact; for common laboratory
strains of
E. coli the transfer of
the entire bacterial chromosome
takes about 100 minutes. The
transferred DNA can be integrated
into the recipient genome via
recombination.
A culture of cells containing
non-integrated F plasmids usually
contains a few that have
accidentally become integrated,
and these are responsible for the
low-frequency of chromosomal gene
transfer by such cultures. Strains
of bacteria with an integrated
F-plasmid can be isolated and
grown in pure culture. Because
such strains transfer chromosomal
genes very efficiently, they are
called Hfr (high
frequency of recombination).
The E. coli
genome was originally mapped
by interrupted mating experiments,
in which various Hfr cells in the
process of conjugation were
sheared from recipients after less
than 100 minutes (initially using
a Waring blender) and
investigating which genes were
transferred.
See also
