Bacterial N-acylhomoserine lactone-induced apoptosis in breast carcinoma cells correlated with down-modulation of STAT3

Cell growth is promoted by mitogens and survival factors, which activate intracellular signalling pathways to control cell cycle progression and cellular integrity. Proliferation signals are transmitted through Ras and Rho family small G-proteins coupled to mitogen-activated protein kinase (MAPK) cascades, while survival signals are propagated by lipid-dependent kinases such as phosphatidylinositide 3-kinases (PI3Ks) and protein kinase B (Akt/PKB). Recently, signal transducer and activator of transcription (STAT) proteins were identified as positive regulators of proliferation in a variety of cell types. Persistent activation of these pathways is associated with tumour cell growth, whereas their inhibition can halt proliferation and precipitate apoptotic cell death. The human pathogen Pseudomonas aeruginosa uses quorum-sensing signal molecules (QSSMs) to regulate virulence gene expression. QSSMs also suppress host immune responses although the mechanism of suppression is unknown. Here, we demonstrate that the QSSM N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) from P. aeruginosa blocks proliferation and induces apoptosis in human BC cell lines. Analyses of signalling events reveal that OdDHL has little or no effect on MAPK cascades, partially inhibits the Akt/PKB pathway and ablates STAT3 activity. Pharmacological inhibition of each pathway independently indicates that STAT3 activity is critical for BC cell proliferation and survival, while a constitutively active STAT3 confers resistance to OdDHL. These results support the notion of OdDHL as a bioactive molecule in eukaryotic systems and a paradigm for a novel class of antiproliferative compounds.

Recent progress in the design of selectin inhibitors

The selectins are a family of cell-adhesion proteins that mediate the early stages of leukocyte recruitment from the blood stream to sites of tissue damage through recognition of the carbohydrate epitope sialyl Lewis(x) (sLe(x)). Current development of small molecule based inhibitors of this process and their clinical potential to address numerous acute and chronic diseases are explored.

The Pseudomonas aeruginosa quinolone signal molecule overcomes the cell density-dependency of the quorum sensing hierarchy, regulates rhl-dependent genes at the onset of stationary phase and can be produced in the absence of LasR

In Pseudomonas aeruginosa, diverse exoproduct virulence determinants are regulated via N-acylhomoserine lactone-dependent quorum sensing. Here we show that 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS) is also an integral component of the quorum sensing circuitry and is required for the production of rhl-dependent exoproducts at the onset of stationary phase. Analysis of spent P. aeruginosa culture supernatants revealed that PQS is produced at the end of exponential phase in the parent strain and in the late stationary phase of a lasR mutant. Mutants defective in both PQS production (pqsR-) and response (pqsE-) produced substantially reduced levels of exoproducts but retained wild-type N-butanoyl homoserine lactone (C4-HSL) levels. In the wild type, provision of exogenous PQS at the time of inoculation significantly increased PA-IL lectin, pyocyanin and elastase production during early stationary phase and promoted biofilm formation. Exogenous PQS but not PQS derivatives lacking the 3-hydroxy group overcame the cell density but not growth phase-dependent production of exoproducts. PQS also overcame the transcriptional and post-transcriptional repression of lecA (which codes for the PA-IL lectin) mediated via the negative regulators MvaT and RsmA respectively. Increased expression of lecA in the presence of exogenous PQS can be explained partially by increases in RhlR, RpoS and C4-HSL levels. A refined model for quorum sensing in P. aeruginosa is presented.

Synthetic analogues of the bacterial signal (quorum sensing) molecule N-(3-oxododecanoyl)-L-homoserine lactone as immune modulators

Comparative immune modulatory activity for a range of synthetic analogues of a Pseudomonas aeruginosa signal molecule, N-(3-oxododecanoyl)-l-homoserine lactone (3O, C(12)-HSL), is described. Twenty-four single or combination systematic alterations of the structural components of 3O, C(12)-HSL were introduced as described. Given the already defined immunological profile of the parent compound, 3O, C(12)-HSL, these compounds were assayed for their ability to inhibit murine and human leucocyte proliferation and TNF-alpha secretion by lipopolysaccharide (LPS) stimulated human leucocytes in order to provide an initial structure-activity profile. From IC(50) values obtained with a murine splenocyte proliferation assay, it is apparent that acylated l-homoserine lactones with an 11-13 C side chain containing either a 3-oxo or a 3-hydroxy group are optimal structures for immune suppressive activity. These derivatives of 3O, C(12)-HSL with monounsaturation and/or a terminal nonpolar substituent on the side chain were also potent immune suppressive agents. However, structures lacking the homoserine lactone ring, structures lacking the l-configuration at the chiral center, and those with polar substituents were essentially devoid of activity. The ability of compounds selected from the optimal activity range to modulate mitogen-driven human peripheral blood mononuclear cell proliferation and LPS-induced TNF-alpha secretion indicates the suitability of these compounds for further investigation in relation to their molecular mechanisms of action in TNF-alpha driven immunological diseases, particularly autoimmune diseases such as psoriasis, rheumatoid arthritis, and type 1 (autoimmune) diabetes.

