User:Parismm/N-Acyl homoserine lactone

Quorum sensing by the means of AHLs contributes to regulate the transcription of specific genes and therefore expression of specific phenotypes, including growth, virulence, biofilm formation, bioluminescence, production of exopolysaccharide (EPS). AHLs are used by over 50 gram-negative bacteria species (including several pathogenic species) use AHLs as autoinducers and the means of their communication in Quorum sensing.

Structure:[edit]
Main structural components of AHLs are the hydrophilic sections of homoserine lactone ring (biosynthesized from S-adenosylmethionine) and the central amide group, as well as the hydrophobic section strain-specific side hydrocarbon chain with varieties in length and level of of oxygenation with a 3-oxo group. Hydrophilic sections are able to form hydrogen-bond network within the receptor binding site and the hydrophobic site contributes to diffusion through the cell membranes and binding within the hydrophobic pocket. The length of the acyl chain generally ranges from 4 to 18 carbons and can be modified by a 3-oxo substituent, a 3-hydroxy substituent, or varied degrees of unsaturation.

Following strategies were applied to disrupt the virulence programs of bacteria through antagonizing quorum sensing:


 * 1) quorum quenching: inhibiting the production or reducing the concentration of the autoinducers in the extracellular matrix
 * 2) Applying catalytic antibodies
 * 3) interfering with the ligand-receptor interaction by adding antagonistic analogues

The two critical factors in AHL-based QS are concentration and structure of AHL.

Three types of AHLs synthetases are LuxI (the most widely distributed and studied), LuxM, and Hdts. The corresponding AHL receptors are LuxR-type proteins. The substrates of LuxI-type AHL synthetases are Sadenosyl-L-methionine (SAM) and acyl-acyl carrier protein (acylACP) which determines the AHLs variety. When the concentration of AHLs in the environment reaches a critical threshold (quorum), the expression of QS-dependent target genes is regulated by an autoinducer-receptor complex.

Studies have not yet demonstrated a correlation between AHL synthase and AHL type. The C-terminal region, which determines the type of substrates the synthetase can recognize and the subsequent acyl-chain length, is not conserved in AHL synthase family. Furthermore, according to the studies so far, there is not a significant correlation of the primary sequence composition and acyl-chain types. Moreover, there is no evidence that the distribution of AHL synthase and the species are correlated.

Mechanism[edit]
In the process of quorum sensing, first LuxI protein synthesizes and catalyzes the formation of an acylated homoserin-lactone molecule. AHL can pass through cell membrane down the gradient to the environmental space. When the concentration of this autoinducer reaches a specific degree (quorum), cognate LuxR protein binds to the AHL and directs all regulate the transcription of the target genes. As a result, gene expression of an entire population will be coordinated.

Several studies have been investigating on the potential AHLs effective in infection and resistance to antibiotics. LuxR–LuxI system mediated by AHLs is the best screened QS system in multi-drug resistant bacteria species.

Determining the antibiotic susceptibility of P. aeruginosa, E. coli and K. pneumoniae species isolated from various clinics and showing diferent resistance profles, investigating which signal molecules are efective in infection and elucidating the relationship between the signal molecules used and resistance profles are among the main goals in preventing the problem of resistance.

Impact of AHLs on Plants[edit]
Application of AHLs have been reported to activate the auxin-responsible GH3 promoter (upregulate auxin-related genes), and down-regulate the genes related to cytokinin (change in the ratio between auxin and cytokinin could promote the growth)

enhanced the nodulation in roots

change in transpiration rate by increasing stomata opening and consequently higher water and mineral flow through the plant

AHLs and Nitrogen cycle
AHL-mediated Quorum sensing has an important role in the significant process of Nitrogen cycle, for example it influnces the efficiency of and regulates the functions involved in nitrificationa and denitrification. some Bacterial species with ammonia-oxidizing bacteria that use C6- to C14- HSLs like Nitrosomonas europaea, Nitrosospira multiformis, Nitrosospira briensis. nitrite-oxidizing bacteria (NOB) also use C8- or C10- AHLs such as Nitrobacter winogradskyi, Nitrobacter vulgaris, and Nitrospira moscoviensis. Candidatus Jettenia caeni with anaerobic ammonium oxidation (annamox) mode uses C6- and C8- HSLs. Moreover, Pseudomonas aeruginosa and Paracoccus denitrificans as two denitrifier bacteria also use C4-HSL and C16-HSL, respectively. In some of the nitrifying and denitrifying bacteria like Nitrobacter hamburgensis AHLs were not found although putative AHL synthetase and receptor proteins.

Peer review #2: be sure to italicize Latin names. And this reads a little more like a journal article than an encyclopedia entry: be sure to only discuss what has been well-verified. This should not be a snapshot of the cutting edge of a particular sub-field of science, rather a review of what has been established so far, speaking in particular about the information associated with reference #4.