A tangled web: regulatory connections between quorum sensing and cyclic Di-GMP

Abstract

Bacteria sense and respond to environmental cues to control important developmental processes. Two widely conserved and important strategies that bacteria employ to sense changes in population density and local environmental conditions are quorum sensing (QS) and cyclic di-GMP (c-di-GMP) signaling, respectively. The importance of these pathways in controlling a broad variety of functions, including virulence, biofilm formation, and motility, has been recognized in many species. Recent research has shown that these pathways are intricately intertwined. Here we review the regulatory connections between QS and c-di-GMP signaling. We propose that the integration of QS with c-di-GMP allows bacteria to assimilate information about the local bacterial population density with other physicochemical environmental signals within the broader c-di-GMP signaling network.

Fig 1

C-di-GMP signaling integrates information about local cell density through QS. The synthesis and degradation of c-di-GMP are controlled by multiple environmental signals to modulate downstream phenotypic changes. In this review, we argue that the information regarding local cell density transmitted by QS pathways is but one of many environmental signals that are ultimately integrated into the c-di-GMP signaling network composed of multiple signaling pathways (not depicted here) to allow bacteria to appropriately adapt and respond to different environments.

Fig 2

The QS system of Xanthomonas campestris modulates c-di-GMP levels in the cell. The AI signal, DSF, depicted as blue four-pointed stars, is synthesized by RpfF protein (red) and sensed by RpfC (blue), a membrane-bound histidine kinase protein. The double lines in Fig. 2 to 4 indicate the inner membrane. At low cell densities, when the concentration of DSF is low, RpfC interacts with RpfF and decreases its activity. In this state, the HD-GYP protein RpfG (green) is unphosphorylated and inactive. This leads to an increase in c-di-GMP levels (depicted by red triangles). C-di-GMP binds to the transcription factor Clp (gray) to abrogate its ability to bind DNA. At high cell density, RpfC binds to DSF, leading to phosphorylation of RpfG, which activates its PDE activity and decreases the c-di-GMP pool. RpfF no longer binds to RpfC and produces more DSF. The decrease in c-di-GMP activates Clp, which then induces target gene expression either directly or through modulation of other transcription factors. Clp can also promote biofilm dispersal through an unknown mechanism. RpfG also binds to and inhibits the activity of other GGDEF proteins.

Fig 3

Control of c-di-GMP by the QS system of Vibrio cholerae. QS-mediated control of c-di-GMP in V. cholerae occurs at multiple levels. At low cell density, the levels of AIs AI-2 (brown double pentagons) and CAI-1 (orange double triangles) are low, causing the LuxPQ (pink) and CpqS (blue) histidine kinase receptors to ultimately phosphorylate the response regulator LuxO (gray; not all steps in this pathway are shown). Phosphorylated LuxO activates the expression of qrr sRNAs, which repress HapR expression by destabilization of hapR mRNA. HapR (orange) is the master high-cell-density regulator of V. cholerae. Qrr sRNAs also activate expression of VCA0939, a GGDEF domain-containing protein, by stabilizing its transcript. In the low-cell-density state, the levels of c-di-GMP (red triangles) in the cell are high. VpsR (purple) and VpsT (green), two transcriptional activators that directly bind to c-di-GMP, positively regulate biofilm genes. Also, expression of AphA (blue), the master QS low-cell-density regulator, is induced by VpsR and c-di-GMP to activate low-cell-density-expressed genes. Virulence factor expression is also induced by AphA but thought to be repressed by c-di-GMP, and this contradiction is not currently understood. At high cell densities, the increase in AI-2 and CAI-1 levels reverses the flow of phosphate in the QS cascade, leading to decreased qrr sRNA expression. The lack of qrr sRNAs increases HapR protein, which then regulates the transcription of multiple GGDEF, EAL, and HD-GYP enzymes, both positively and negatively, to decrease c-di-GMP levels in the cell. HapR also directly represses vpsT and aphA expression, decreasing biofilm formation, virulence factor expression, and low-cell-density gene expression. The expression of high-cell-density genes is increased by the presence of HapR.

Fig 4

The ScrABC system of Vibrio parahaemolyticus. The Scr QS system of V. parahaemolyticus utilizes c-di-GMP to transmit the cell density information to the control of downstream gene expression. In this system, at low cell densities, the levels of S-signal (depicted by yellow circles) synthesized by ScrA (orange) are low. ScrB (green), the receptor for the S-signal, is in its unbound state, and ScrC (purple), the membrane-bound GGDEF and EAL domain protein, acts as a DGC, increasing c-di-GMP levels in the cell (red triangles). High c-di-GMP levels are sensed by CpsQ (blue), a c-di-GMP binding transcription factor, which activates downstream cps gene expression and induces biofilm development. CpsR (pink) is a transcription factor that induces expression of CpsQ, while CpsS (red) represses CpsR. The predicted connections between c-di-GMP, CpsS, and CpsR are depicted by dashed lines. At high cell densities, S-signal increases and ScrB binds the S-signal, changing its interaction with ScrC to convert it to a PDE to reduce c-di-GMP levels. This reduction in c-di-GMP levels leads to an increase in laf gene and lateral flagellar expression, type III secretion, and lipoprotein and chitin binding protein levels.