Soon after the discovery of lactonase (AiiA) produced by soil bacilli [31
], other QQ enzymes from a wide variety of bacteria have been confirmed, namely AttM (A. tumefaciens
), AhlD (Arthrobacter
sp.), QsdA (R. erythropolis
), PvdQ and QuiP (P. aeruginosa
). QQ bacteria can be divided into three phyla, i.e.
, Firmicutes (Bacillus
sp. and S. silvestris
), Actinobacteria (Arthrobacter
, M. testaceum
, R. erythropolis
, M. avium
) and Proteobacteria (Agrobacterium
, V. paradoxus
, R. solanacearum
sp., P. aeruginosa
sp. and Delftia
sp.). Interestingly, Anabaena
sp., a member of cyanobacteria, has been discovered to exhibit lactonase activity. Similarly, Tenacibaculum maritimum
, a member of Bacteroidetes, produces acylase. Several recent papers reported the discovery of novel QQ enzymes via the metagenomic approach (). All these show that the genes encoding QQ enzymes are widely conserved among many prokaryotic microorganisms [29
QQ enzymes have been thought to solely play an important role in interfering QS. The well studied AiiA lactonases of B. thuringiensis
has been shown to quench the virulence of the phytopathogen E. carotovora
by inactivating its AHL signals [113
]. Czajkowski and Jafra have reported that bacteria produce QQ enzymes in order to ensure success in competition for the limited natural resources [114
]. Park et al.
revealed that the AiiA lactonase of B. thuringiensis
plays an important role in rhizosphere competence of B. thuringiensis
]. The aiiA
-defective mutant has a relatively lower survival rate, competency and adaptability compared to the wild type [96
]. These findings collectively suggest that the AiiA lactonase plays an important role in microbial competition. However, Kaufmann et al.
proposed that the lactonase of Bacillus
sp. plays a crucial role in controlling the toxicity effect of AHLs and prevents the formation of tetramic acid derivatives, i.e.
, nonenzymatical products of Claisen-like condensation reaction of the 3-oxo-AHLs [97
]. Tetramic acid derivatives are bactericidal agents which act against Gram-positive bacteria. Furthermore, tetramic acid derivatives are able to chelate diverse metal cations, such as iron, forming metal complexes believed to act as primordial siderophore. Thus, by degrading 3-oxo-AHLs, the toxicity of AHLs is abated, formation of tetramic acid derivatives prevented, competition for iron in the natural environment decreased, signalling pathway of competitors interfered and bacterial survival in the natural environment enhanced [97
]. An interesting opinion that contradicts this suggestion has been posted by Zhou et al.
, who suggested that Bacillus
species, especially B. cereus
, have different needs in terms of the ecological environment and nutritional sources compared to those of Gram-negative bacteria, e.g., E. carotovora. B. cereus
has a preference for protein and amino acid substrates. On the other hand, E. carotovora
prefers nutrients derived from plants. Therefore, it is unlikely there is competition between these two bacteria [115
Destruction of the AHL structure is not the only means for Bacillus
to exert its QQ effect. It has oxidoreductase capable of inactivating the AHL molecule by oxidizing the acyl chain at the ω-1, ω-2, and ω-3 carbons [78
]. This mechanism of QQ decreases the QS activity but not as much as lactonolysis which inhibits QS completely. It is hypothesized that this oxidoreductase makes acyl homoserine more membrane permeable, thus preventing the accumulation of degraded AHL products inside the cell. Furthermore, it makes acyl homoserine and AHLs more water soluble, thereby enhancing their diffusion out of the cell. As 3-oxo-AHLs are converted into tetramic acid derivatives nonenzymatically, oxidizing the acyl chain helps to detoxify the AHLs before the conversion takes place. The oxidization of the acyl chain might be the first of AHL metabolic pathway [78
is first bacterium shown to metabolize different AHLs as the sole carbon, nitrogen, and energy sources [77
]. Later, Park et al.
] reported a Gram-positive bacterium, Arthrobacter
sp. ISN110, that produces AhlD lactonase and can degrade and use various AHLs for energy and growth. Yoon et al.
] also reported that another Gram-positive bacterium, N. kongjuensis
, is able to grow on C6-HSL and use the AHL degradation products as the carbon source [58
The phytopathogen A. tumefaciens
expresses two lactonases, i.e.
, AttM and AiiB, which serve as the modulators of QS-regulated conjugation and transfer of tumour inducing (Ti) plasmid [48
] by regulating the level of 3-oxo-C8-HSL. Expression of aiiB
is induced by plant signals such as opines, agrocinopines A and B. On the other hand, expression of attM
, which is part of the attKLM
operon, is induced by succinic semialdehyde, γ-hydroxybutyrate, γ-butyrolactone (GBL), salicylic acid and γ-aminobutyrate [93
]. AiiB modulates the conjugation frequency of the Ti plasmid and the emergence of tumour. AttM lactonase contributes to the fitness of A. tumefaciens
in the plant tumour. Both AiiB and AttM modulate the level of 3-oxo-C8-HSL. The QS pathways and the QQ enzymes of A. tumefaciens
combine to contribute to optimal expression of virulence functions in A. tumefaciens
]. Thus, AttM and AiiB play an important role in the regulatory machinery of QS in A. tumefaciens
. However, this view has been challenged by Khan and Farrand who showed that AttM lactonase (or BlcC for γ-butyrolactone catabolism by Khan and Farrand) does not degrade AHL bound to TraR and the overexpression or null mutation of blcC
does not significantly affect the conjugation competence and transfer of Ti plasmid [94
]. Besides, the blc
operon is not widely distributed in the genus of Agrobacterium. Agrobacterium
spp. that harbour blcC
exhibit a significantly better growth on minimal medium with GBL as the sole source of carbon. Thus, the function of the blc
operon concerns the catabolism of butyryl compounds rather than the QS-regulated Ti plasmind conjugative transfer. BlcC degrades GBL to a product which may be eventually converted to succinic acid, an intermediate in the citric acid cycle [94
has been shown by Huang et al.
