ABSTRACT
The WalRK two-component system plays important roles in maintaining cell wall homeostasis and responding to antibiotic stress in low-GC Gram-positive bacteria. In the major human pathogen, Streptococcus pneumoniae, phosphorylated WalR
Spn
(VicR) response regulator positively controls the transcription of genes encoding the essential PcsB division protein and surface virulence factors. WalR
Spn
is phosphorylated by the WalK
Spn
(VicK) histidine kinase. Little is known about the signals sensed by WalK histidine kinases. To gain information about WalK
Spn
signal transduction, we performed a kinetic characterization of the WalRK
Spn
autophosphorylation, phosphoryltransferase, and phosphatase reactions. We were unable to purify soluble full-length WalK
Spn
. Consequently, these analyses were performed using two truncated versions of WalK
Spn
lacking its single transmembrane domain. The longer version (Δ35 amino acids) contained most of the HAMP domain and the PAS, DHp, and CA domains, whereas the shorter version (Δ195 amino acids) contained only the DHp and CA domains. The autophosphorylation kinetic parameters of Δ35 and Δ195 WalK
Spn
were similar [Km
(ATP) ≈ 37 μM; k
cat ≈ 0.10 min−1] and typical of those of other histidine kinases. The catalytic efficiency of the two versions of WalK
Spn
∼P were also similar in the phosphoryltransfer reaction to full-length WalR
Spn
. In contrast, absence of the HAMP-PAS domains significantly diminished the phosphatase activity of WalK
Spn
for WalR
Spn
∼P. Deletion and point mutations confirmed that optimal WalK
Spn
phosphatase activity depended on the PAS domain as well as residues in the DHp domain. In addition, these WalK
Spn
DHp domain and ΔPAS mutations led to attenuation of virulence in a murine pneumonia model.
The characterization of conserved binding motifs and potential target genes for M. tuberculosis MtrAB reveals a link between the two-component system and the drug resistance of M. smegmatis
