Homoserine Lactone
Encyclopedia
N-Acyl homoserine lactones (AHLs or N-AHLs) are a class of signaling molecule
s involved in bacterial quorum sensing
. Quorum sensing is a method of communication between bacteria that enables the coordination of group-based behavior based on population density. They signal changes in gene expression
, such as switching between the flagella gene and the gene for pili
for the development of a biofilm
.
s are produced within the cell
and are released into the environment. The resulting concentration of signaling molecules in the environment is dependent upon population density
. Once the population density has reached a particular threshold, gene expression can begin. This allows bacteria to coordinate group-based behavior. N-AHLs produced by different bacteria differ in the length of the R-group side-chain. Chain lengths vary from 4 to 18 carbon
atoms and in the substitution of a carbonyl
at the third carbon. It has also been suggested that N-AHLs alter local surface tension enough to create Marangoni flows
which facilitate swarming and colony motility.
in the luminescent bacteria Vibrio fischeri
. Similar pathways occur in other luminescent bacteria. In Vibrio fischeri, AHL binds to the protein product of the LuxR gene and activates it. The C-terminal domain of activated LuxR relieves the repression exerted by H-NS nucleoid proteins that bind to the promoters of LuxR, LuxI and the LuxCDABEG operon
, as well as to A-T-rich stretches within that operon and other genomic regions. The product of LuxI catalyses the synthesis of AHL. Thus, AHL acts as an autoinducer. Transcription of the LuxCDABEG operon results in luminescence due to the expression of LuxA and LuxB, which form a protein known as a luciferase
and the expression of LuxC, D, E, and G, which are involved in the synthesis of the luciferase's substrate, tetradecanal. This is an important feature of quorum sensing
, as it makes little sense for one cell to waste the energy producing light, as the resulting light will be so faint that it will be more or less undetectable. Instead, once the bacterial population has reached a specific size, only then does light production commence.
(CNBR) with a methionine
residue in a protein. This reaction is important for chemical sequencing of proteins, as the Edman degradation
process is unable to sequence more than 70 consecutive residues.
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s involved in bacterial quorum sensing
Quorum sensing
Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest...
. Quorum sensing is a method of communication between bacteria that enables the coordination of group-based behavior based on population density. They signal changes in gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
, such as switching between the flagella gene and the gene for pili
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...
for the development of a biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
.
Mechanism
Signaling moleculeSignaling molecule
A signaling molecule is a chemical involved in transmitting information between cells. Such molecules are released from the cell sending the signal, cross over the gap between cells by diffusion, and interact with specific receptors in another cell, triggering a response in that cell by activating...
s are produced within the 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....
and are released into the environment. The resulting concentration of signaling molecules in the environment is dependent upon population density
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
. Once the population density has reached a particular threshold, gene expression can begin. This allows bacteria to coordinate group-based behavior. N-AHLs produced by different bacteria differ in the length of the R-group side-chain. Chain lengths vary from 4 to 18 carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms and in the substitution of a carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
at the third carbon. It has also been suggested that N-AHLs alter local surface tension enough to create Marangoni flows
Marangoni effect
The Marangoni effect is the mass transfer along an interface between two fluids due to surface tension gradient...
which facilitate swarming and colony motility.
Example
One example of the involvement of AHLs in quorum sensing is in the regulation of the bioluminescent protein luciferaseLuciferase
Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...
in the luminescent bacteria Vibrio fischeri
Vibrio fischeri
Vibrio fischeri is a gram-negative rod-shaped bacterium found globally in marine environments. V. fischeri has bioluminescent properties, and is found predominantly in symbiosis with various marine animals, such as the bobtail squid. It is heterotrophic and moves by means of flagella. Free living...
. Similar pathways occur in other luminescent bacteria. In Vibrio fischeri, AHL binds to the protein product of the LuxR gene and activates it. The C-terminal domain of activated LuxR relieves the repression exerted by H-NS nucleoid proteins that bind to the promoters of LuxR, LuxI and the LuxCDABEG operon
Operon
In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...
, as well as to A-T-rich stretches within that operon and other genomic regions. The product of LuxI catalyses the synthesis of AHL. Thus, AHL acts as an autoinducer. Transcription of the LuxCDABEG operon results in luminescence due to the expression of LuxA and LuxB, which form a protein known as a luciferase
Luciferase
Luciferase is a generic term for the class of oxidative enzymes used in bioluminescence and is distinct from a photoprotein. One famous example is the firefly luciferase from the firefly Photinus pyralis. "Firefly luciferase" as a laboratory reagent usually refers to P...
and the expression of LuxC, D, E, and G, which are involved in the synthesis of the luciferase's substrate, tetradecanal. This is an important feature of quorum sensing
Quorum sensing
Quorum sensing is a system of stimulus and response correlated to population density. Many species of bacteria use quorum sensing to coordinate gene expression according to the density of their local population. In similar fashion, some social insects use quorum sensing to determine where to nest...
, as it makes little sense for one cell to waste the energy producing light, as the resulting light will be so faint that it will be more or less undetectable. Instead, once the bacterial population has reached a specific size, only then does light production commence.
Edman degradation
Homoserine lactone is also a product of the proteolytic reaction of cyanogen bromideCyanogen bromide
Cyanogen bromide is a pseudohalogen compound with the formula CNBr. It is a colorless solid that is widely used to modify biopolymers, fragment proteins and peptides, and synthesize other compounds.-Synthesis, basic properties, and structure:...
(CNBR) with a methionine
Methionine
Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
residue in a protein. This reaction is important for chemical sequencing of proteins, as the Edman degradation
Edman degradation
Edman degradation, developed by Pehr Edman, is a method of sequencing amino acids in a peptide. In this method, the amino-terminal residue is labeled and cleaved from the peptide without disrupting the peptide bonds between other amino acid residues....
process is unable to sequence more than 70 consecutive residues.