The growth response of Escherichia coli to neurotransmitters and related catecholamine drugs requires a functional enterobactin biosynthesis and uptake system

The neurotransmitter norepinephrine (NE) stimulates the growth of low inocula of Escherichia coli in a minimal medium (SAPI) supplemented with serum (SAPI+serum) and induces the production of an "autoinducer" (AI) which, in turn, promotes E. coli growth in the absence of NE. Given the importance of NE, epinephrine, and their corresponding adrenergic agonists and antagonists in clinical medicine, we sought to investigate the molecular basis for these observations. Using a variety of NE precursors, metabolites, and therapeutic agents, we demonstrated that their ability to stimulate E. coli growth in SAPI+serum is dependent on the presence of a catechol (1,2-dihydroxybenzene) moiety with maximal activity requiring a two-carbon substituent incorporating a terminal primary amine. Serum contains the iron-binding glycoprotein, transferrin, and when SAPI+serum was supplemented with sufficient Fe(3+) to saturate transferrin, growth inhibition was relieved. Other metal cations, including Mg(2+), Ca(2+), and Zn(2+), had no effect. These data suggested that the stimulation of E. coli growth by NE in SAPI+serum may involve the catecholate siderophore, enterobactin, a cyclic triester of 2,3-dihydroxybenzoylserine. Consistent with this hypothesis, E. coli strains with mutations in ferrienterobactin transport (fepA or tonB) or enterobactin biosynthesis (entA) did not respond to NE. Furthermore, NE induced expression of the ferrienterobactin receptor, FepA, during growth in SAPI+serum. The enterobactin degradation product, 2,3-dihydroxybenzoylserine (DBS) was as effective as NE in stimulating the growth of E. coli and mutations in fepA or tonB abolished the DBS-dependent growth stimulation. In contrast to NE, however, DBS stimulated the growth of the entA mutant. Moreover, after growth in an iron-limited M9 medium in the absence of NE, ethyl acetate extracts of the E. coli entA(+) parent but not of the entA mutant contained AI, i.e., stimulated the growth of E. coli in SAPI+serum. Taken together, these data show that when low numbers of E. coli are inoculated into SAPI+serum, NE, DBS, and related catecholamines induce the enterobactin iron uptake system. This, in turn, facilitates iron sequestration from transferrin and indicates that the AI present in NE-conditioned SAPI+serum medium is enterobactin and its DBS breakdown products.

N-acylhomoserine lactones undergo lactonolysis in a pH-, temperature-, and acyl chain length-dependent manner during growth of Yersinia pseudotuberculosis and Pseudomonas aeruginosa

