Figure 6. Schematic representation of possible thermosensing by specific SKs from Archaea and Bacteria.

DesK/DesR from B. subtilis. DesK senses temperature through its TMDs which manifest as coiled-coil interactions of the DesK homodimer³⁵, thereby regulating the phosphotransfer activities of DesK with DesR. DesR regulates the expression of desaturase genes which function to control membrane fluidity. LtrK/LtrR from M. burtonii. The cytoplasmic domain of LtrK has high activity at 0 °C, highest activity at 10 °C and minimal activity at 30 °C, consistent with LtrK/LtrR being able to regulate gene expression in response to growth temperature. LtrK also possesses an extracellular CHASE domain which is likely to detect an environmental signal (other than temperature). LtrR has an N-terminal HTH domain, typical of Archaea, which is in the opposite orientation to HTH domains from most Bacteria. Also typical of Archaea, M. burtonii possesses ether-linked isoprenoid lipids attached to a glycerol-1-phosphate backbone (membrane shaded grey), whereas Bacteria contain ester-linked fatty acids attached to a glycerol-3-phosphate backbone (shaded white). M. burtonii lipid unsaturation increases at low growth temperatures⁷⁸. The genes regulated by LtrR have not been determined. VirA/VirG from A. tumefaciens. VirA senses temperature via its cytoplasmic domain¹⁰. Distinct structural domains of VirA are also capable of directly or indirectly detecting sugars, phenolic compounds and acidity levels⁷⁶. VirG regulates genes involved in plant tumour formation.