Competence (biology)
Encyclopedia
In microbiology
, genetics
, cell biology
and molecular biology
, competence is the ability of a cell
to take up extracellular ("naked") DNA
from its environment. 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. This article primarily deals with natural competence in bacteria. Information about artificial competence is provided in the article Transformation (genetics)
.
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 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"), Neisseria gonorrhoeae
and Haemophilus influenzae
. Areas of active research include the mechanisms of DNA transport, the regulation of competence in different bacteria, and the evolutionary function of competence.
of other cells, but in the laboratory it 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 genotype
the cell is said to have been transformed. Artificial competence and transformation are used as research tools in many organisms (see Transformation (genetics)
).
In almost all naturally competent bacteria components of extracellular filaments called type 4 pili (a type of fimbria
) are involved in the transformation process, and DNA may enter the cells via DNA translocase. Some bacteria cut the DNA into short pieces before transporting it; others can take up very long intact fragments and circular plasmids. The details of the uptake machinery are not yet fully characterized in any system.
s have been discovered which regulate competence; an example is sxy (also known as tfoX) which has been found to be regulated in turn by a 5' non-coding RNA element
. 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 cells. 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), 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.
ary functions of natural competence are controversial. Competence has conventionally been viewed as a mechanism that cells evolved to provide themselves with novel genetic information. However the theoretical difficulties associated with the evolution of sex suggest that this explanation is problematic. Cells that take up DNA inevitably acquire the nucleotides the DNA consists of, and, because nucleotides are needed for DNA and RNA
synthesis and are expensive to synthesize, these may make a significant contribution to the cell's energy budget. In principle, competence could also allow cells to replace heavily damaged DNA in the cell's genome if needed.
Regardless of the nature of selection for competence, the composite nature of bacterial genomes provides abundant evidence that the lateral gene transfer caused by competence contributes to the genetic diversity that makes evolution possible.
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
, genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, cell biology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
and molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
, competence is the ability of a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
to take up extracellular ("naked") DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
from its environment. Competence may be differentiated between natural competence, a genetically specified ability of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
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. This article primarily deals with natural competence in bacteria. Information about artificial competence is provided in the article Transformation (genetics)
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
.
History
Natural competence was discovered by Frederick GriffithFrederick Griffith
Frederick Griffith was a British bacteriologist whose focus was the epidemiology and pathology of bacterial pneumonia. In January 1928 he reported what is now known as Griffith's Experiment, the first widely accepted demonstrations of bacterial transformation, whereby a bacterium distinctly...
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
Oswald Avery
Oswald Theodore Avery ForMemRS was a Canadian-born American physician and medical researcher. The major part of his career was spent at the Rockefeller University Hospital in New York City...
, Colin MacLeod
Colin MacLeod
Colin Munro MacLeod was a Canadian-American geneticist.- Biography :Born in Port Hastings, Nova Scotia, Canada MacLeod entered McGill University at the age of 16 , and completed his medical studies by age 23.In his early years as a research scientist, MacLeod, together with Oswald Avery and...
, and Maclyn McCarty
Maclyn McCarty
Maclyn McCarty was an American geneticist.Maclyn McCarty, who devoted his life as a physician-scientist to studying infectious disease organisms, was best known for his part in the monumental discovery that DNA, rather than protein, constituted the chemical nature of a gene...
demonstrated that this 'transforming factor' was pure DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
. This was the first 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
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
, Streptococcus pneumoniae
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...
(Griffith's "pneumococcus"), Neisseria gonorrhoeae
Neisseria gonorrhoeae
Neisseria gonorrhoeae, also known as gonococci , or gonococcus , is a species of Gram-negative coffee bean-shaped diplococci bacteria responsible for the sexually transmitted infection gonorrhea.N...
and Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...
. Areas of active research include the mechanisms of DNA transport, the regulation of competence in different bacteria, and the evolutionary function of competence.
Mechanisms of DNA uptake
In the natural world DNA usually becomes available by death and lysisLysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
of other cells, but in the laboratory it is provided by the researcher, often as a genetically engineered fragment or plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
. During uptake, DNA is transported across the cell membrane(s)
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, and the cell wall
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
if one is present. Once the DNA is inside the cell it may be degraded to nucleotides, which are reused for DNA replication
DNA synthesis
DNA synthesis commonly refers to:*DNA replication - DNA biosynthesis *Polymerase chain reaction - enzymatic DNA synthesis *Oligonucleotide synthesis - chemical synthesis of nucleic acids...
and other metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
functions. Alternatively it may be recombined
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
into the cell’s genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
by its DNA repair
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
enzymes. If this recombination changes the cell’s genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
the cell is said to have been transformed. Artificial competence and transformation are used as research tools in many organisms (see Transformation (genetics)
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...
).
In almost all naturally competent bacteria components of extracellular filaments called type 4 pili (a type of fimbria
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
) are involved in the transformation process, and DNA may enter the cells via DNA translocase. Some bacteria cut the DNA into short pieces before transporting it; others can take up very long intact fragments and circular plasmids. The details of the uptake machinery are not yet fully characterized in any system.
Regulation of competence
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 factorTranscription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
s have been discovered which regulate competence; an example is sxy (also known as tfoX) which has been found to be regulated in turn by a 5' non-coding RNA element
Sxy 5' UTR element
The Sxy 5' UTR element is an RNA element that controls expression of the sxy gene in H. influenzae. The sxy gene is a transcription factor that regulates competence which is the ability of bacteria to take up DNA from their environment. When the sxy gene is deleted the bacterium loses the ability...
. In bacteria capable of forming spores
Endospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
, conditions inducing sporulation often overlap with those inducing competence. Thus cultures or colonies containing sporulating cells often also contain competent cells. 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
Neisseriaceae
The Neisseriaceae are a family of Proteobacteria, given their own order. While many organisms in the family are mammalian commensals or part of the normal flora, the genus Neisseria includes 2 important human pathogens, specifically those responsible for gonorrhea and many cases of meningitis...
and Pasteurellaceae
Pasteurellaceae
Pasteurellaceae comprise a large and diverse family of Gram-negative Proteobacteria with members ranging from important pathogens such as Haemophilus influenzae to commensals of the animal and human mucosa. Most members live as commensals on mucosal surfaces of birds and mammals, especially in the...
preferentially take up DNA fragments containing short DNA sequences, termed DNA uptake sequence (DUS), that are very frequent in their own genomes. Neisserial genomes contain thousands of copies of the preferred sequence GCCGTCTGAA, and Pasteurellacean
Pasteurellaceae
Pasteurellaceae comprise a large and diverse family of Gram-negative Proteobacteria with members ranging from important pathogens such as Haemophilus influenzae to commensals of the animal and human mucosa. Most members live as commensals on mucosal surfaces of birds and mammals, especially in the...
genomes contain either AAGTGCGGT or ACAAGCGGT.
Evolutionary functions and consequences of competence
The evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary functions of natural competence are controversial. Competence has conventionally been viewed as a mechanism that cells evolved to provide themselves with novel genetic information. However the theoretical difficulties associated with the evolution of sex suggest that this explanation is problematic. Cells that take up DNA inevitably acquire the nucleotides the DNA consists of, and, because nucleotides are needed for DNA and RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
synthesis and are expensive to synthesize, these may make a significant contribution to the cell's energy budget. In principle, competence could also allow cells to replace heavily damaged DNA in the cell's genome if needed.
Regardless of the nature of selection for competence, the composite nature of bacterial genomes provides abundant evidence that the lateral gene transfer caused by competence contributes to the genetic diversity that makes evolution possible.