In gram-negative bacterial pathogens, such as Pseudomonas aeruginosa and Yersinia pseudotuberculosis, cell-to-cell communication via the N-acylhomoserine lactone (AHL) signal molecules is involved in the cell population density-dependent control of genes associated with virulence. This phenomenon, termed quorum sensing, relies upon the accumulation of AHLs to a threshold concentration at which target structural genes are activated. By using biosensors capable of detecting a range of AHLs we observed that, in cultures of Y. pseudotuberculosis and P. aeruginosa, AHLs accumulate during the exponential phase but largely disappear during the stationary phase. When added to late-stationary-phase, cell-free culture supernatants of the respective pathogen, the major P. aeruginosa [N-butanoylhomoserine lactone (C4-HSL) and N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL)] and Y. pseudotuberculosis [N-(3-oxohexanoyl)homoserine lactone (3-oxo-C6-HSL) and N-hexanoylhomoserine lactone (C6-HSL)] AHLs were inactivated. Short-acyl-chain compounds (e.g., C4-HSL) were turned over more extensively than long-chain molecules (e.g., 3-oxo-C12-HSL). Little AHL inactivation occurred with cell extracts, and no evidence for inactivation by specific enzymes was apparent. This AHL turnover was discovered to be due to pH-dependent lactonolysis. By acidifying the growth media to pH 2.0, lactonolysis could be reversed. By using carbon-13 nuclear magnetic resonance spectroscopy, we found that the ring opening of homoserine lactone (HSL), N-propionyl HSL (C3-HSL), and C4-HSL increased as pH increased but diminished as the N-acyl chain was lengthened. At low pH levels, the lactone rings closed but not via a simple reversal of the ring opening reaction mechanism. Ring opening of C4-HSL, C6-HSL, 3-oxo-C6-HSL, and N-octanoylhomoserine lactone (C8-HSL), as determined by the reduction of pH in aqueous solutions with time, was also less rapid for AHLs with more electron-donating longer side chains. Raising the temperature from 22 to 37 degrees C increased the rate of ring opening. Taken together, these data show that (i) to be functional under physiological conditions in mammalian tissue fluids, AHLs require an N-acyl side chain of at least four carbons in length and (ii) that the longer the acyl side chain the more stable the AHL signal molecule.

Homochiral 4-azalysine building blocks: syntheses and applications in solid-phase chemistry

Anomalous amino acids not only play central roles as mimics of natural amino acids but also offer opportunities as unique building blocks for combinatorial chemistry. This paper describes the chiral syntheses and solid-phase applications of a versatile atypical amino acid, 4-azalysine (2,6-diamino-4-azahexanoic acid) 1. The syntheses of differentially protected 4-azalysine derivatives 28a-e have been developed by two efficient and inexpensive routes that start either from Garner's aldehyde 16 or the chiron (S)-N(alpha)-Cbz-2,3-diaminopropionic acid 23. Both approaches employ the convergent modular concept and exploit reductive amination of aldehydes with amines as the key step for the fusion of the two segments. In the first route, the overall process inverts the chirality of the starting material, L-serine, and thus provides an excellent route to the unnatural D-isomers. The alternative route starting from L-asparagine provides a shorter and high-yielding route to orthogonally protected 4-azalysine derivatives. The corresponding N(2)-Fmoc-4-azalysines 31a-e, readily derived from the key intermediate 27, are compatible with the Fmoc-based solid-phase peptide synthesis (SPPS) and solid-phase organic chemistry (SPOC) protocols. Furthermore, the utility and versatility of another key structure, tris-Boc-4-azalysine 2 in the engineering of novel high-loading dendrimeric polystyrene resins 33 and 36, have been demonstrated. Following derivatization with the Rink amide linker 34, the stability and robustness of these resin-bound dendrimers 35 and 37 in the synthesis of small molecules using a range of reaction conditions (e.g., Mitsunobu and Suzuki reactions) have been effectively illustrated.

raiIR genes are part of a quorum-sensing network controlled by cinI and cinR in Rhizobium leguminosarum

Analysis of N-acyl-L-homoserine lactones (AHLs) produced by Rhizobium leguminosarum bv. viciae indicated that there may be a network of quorum-sensing regulatory systems producing multiple AHLs in this species. Using a strain lacking a symbiosis plasmid, which carries some of the quorum-sensing genes, we isolated mutations in two genes (raiI and raiR) that are required for production of AHLs. The raiIR genes are located adjacent to dad genes (involved in D-alanine catabolism) on a large indigenous plasmid. RaiR is predicted to be a typical LuxR-type quorum-sensing regulator and is required for raiI expression. The raiR gene was expressed at a low level, possibly from a constitutive promoter, and its expression was increased under the influence of the upstream raiI promoter. Using gene fusions and analysis of AHLs produced, we showed that expression of raiI is strongly reduced in strains carrying mutations in cinI or cinR, genes which determine a higher-level quorum-sensing system that is required for normal expression of raiIR. The product of CinI, N-(3-hydroxy-7-cis tetradecenoyl) homoserine lactone, can induce raiR-dependent raiI expression, although higher levels of expression are induced by other AHLs. Expression of raiI in a strain of Agrobacterium that makes no AHLs resulted in the identification of N-(3-hydroxyoctanoyl)-L-homoserine lactone (3OH,C(8)-HSL) as the major product of RaiI, although other AHLs that comigrate with N-hexanoyl-, N-heptanoyl-, and N-octanoyl-homoserine lactones were also made at low levels. The raiI gene was strongly induced by 3OH,C(8)-HSL (the product of RaiI) but could also be induced by other AHLs, suggesting that the raiI promoter can be activated by other quorum-sensing systems within a network of regulation which also involves AHLs determined by genes on the symbiotic plasmid. Thus, the raiIR and cinIR genes are part of a complex regulatory network that influences AHL biosynthesis in R. leguminosarum.

