Signal Sensing and Quenching

To gain maximal benefit in a competitive environment, single-celled bacteria have adopted a community genetic regulatory mechanism, known as quorum sensing. Many bacteria use quorum sensing signaling systems to synchronize gene expression and coordinate diverse biological functions, including virulence and biofilm development. Investigation of the molecular mechanisms of quorum sensing could have enormous implications in understanding microbial ecology and pathogenesis, and in developing novel approaches to control infectious diseases and biofilm-associated problems.

Quorum sensing bacteria produce, detect and respond to small signal molecules to coordinate community behaviors in a population density–dependent manner. Different bacterial pathogens may produce and respond to different quorum sensing signals. One ongoing research interest in Dr. Zhang’s group is identification and characterization of new signaling molecules. The group has completed structural analysis of a diffusible signal factor (DSF) that regulates virulence and biofilm dispersal in Xanthomonas campestris (Fig. 1). DSF was identified as methyl dodecenoic acid, which represents a new class of QS signals that seems to be conserved in a range of bacterial species. More recently, a DSF analogue has been identified and purified from Burkholderia cenocepacia, and structurally determined as dodecenoic acid (designated as BDSF). Similar to DSF, BDSF appears to play significant roles in both intraspecies and interspecies communications.

The another focus of Dr. Zhang’s group is the systems biology of microbial quorum sensing. His major curiosities are: (1) the signaling mechanism; (2) the quorum sensing regulon; and (3) the quorum sensing regulatory network and regulatory mechanisms. By using genetic and biochemical analysis, the group has unveiled that DSF signal production is auto-regulated by a novel post-translational mechanism, which involves protein-protein interaction between the DSF synthase RpfF and sensor RpfC. By using an integrative genomic and genetic approach, they have further demonstrated that DSF regulates a wide range of genes encoding various biological functions, including virulence, biofilm dispersal, drug resistance and survival competence, through a hierarchical signaling network (Fig. 1)

Prokaryote-prokaryote and prokaryote-eukaryote interactions are ubiquitous in natural ecosystems. Given that diverse bacterial species use quorum sensing-coordinated community biological activities to boost their competitive advantages, for example, production of antibiotics and virulence factors, it is rational that competitors may also have evolved certain mechanisms to disarm the quorum sensing systems of microbes to gain the upper hand in competition. Prompted by this consideration, the group has discovered two types of widely conserved novel enzymes that degrade AHL-type quorum sensing signals (AHL-lactonase and AHL-acylase), and demonstrated that quenching quorum-sensing by inactivation of quorum sensing signal blocks bacterial infections and biofilm formation (Fig. 2). Kinetics and specificity analyses show that AHL-lactonase is a potent and highly specific enzyme. Their latest results indicate that the enzymes that degrades AHL-type QS signals are widely conserved in mammalian species. The group is currently testing the potentials of the quorum quenching enzymes as therapeutical proteins, and will continue to screen and characterize novel signal interference molecules to control and prevent the QS-mediated bacterial and fungal infections and biofilm formation.