] to produce two acylases, PvdQ and QuiP. They suggested that QuiP may play a role to distinguish subpopulation spatially, especially in the biofilm state [70
]. The gene that encodes PvdQ acylase is located in the pvd
locus, essential for the regulation of pyoverdine biosynthesis [98
]. PvdQ might play a role in the utilization of AHL, maturation of pyoverdine siderophore and regulation of 3-oxo-C12-HSL-regulated physiological functions [68
]. The last possible role is supported by Sio et al.
] who provided experimental evidence that PvdQ modulates QS-regulated virulence phenotypes. Under iron-limiting conditions, deletion of pvdQ
led to attenuation of virulence, decreased swarming motility and failure to form biofilm. Hence, PvdQ might be involved in the biosynthesis of pyoverdine and regulation of iron homeostasis. Its role in iron sequestration precedes its acylase activity under iron-limiting conditions [99
According to Wang et al.
] and Yeung et al.
mutant and pvdQ
mutant exhibit reduced swarming motility, suggesting that a specific concentration of 3-oxo-C12-HSL is crucial for the swarming motility. The degraded AHL products of PvdQ may serve as a signal during swarmer cell differentiation [118
]. In 2011, Wang et al.
] made an interesting proposal that as both biofilm formation and swarming motility are dependent on a functional flagellum, PvdQ may play a role in regulating the flagellum-dependent motions. This in turn facilitates the decision-making mechanism between biofilm formation and swarming motility [100
]. Wang et al.
also suggested that PvdQ might play an important role in antibiotic resistance by altering the membrane permeability [100
]. PvdQ may change the outer membrane permeability by up-regulating the lipopolysaccharide-related operon [100
], and alter the expression of the outer membrane TonB-dependent receptors, OprF (which is involved in cell adhesion, binding of gamma interferon and activation of QS-pathway) [119
] and OprD (which facilitates the transport of basic amino acids and imipenem) [121
]. In a recent study, Hannauer et al.
demonstrated that pvdQ
mutant produces pyoverdine I precursors with a myristic or a myristoleic acid chain and an unformed chromophore [101
]. This leads to the suggestion that PvdQ plays a role in removing the acylated fatty acid chain or the non-fluorescent precursor prior to the cyclization of chromophore [101
]. These acylases might be important in preventing premature production of virulence factors that could trigger the immune response of the host. In summary, PvdQ might play a role in the utilization of AHL, regulation of QS-dependent phenotypes, biosynthesis of pyoverdine and elevation of antibiotic resistance.
Like P. aeruginosa
and A. tumefaciens
, R. solanacearum
exhibits both QS and QQ systems. It possesses aac
gene which encodes acylase that degrades long chain AHLs. According to Chen et al.
, R. solanacearum
metabolizes AHL degradation product, i.e.
, fatty acids, by β-oxidation during cultivation [72
]. Thus, acylase may be involved in the metabolism of AHLs. This enzyme may modulate QS pathways by using a unique signal turnover mechanism [32
]. Lin et al.
proposed that the acylase of Ralstonia
plays an important role during oligotrophic nutrient scavenging from the natural environment [32
Interestingly, R. erythropolis
possesses three different types of QQ enzymes, namely oxidoreductase, acylase and lactonase [55
]. However, the roles of all these QQ enzymes remain unclear. Several neighboring sequences of the lactonase-encoding gene play an important role in the metabolism of fatty acids, such as acyl coenzyme A synthase and FadR peptide analogous to a fatty acid biosynthesis regulatory protein. Thus, the lactonase produced by R. erythropolis
might be involved in fatty acid metabolism [55
is a member of the Cytophaga-Flavobacterium-Bacteroides (CFB) group whose lactonase degrades AHL. It uses the degraded product for growth and energy, and might play a role in providing protection to plants from pathogens [59
]. Hence, Chryseobacterium
and its plant host live in symbiosis. Similarly, the endophytic Gram-positive bacterium M. testaceum
that produces AiiM lactonase may play a role in interfering with the QS pathways of pathogens, thereby providing protection to the host against pathogens invasion [52
]. AiiC acylase produced by Anabaena
, a member of cyanobacteria, is believed to play a role in interfering with the communication system within the complex microbial communities [66
]. AiiC might be important in controlling the exogenous AHLs, but the rationale behind it remains unknown. AHLs have been shown to inhibit nitrogen fixation pathway in Anabaena
. In addition, with the presence of nitrogen source from the natural environment, 3-oxo-C10-HSL exhibits cytotoxic effect on Anabaena
]. Thus, the presence of QQ enzyme might have an important role in defense mechanism.
In summary, several roles of QQ enzymes have been postulated, from interference of QS to metabolism of AHL as the source of carbon and nitrogen, detoxification, regulation of physiological functions, and symbiotic interaction with the host. To date, there is no experiment showing a conclusive link between QQ and the ability to gain a competitive advantage [122
]. There are still a number of bacteria in which the roles of their QQ enzymes remain enigmatic. Hence, further investigation is needed to gain insight into the role of QQ enzymes.