N-acylhomoserine lactones, known as autoinducers (AIs), are widely conserved signal molecules present in quorum-sensing systems of many Gram-negative bacteria. AIs are involved in the regulation of diverse biological functions, including expression of pathogenic genes in the plant pathogens Pseudomonas solanacearum, several Erwinia species, and the human pathogen Pseudomonas aeruginosa. A bacterial isolate, Bacillus sp. 240B1, is capable of enzymatic inactivation of AIs. The gene (aiiA) for AI inactivation from Bacillus sp. 240B1 has been cloned and shown to encode a protein of 250 amino acids. Sequence alignment indicates that AiiA contains a ''HXHXDH'' zinc-binding motif that is conserved in several groups of metallohydrolases. Site-directed mutagenesis showed that conserved aspartate and most histidine residues are required for AiiA activity. Expression of aiiA in transformed Erwinia carotovora strain SCG1 significantly reduces the release of AI, decreases extracellular pectolytic enzyme activities, and attenuates pathogenicity on potato, eggplant, Chinese cabbage, carrot, celery, cauliflower, and tobacco. Our results indicate that the AI-inactivation approach represents a promising strategy for prevention of diseases in which virulence is regulated by AIs.
With the emergence of antibiotic-resistant strains of bacteria, the available options for treating bacterial infections have become very limited, and the search for a novel general antibacterial therapy has received much greater attention. Quorum quenching can be used to control disease in a quorum sensing system by triggering the pathogenic phenotype. The interference with the quorum sensing system by the quorum quenching enzyme is a potential strategy for replacing traditional antibiotics because the quorum quenching strategy does not aim to kill the pathogen or limit cell growth but to shut down the expression of the pathogenic gene. Quorum quenching enzymes have been identified in quorum sensing and non-quorum sensing microbes, including lactonase, acylase, oxidoreductase and paraoxonase. Lactonase is widely conserved in a range of bacterial species and has variable substrate spectra. The existence of quorum quenching enzymes in the quorum sensing microbes can attenuate their quorum sensing, leading to blocking unnecessary gene expression and pathogenic phenotypes. In this review, we discuss the physiological function of quorum quenching enzymes in bacterial infection and elucidate the enzymatic protection in quorum sensing systems for host diseases and their application in resistance against microbial diseases.
In recent years, white polymer light-emitting diodes (WPLEDs) have received great attention because of their potential application in full-color flat-panel displays and solid-state lighting. A variety of approaches have been proposed for the realization of white emission in PLEDs.[1] One of the successful approaches in small-molecule organic light-emitting diodes (OLEDs) fabricated by thermal deposition is to use a multilayer device system consisting of two or more active layers, where each layer emits a primary color. [2,3] The highest device performance in multilayer phosphorescent OLEDs reached external quantum efficiency (QE) and power efficiency of 18.7 % and 37.6 lm W -1 , respectively, at a luminance of 500 cd m -2 with Commission Internationale de L'Éclairage (CIE) coordinates of (0.40,0.41), as reported by the Forrest group.[4]However, it is very difficult to fabricate multilayer PLEDs by solution processing because of the intermixing of different layers as a result of dissolution of the previously deposited layer. The most widely used approach for the manufacturing of PLEDs is to use the single-layer polymer blend system, [5][6][7][8][9] where the emitting layer consists of green and red emitters (small molecule or polymer) blended into a wide-gap bluelight-emitting polymer host and spin-coated onto an indium tin oxide/poly-(3,4-ethylenedioxythiophene) (ITO/PEDOT) substrate. Like most blended devices, the phase behavior of the guest and host is very sensitive to the driving voltage and the operating and shelf life; as a result, the color coordinates are not very stable. [5][6][7] Gong et al. [10] reported the first polymer multilayer white-light-emitting devices with a triplet phosphore doped into a blue-green polyfluorene host with water-soluble polyelectrolytes as the hole-transport layer (HTL) and electron-transfer layer (ETL). Recently, efforts have been made to prepare a single-component white polymeric emitter based on insufficient energy transfer, because phase segregation of chromophores can be significantly reduced by incorporating RGB (red-green-blue) chromophores into a single polymer chain. Lee et al. first reported a single fluorene-based copolymer composed of blue-, green-, and red-light-emitting units (although the RGB chromophores were not in full conjugation in the main chain) with a maximum brightness of 820 cd m -2 at 11 V with CIE coordinates of (0.33,0.35).[11] At almost the same time, Wang and co-workers adopted a slightly different synthetic strategy by which a green-emitting component was attached to the pendant chain and a red-emitting component was incorporated into the blue-emitting polyfluorene backbone.[12] The electroluminescent device exhibited a luminance efficiency of 1.59 cd A -1and CIE coordinates of (0.31,0.34). A similar strategy with two chromophores for producing white-light-emitting polymers has been reported, with a luminous efficiency (LE) of 3.8 cd A -1 and CIE coordinates of (0.32,0.36), [13] and a luminous efficiency of 7.3 cd A -1 and CIE coordinates of (0.3...
Xanthomonas campestris pathovar campestris causes black rot, a vascular disease on cruciferous plants, including Arabidopsis thaliana. The gene XC1553 from X. campestris pv. campestris strain 8004 encodes a protein containing leucine-rich repeats (LRRs) and appears to be restricted to strains of X. campestris pv. campestris. LRRs are found in a number of type III-secreted effectors in plant and animal pathogens. These prompted us to investigate the role of the XC1553 gene in the interaction between X. campestris pv. campestris and A. thaliana. Translocation assays using the hypersensitive-reaction-inducing domain of X. campestris pv. campestris AvrBs1 as a reporter revealed that XC1553 is a type III effector. Infiltration of Arabidopsis leaf mesophyll with bacterial suspensions showed no differences between the wild-type strain and an XC1553 gene mutant; both strains induced disease symptoms on Kashmir and Col-0 ecotypes. However, a clear difference was observed when bacteria were introduced into the vascular system by piercing the central vein of leaves. In this case, the wild-type strain 8004 caused disease on the Kashmir ecotype, but not on ecotype Col-0; the XC1553 gene mutant became virulent on the Col-0 ecotype and still induced disease on the Kashmir ecotype. Altogether, these data show that the XC1553 gene, which was renamed avrAC Xcc8004 , functions as an avirulence gene whose product seems to be recognized in vascular tissues.
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