Galactomannanases Man2 and Man5 from Thermotoga species: growth physiology on galactomannans, gene sequence analysis, and biochemical properties of recombinant enzymes

The enzymatic hydrolysis of mannan-based hemicelluloses is technologically important for applications ranging from pulp and paper processing to food processing to gas and oil well stimulation. In many cases, thermostability and activity at elevated temperatures can be advantageous. To this end, the genes encoding beta-mannosidase (man2) and beta-mannanase (man5) from the hyperthermophilic bacteria Thermotoga neapolitana 5068 and Thermotoga maritima were isolated, cloned, and expressed in Escherichia coli. The amino acid sequences for the mannosidases from these organisms were 77% identical and corresponded to proteins with an M(r) of approximately 92 kDa. The translated nucleotide sequences for the beta-mannanase genes (man5) encoded polypeptides with an M(r) of 76 kDa that exhibited 84% amino acid sequence identity. The recombinant versions of Man2 and Man5 had similar respective biochemical and biophysical properties, which were also comparable to those determined for the native versions of these enzymes in T. neapolitana. The optimal temperature and pH for the recombinant Man2 and Man5 from both organisms were approximately 90 degrees C and 7.0, respectively. The presence of Man2 and Man5 in these two Thermotoga species indicates that galactomannan is a potential growth substrate. This was supported by the fact that beta-mannanase and beta-mannosidase activities were significantly stimulated when T. neapolitana was grown on guar or carob galactomannan. Maximum cell densities increased by at least tenfold when either guar or carob galactomannan was added to the growth medium. For T. neapolitana grown on guar at 83 degrees C, Man5 was secreted into the culture media, whereas Man2 was intracellular. These localizations were consistent with the presence and lack of signal peptides for Man5 and Man2, respectively. The identification of the galactomannan-degrading enzymes in these Thermotoga species adds to the list of biotechnologically important hemicellulases produced by members of this hyperthermophilic genera.

Haemodynamic effects of the bacterial quorum sensing signal molecule, N-(3-oxododecanoyl)-L-homoserine lactone, in conscious, normal and endotoxaemic rats

N-acylhomoserine lactones (AHLs) are small, diffusible signalling molecules, employed by Gram-negative bacteria to coordinate gene expression with cell population density. Recent in vitro findings indicate that AHLs may function as virulence determinants per se, through modification of cytokine production by eukaryotic cells, and by stimulating the relaxation of blood vessels. In the present study, we assessed the influence of AHLs on cardiovascular function in conscious rats, and draw attention to the ability of the N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C12-HSL), a signal molecule produced by P. aeruginosa, to cause marked bradycardia. This bradycardic effect was blocked by atropine and atenolol, and did not occur in vitro. Furthermore, modification of the acyl side chain length resulted in the loss of activity, whereas removal of the homoserine lactone ring, did not. The bradycardic effect of 3-oxo-C12-HSL was also observed in endotoxaemic animals, albeit attenuated. In normal rats, 3-oxo-C12-HSL caused initial mesenteric and hindquarters vasoconstriction, but only slight, and delayed signs of vasodilatation in the renal and mesenteric vascular beds. Furthermore, administration of 3-oxo-C12-HSL (pre-treatment or 2 h post-treatment) together with LPS, did not modify the established regional haemodynamic effects of the LPS, 6 h after the onset of its infusion. Our observations do not provide any clear evidence for an ability of 3-oxo-C12-HSL to modify the haemodynamic responses to LPS infusion. However, they are not inconsistent with the hypothesis that some of the cardiovascular sequelae of bacterial infection may be modulated by an influence of bacterial quorum sensing signalling molecules on the host.

