Two-Component Systems in Francisella Species

Abstract

Bacteria alter gene expression in response to changes in their environment through various mechanisms that include signal transduction systems. These signal transduction systems use membrane histidine kinase with sensing domains to mediate phosphotransfer to DNA-binding proteins that alter the level of gene expression. Such regulators are called two-component systems (TCSs). TCSs integrate external signals and information from stress pathways, central metabolism and other global regulators, thus playing an important role as part of the overall regulatory network. This review will focus on the knowledge of TCSs in the Gram-negative bacterium, Francisella tularensis, a biothreat agent with a wide range of potential hosts and a significant ability to cause disease. While TCSs have been well-studied in several bacterial pathogens, they have not been well-studied in non-model organisms, such as F. tularensis and its subspecies, whose canonical TCS content surprisingly ranges from few to none. Additionally, of those TCS genes present, many are orphan components, including KdpDE, QseC, QseB/PmrA, and an unnamed two-component system (FTN_1452/FTN_1453). We discuss recent advances in this field related to the role of TCSs in Francisella physiology and pathogenesis and compare the TCS genes present in human virulent versus. environmental species and subspecies of Francisella.

Keywords: Francisella; PmrA; QseB; QseC; response regulator; sensor histidine kinase; tularemia; two-component system (TCS).

Figure 1

Francisella non-classical two-component system (TCS) QseC and QseB/PmrA. The domain organization of the sensor kinase (SK) QseC as a transmembrane protein is shown, along with its critical domains (described in detail in Figure 3) and autophosphorylation phosphoacceptor site on Hisitidine 259. The sensing domain is located in the periplasmic region (P), between the inner membrane (IM) and outer membrane (OM). The signal or ligand is shown in the periplasm as well. The response regulator (RR) QseB/PmrA is shown as pink curved shape, along with the phosphorylated aspartate phosphoacceptor site (P). The phosphorelay from the SK to the RR is shown by the solid curved arrow. Dimerization of the phosphorylated response regulator enables promoter binding. The auto-regulation of the RR QseB/PmrA expression and the regulation of SK QseC expression along with expression of the TCS regulon, is illustrated in the lower part of the figure, with dotted-line arrows to the resulting proteins.

Figure 2

Sylvatic cycle of Francisella tularensis, illustrating the transmission cycles and the relevant biting insects depending on the region (Art by Brad Gilleland, UGA College of Veterinary Medicine. © 2004 - 2019 University of Georgia Research Foundation, Inc.). Printed by Permission of the University of Georgia Research Foundation Inc.

Figure 3

Predicted protein domains of selected Francisella TCS genes. (A) QseB. Domain arrangement of Francisella QseB/PmrA (FTT_1557c). This 228 amino acid protein is a response regulator. The critical aspartate that receives the phosphorylation is D51 (*). The dimerization domain is from residues 3–113. The DNA binding domain is from residues 147–221 (https://www.uniprot.org/uniprot/Q5NER1; http://pfam.xfam.org/protein/Q5NER1). (B) QseC. Domain arrangement of Francisella QseC (FTT_0094c). This 475 amino acid protein is a sensor Histidine kinase. This protein is predicted to have two transmembrane domains (Residues 12–36 and 172–195), flanking the sensing domain (33–171). The Histidine kinase domain is located between residues 249–470. The HisKA domain is identified as between 249–314 (also identified as the dimerization domain), while the HATPase_c domain is identified between residues 363–470. The critical phosphoacceptor Histidine involved in phosphorelay is His259 (*) (https://www.uniprot.org/uniprot/Q5NIH6; http://pfam.xfam.org/protein/Q5NIH6). (C) KdpD. Domain arrangement for Francisella KdpD (FTT_1736c). This 893 (100.9 kDa) protein is a more complex sensor Histidine kinase than QseC. Unlike QseC, it has a long N-terminal domain that is annotated to contain a KdpD domain (from residues 21–230). KdpD also is annotated to have three transmembrane domains (residues 405–435, 442–461, and 467–495), comprising DUF4118. The C-terminal hisitidine kinase domain is annotated from residues 671–890, with the HisKA domain from residues 664–732, and the HATPase_c domain from residues 776–890. The critical Histidine for phosphorelay is at Histidine 674 (*) (https://www.uniprot.org/uniprot/Q5NEA7, http://pfam.xfam.org/protein/Q5NEA7).

Figure 4

Sensor Kinases and Response Regulators in Francisella species. (A) There are no tandemly arranged TCS in Ft Schu S4 (Larsson et al., 2005). Both the SK FTT1544 and RR FTT1735c (kdpE) are pseudogenes. FTT1543, KdpD, PmrA/QseB, and QseC are orphan TCS components in F. tularensis Schu S4. (B) There are no tandemly arranged TCSs in F. holarctica LVS. LVS (Live Vaccine Strain) has only one RR (FTL0552/PmrA/QseB) and one orphan SK (QseC). F. holarctica FSC200 has a similar set of genes as LVS, and has the same pseudogenes (Alkhuder et al., 2010). (C) There are two complete and one uncoupled TCS in F. novicida. FTN1452/FTN1453 and KdpDE form the two tandemly arranged TCS, while FTN1465 (PmrA/QseB) and QseC are orphan members. (D) F. philomiragia has three RR and three SK genes, similar to F. novicida. As in F. novicida and LVS, PmrA/QseB, and QseC appear to be orphan TCS components.