Autoinducer-2
Encyclopedia
Autoinducer-2 a furanosyl borate diester, is a member of a family of signaling molecule
s used in quorum sensing
. AI-2 is unique in that it is one of only a few known biomolecules incorporating boron
. First identified in the marine bacterium Vibrio harveyi
, AI-2 is produced and recognized by many Gram-negative
and Gram-positive bacteria. AI-2 is sythesized by the reaction of 1-deoxy-3-dehydro-D-ribulose
with boric acid
.
AI-2 is sensed by the Lsr transport cassette and is actively transported into the cell, where it is phosphorylated by . Then, Phospho-AI-2 binds the transcriptional repressor protein, LsrR, which subsequently is released from the promoter/operator region of the lsr operon – and transcription of the lsr genes is initiated. AI-2 signalling is also regulated by glucose and cAMP/CRP via the lsr operon. In the presence of glucose, low levels of cAMP/CRP result in almost no lsr operon (lsrABCDFG) expression. Without glucose, cAMP-CRP is needed to stimulate the lsr expression, while LsrR represses its expression in the absence of the inducer, phospho-AI-2. As AI-2 accumulates, more AI-2 is taken in via LsrABCD, phosphorylated via LsrK, and the lsr transcription is de-repressed, enabling even more AI-2 uptake.
Doubts have been expressed regarding AI-2's status as a universal signal. The gene responsible for its production is the widespread luxS; this gene has an important role in the recycling of S-adenosyl-L-methionine, with AI-2 being a metabolic by-product of that process. While it is certainly true that some bacteria respond to AI-2, it is not yet clear that it is always being produced for purposes of signalling.
Signaling 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 used in 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...
. AI-2 is unique in that it is one of only a few known biomolecules incorporating boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
. First identified in the marine bacterium Vibrio harveyi
Vibrio harveyi
Vibrio harveyi is a species of Gram-negative, bioluminescent, marine bacteria in the genus Vibrio. V. harveyi are rod-shaped, motile , facultatively anaerobic, halophilic, and competent for both fermentative and respiratory metabolism. They do not grow at 4°C or above 35°C. V...
, AI-2 is produced and recognized by many Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
and Gram-positive bacteria. AI-2 is sythesized by the reaction of 1-deoxy-3-dehydro-D-ribulose
Ribulose
Ribulose is a ketopentose — a monosaccharide containing five carbon atoms, and including a ketone functional group. It has chemical formula 5105. Two enantiomers are possible, D-ribulose and L-ribulose . D-Ribulose is the diastereomer of D-xylulose.Ribulose sugars are composed in the...
with boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
.
AI-2 is sensed by the Lsr transport cassette and is actively transported into the cell, where it is phosphorylated by . Then, Phospho-AI-2 binds the transcriptional repressor protein, LsrR, which subsequently is released from the promoter/operator region of the lsr operon – and transcription of the lsr genes is initiated. AI-2 signalling is also regulated by glucose and cAMP/CRP via the lsr operon. In the presence of glucose, low levels of cAMP/CRP result in almost no lsr operon (lsrABCDFG) expression. Without glucose, cAMP-CRP is needed to stimulate the lsr expression, while LsrR represses its expression in the absence of the inducer, phospho-AI-2. As AI-2 accumulates, more AI-2 is taken in via LsrABCD, phosphorylated via LsrK, and the lsr transcription is de-repressed, enabling even more AI-2 uptake.
Doubts have been expressed regarding AI-2's status as a universal signal. The gene responsible for its production is the widespread luxS; this gene has an important role in the recycling of S-adenosyl-L-methionine, with AI-2 being a metabolic by-product of that process. While it is certainly true that some bacteria respond to AI-2, it is not yet clear that it is always being produced for purposes of signalling.