The biocontrol strain Pseudomonas fluorescens F113 produces the Rhizobium small bacteriocin, N-(3-hydroxy-7-cis-tetradecenoyl)homoserine lactone, via HdtS, a putative novel N-acylhomoserine lactone synthase

Several different species of Pseudomonas: produce N:-acylhomoserine lactones (AHLs), quorum-sensing signal molecules which are involved in the cell-density-dependent control of secondary metabolite and virulence gene expression. When Pseudomonas fluorescens F113 was cross-streaked against AHL biosensors capable of sensitively detecting either short (C(4)-C(8)) or long (C(10)-C(14)) acyl chain AHLs, no activity was detectable. However, by extracting cell-free stationary-phase culture supernatants with dichloromethane followed by reverse-phase HPLC, three distinct fractions were obtained capable of activating the AHL biosensors. Three AHLs were subsequently characterized using high-resolution MS and chemical synthesis. These were (i) N:-(3-hydroxy-7-cis-tetradecenoyl)homoserine lactone (3OH, C(14:1)-HSL), a molecule previously known as the Rhizobium leguminosarum small bacteriocin as a consequence of its growth inhibitory properties, (ii) N:-decanoylhomoserine lactone (C(10)-HSL) and (iii) N:-hexanoylhomoserine lactone (C(6)-HSL). A gene (hdtS) capable of directing synthesis of all three P. fluorescens AHLs in Escherichia coli was cloned and sequenced. In vitro transcription/translation of hdtS yielded a protein of approximately 33 kDa capable of directing the synthesis of 3OH, C(14:1)-HSL, C(10)-HSL and C(6)-HSL in E. coli. HdtS does not belong to either of the known AHL synthase families (LuxI or LuxM) and is related to the lysophosphatidic acid acyltransferase family. HdtS may therefore constitute a member of a third protein family capable of AHL biosynthesis.

The Pseudomonas aeruginosa quorum-sensing signal molecule, N-(3-oxododecanoyl)-L-homoserine lactone, inhibits porcine arterial smooth muscle contraction

The Pseudomonas aeruginosa quorum sensing molecule N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL) has been shown to suppress cytokine production in macrophages. We have examined the effect of OdDHL and related compounds on constrictor tone of porcine blood vessels. OdDHL (1-30 microM) caused a concentration-dependent inhibition of U46619-induced contractions of the coronary artery through a largely endothelium-independent mechanism, but was markedly less effective in the pulmonary artery. Quantitively similar effects to those produced by OdDHL were observed with N-(3-oxododecanoyl)-L-homocysteine thiolactone, a thiolactone derivative, while N-3-oxododecanamide, a lactone-free acyl analogue, possessed 1/3rd the potency as a vasorelaxant. Neither N-butanoyl-L-homoserine lactone nor L-homoserine lactone (up to 30 microM) were active. Our findings indicate that OdDHL inhibits vasoconstrictor tone of both pulmonary and coronary blood vessels from the pig. The vasorelaxant action of OdDHL appears to be primarily determined by the N-acyl chain length, with a minor contribution by the homoserine lactone moiety.

Construction and analysis of luxCDABE-based plasmid sensors for investigating N-acyl homoserine lactone-mediated quorum sensing

Plasmid reporter vectors have been constructed which respond to activation of LuxR and its homologues LasR and RhlR (VsmR) by N-acyl homoserine lactones (AHLs). The expression of luxCDABE from transcriptional fusions to PluxI, PlasI and PrhlI respectively, occurs in the presence of activating AHLs. A profile of structure/activity relationships is seen where the natural ligand is most potent. The characterisation of individual LuxR homologue/AHL combinations allows a comprehensive evaluation of quorum sensing signals from a test organism.

In vitro biosynthesis of the Pseudomonas aeruginosa quorum-sensing signal molecule N-butanoyl-L-homoserine lactone

In Pseudomonas aeruginosa, synthesis of the quorum-sensing signal molecules N-butanoyl-L-homoserine lactone (BHL) and N-hexanoyl-L-homoserine lactone (HHL) requires the Luxl homologue Rhll(Vsml). By using thin-layer chromatography in conjunction with high-performance liquid chromatography (HPLC) and mass spectrometry, we show that purified Rhll can catalyse the biosynthesis of BHL and HHL using either S-adenosylmethionine (SAM) or homoserine lactone (HSL) but not homoserine as the source of the homoserine lactone moiety. As we were unable to detect homoserine lactone in cytoplasmic extracts of Escherichia coli, we conclude that SAM is the natural substrate for Rhll-directed N-acylhomoserine lactone (AHL) biosynthesis. The N-acyl chain of BHL and HHL can be supplied by the appropriately charged coenzyme A derivative (either n-butanoyl-CoA or n-hexanoyl-CoA). The specificity of Rhll for charged CoA derivatives is demonstrated as Rhll was unable to generate AHLs detectable in our bioassays from acetyl-CoA, malonyl-CoA, n-octanoyl-CoA, n-decanoyl-CoA, DL-beta-hydroxybutanoyl-CoA or crotonoyl-CoA. Rhll was also unable to use N-acetyl-S-3-oxobutanoylcysteamine, a chemical mimic for 3-oxobutanoyl-CoA. Furthermore, the Rhll-catalysed synthesis of BHL and HHL was most efficiently driven when NADPH was included in the reaction mixture.

