Two-component systems required for virulence in Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa is a versatile opportunistic pathogen capable of infecting a broad range of hosts, in addition to thriving in a broad range of environmental conditions outside of hosts. With this versatility comes the need to tightly regulate its genome to optimise its gene expression and behaviour to the prevailing conditions. Two-component systems (TCSs) comprising sensor kinases and response regulators play a major role in this regulation. This minireview discusses the growing number of TCSs that have been implicated in the virulence of P. aeruginosa, with a special focus on the emerging theme of multikinase networks, which are networks comprising multiple sensor kinases working together, sensing and integrating multiple signals to decide upon the best response. The networks covered in depth regulate processes such as the switch between acute and chronic virulence (GacS network), the Cup fimbriae (Roc network and Rcs/Pvr network), the aminoarabinose modification of lipopolysaccharide (a network involving the PhoQP and PmrBA TCSs), twitching motility and virulence (a network formed from the Chp chemosensory pathway and the FimS/AlgR TCS), and biofilm formation (Wsp chemosensory pathway). In addition, we highlight the important interfaces between these systems and secondary messenger signals such as cAMP and c-di-GMP.

Keywords: Pseudomonas aeruginosa; Two-component signalling; multikinase network; secondary messengers; virulence.

Figure 1.

The GacS network including the closely affiliated HptB and SagS/BfiS branches. Red ovals show SKs, blue ovals show RRs, the purple oval shows the HptB protein and the grey ovals show other proteins in the system. Arrows show stimulatory interactions, while blunt-ended lines show inhibitory interactions and bulb-ended lines show interactions that can be stimulatory or inhibitory depending on conditions. The primary output of the GacS side of the pathway is the small RNAs RsmY and RsmZ, which sequester the post-transcriptional regulators, RsmA and RsmN. When RsmA and RsmN are sequestered, virulence genes associated with chronic infection are upregulated while those associated with acute virulence genes are downregulated. Conversely, when RsmA and RsmN are free, the acute virulence genes are upregulated and the chronic infection genes are downregulated. The HptB and SagS/BfiS branches of the pathway also regulate RsmY and RsmZ levels, respectively. The role of HsbA differs depending on whether it is phosphorylated (blue arrow) or dephosphorylated (red arrow). Two diguanylate cyclases are controlled by this network, HsbD and SadC.

Figure 2.

Model of the Roc network (A) and Rcs/Pvr network (B). Red ovals indicate the SKs, while the blue ovals are the RRs. The green oval is the unknown component that regulates cupB fimbriae. Arrows specify positive interactions and blunt-ended lines show inhibitory interactions. The bulb-ended line indicates that RcsC can have either stimulatory or inhibitory effects on RcsB depending on conditions.

Figure 3.

The network controlling the aminoarabinose modification of lipid A component of lipopolysaccharide. Five TCSs work together to sense magnesium ions, zinc ions and cationic antimicrobial peptides to regulate the expression of the arnBCADTEF operon which encodes the LPS modification enzymes. The LPS modification enhances resistance to host-derived cationic antimicrobial peptides and to polymyxin antibiotics.

Figure 4.

The Wsp chemosensory pathway. The proteins involved in the pathway are a methyl-accepting protein (WspA), CheW homologues (WspB and WspD), a CheA homologue (WspE), a diguanylate cyclase RR (WspR), a methylesterase RR (WspF) and a methyltransferase (WspC). Mechanical pressure associated with surface growth activates WspA, which promotes the autophosphorylation of WspE. WspE phosphorylates its two RRs, WspR and WspF. Phosphorylated WspR catalyses the synthesis of c-di-GMP (the secondary messenger output of this system). Meanwhile, phosphorylated WspF acts to reset the system by removing methyl groups from WspA, reducing its ability to activate WspE. The methylesterase activity of WspF is opposed by the constitutive methyltransferase activity of WspC.

Figure 5.

The Chp/FimS/AlgR network controls the production and operation of the type 4 pili, involved in surface attachment and twitching motility, and the expression of virulence genes. Surface contact is detected by PilJ (an MCP homologue), it activates signalling by two SKs: ChpA (a CheA homologue) and FimS. FimS phosphorylates its RR, AlgR, leading to the activation of its regulon (T4P genes, virulence genes, the diguanylate cyclase gene mucR and pilY1). ChpA phosphorylates three RRs: ChpB (a CheB homologue that mediates adaptation), PilG which activates the adenylate cyclase (CyaB) and the pilus extension ATPase (PilB), and PilH which may activate the pilus retraction ATPases (PilT/U) and inhibit adenylate cyclase (CyaB). The cAMP produced by CyaB binds to and activates the transcription factor Vfr, leading to the activation of its vast regulon, which includes T4P genes, virulence genes, the fimS/algR TCS and pilY1. After prolonged surface contact, the number of T4P increases due to AlgR and Vfr activity, which promotes the secretion of the outer-membrane surface-associated PilY1 protein. PilY1 signals to the diguanylate cyclase, SadC, which produces c-di-GMP that leads to the upregulation of biofilm genes and the downregulation of the T4P.