Mechanism Of DNA uptake
Regulation Of Competence
Evolutionary functions and consequences of
Horizontal Gene Transfer
Competence is the ability of a Bacterial cell to alter its
genetics material by taking up extracellular DNA
from its environment trough plasmamembren in the
process called transformation.
Competence may be differentiated between natural
competence, a genetically specified ability of bacteria
which is thought to occur under natural conditions as
well as in the laboratory, and induced or artificial
competence, which arises when cells in laboratory
cultures are treated to make them transiently
permeable to DNA.
Competence allows for rapid adaptation and DNA
repair of the cell.
Natural competence was discovered by Frederick
Griffith in 1928, when he showed that a preparation
of killed cells of a pathogenic bacterium contained
something that could transform related non -
pathogenic cells into the pathogenic type.
In 1944 Oswald Avery, Colin MacLeod, and Maclyn
McCarty demonstrated that this 'transforming
factor' was pure DNA.
This was the first History compelling evidence that
DNA carries the genetic information of the cell.
Since then, natural competence has been
studied in a number of different bacteria,
particularly Bacillus subtilis, Streptococcus
pneumoniae (Griffith's "pneumococcus"),
staphylococcus aureus and Haemophilus influenzae.
In the laboratory, DNA is provided by the
researcher, often as a genetically engineered
fragment or plasmid.
During uptake, DNA is transported across the cell
membrane(s), and the cell wall if one is present.
Once the DNA is inside the cell it may be
degraded to nucleotides, which are reused for
DNA replication and other metabolic functions.
Alternatively it may be recombined into the cell's
genome by its DNA repair enzymes.
If this recombination changes the cell's genomtype
the cell is said to have been transformed.
Artificial competence and transformation are used
as research tools in many organisms.
In almost all naturally competent
bacteria components of
extracellular filaments called type
IV pili (a type of fimbria) bind
extracellular double stranded
The DNA is then translocated across the
membrane (or membranes for gram negative
bacteria) through multicomponent protein
complexes driven by the degradation of one strand
of the DNA.
Single stranded DNA in the cell is bound by a
well-conserved protein, DprA(Direct Peptide
Reactivity Assay), which loads the DNA onto
RecA (Bacterial DNA Recombination Protein),
which mediates homologous recombination
through the classic DNA repair pathway.
( RecA is a 38 kilodalton Protein essential for
the repair and maintenance of DNA ).
In laboratory cultures, natural competence is usually
tightly regulated and often triggered by nutritional
shortages or adverse conditions.
However the specific inducing signals and
regulatory machinery are much more variable than
the uptake machinery, and little is known about the
regulation of competence in the natural
environments of these bacteria.
Transcription factors have been discovered which
regulate competence; an Regulation of competence
example is sxy (also known as tfoX) which has been
found to be regulated in turn by a 5' non-coding
In bacteria capable of forming spores,
conditions inducing sporulation often overlap
with those inducing competence.
Thus cultures or colonies containing
sporulating cells often also contain competent
Recent research by Süel et al. has identified an
excitable core module of genes which can
explain entry into and exit from competence
when cellular noise is taken into account.
Most competent bacteria are thought to take up all
DNA molecules with roughly equal efficiencies, but
bacteria in the families Neisseriaceae and
Pasteurellaceae preferentially take up DNA
fragments containing short DNA sequences, termed
DNA uptake sequence (DUS) in Neisseriaceae and
uptake signal sequence (USS) in Pasteurellaceae,
that are very frequent in their own genomes.
Neisserial genomes contain thousands of copies of
the preferred sequence GCCGTCTGAA, and
Pasteurellacean genomes contain either
AAGTGCGGT or ACAAGCGGT.
Most proposals made for the primary evolutionary
function of natural competence as a part of natural
bacterial transformation fall into three categories:
(1) the selective advantage of genetic diversity
(2) DNA uptake as a source of nucleotides (DNA
as “food”) and
(3) the selective advantage of a new strand of DNA
to promote homologous recombinational repair of
damaged DNA (DNA repair)
A secondary suggestion has also been made,
noting the occasional advantage of horizontal gene
Hypothesis of genetic diversity
Genetic diversity is the total number
of genetic characteristics in the genetic makeup of
a species. Diversifying selection is
the hypothesis that two subpopulations of a
species live in different environments that select
for different alleles at a particular locus.
Hypothesis of DNA as food
Researchers are demonstrated that the diets of
organisms can affect the DNA sequences of their
genes. Our hypothesis was that the composition of
this food could alter an organism's DNA.
Hypothesis of repair of DNA damage
The DNA repair hypothesis for the
maintenance of sex states that recombination is
necessary for the repair of double-strand DNA
damage. In a closed (mitotic) genetic system
crossing-over generates homozygosity. Thus,
outcrossing is required to restore heterozygosity
destroyed by recombination.
Horizontal Gene Transfer means transfer gene to
A long-term advantage may occasionally be
conferred by occasional instances of horizontal gene
transfer also called lateral gene transfer, (which might
result from non-homologous recombination after
competence is induced), that could provide for
antibiotic resistance or other advantages.
Regardless of the nature of selection for competence,
the composite nature of bacterial genomes provides
abundant evidence that the horizontal gene transfer
caused by competence contributes to the genetic
diversity that makes evolution possible.