Quorum sensing and Chromobacterium violaceum: exploitation of violacein production and inhibition for the detection of N-acylhomoserine lactones

Quorum sensing relies upon the interaction of a diffusible signal molecule with a transcriptional activator protein to couple gene expression with cell population density. In Gram-negative bacteria, such signal molecules are usually N-acylhomoserine lactones (AHLs) which differ in the structure of their N-acyl side chains. Chromobacterium violaceum, a Gram-negative bacterium commonly found in soil and water, produces the characteristic purple pigment violacein. Previously the authors described a violacein-negative, mini-Tn5 mutant of C. violaceum (CV026) in which pigment production can be restored by incubation with supernatants from the wild-type strain. To develop this mutant as a general biosensor for AHLs, the natural C. violaceum AHL molecule was first chemically characterized. By using solvent extraction, HPLC and mass spectrometry, a single AHL, N-hexanoyl-L-homoserine lactone (HHL), was identified in wild-type C. violaceum culture supernatants which was absent from CV026. Since the production of violacein constitutes a simple assay for the detection of AHLs, we explored the ability of CV026 to respond to a series of synthetic AHL and N-acylhomocysteine thiolactone (AHT) analogues. In CV026, violacein is inducible by all the AHL and AHT compounds evaluated with N-acyl side chains from C4 to C8 in length, with varying degrees of sensitivity. Although AHL compounds with N-acyl side chains from C10 to C14 are unable to induce violacein production, if an activating AHL (e.g. HHL) is incorporated into the agar, these long-chain AHLs can be detected by their ability to inhibit violacein production. The versatility of CV026 in facilitating detection of AHL mixtures extracted from culture supernatants and separated by thin-layer chromatography is also demonstrated. These simple bioassays employing CV026 thus greatly extend the ability to detect a wide spectrum of AHL signal molecules.

Quorum sensing in Aeromonas hydrophila and Aeromonas salmonicida: identification of the LuxRI homologs AhyRI and AsaRI and their cognate N-acylhomoserine lactone signal molecules

Spent culture supernatants from both Aeromonas hydrophila and Aeromonas salmonicida activate a range of biosensors responsive to N-acylhomoserine lactones (AHLs). The genes for a quorum sensing signal generator and a response regulator were cloned from each Aeromonas species and termed ahyRI and asaRI, respectively. Protein sequence homology analysis places the gene products within the growing family of LuxRI homologs. ahyR and asaR are transcribed divergently from ahyI and asaI, respectively, and in both Aeromonas species, the genes downstream have been identified by DNA sequence and PCR analysis. Downstream of both ahyI and asaI is a gene with close homology to iciA, an inhibitor of chromosome replication in Escherichia coli, a finding which implies that in Aeromonas, cell division may be linked to quorum sensing. The major signal molecule synthesized via both AhyI and AsaI was purified from spent culture supernatants and identified as N-(butanoyl)-L-homoserine lactone (BHL) by thin-layer chromatography, high-pressure liquid chromatography analysis, and mass spectrometry. In addition, a second, minor AHL, N-hexanoyl-L-homoserine lactone, was identified. Transcriptional reporter studies with ahyI::luxCDABE fusions indicate that AhyR and BHL are both required for ahyI transcription. For A. salmonicida, although the addition of exogenous BHL gives only a small stimulation of the production of serine protease with comparison to the control culture, the incorporation of a longer-chain AHL, N-(3-oxodecanoyl)-L-homoserine lactone, reduced the final level (by approximately 50%) and delayed the appearance (from an A650 of 0.9 in the control to an A650 of 1.2 in the test) of protease in the culture supernatant. These data add A. hydrophila and A. salmonicida to the growing family of gram-negative bacteria now known to control gene expression through quorum sensing.