Two-component systems provide a means for bacteria to sense and adapt to environmental signals in order to survive in a continuously changing environment. Understanding of the mechanism by which these systems function is important in combating bacterial infections because many bacterial two-component systems are associated with virulence. The plant pathogenic bacterium Pseudomonas syringae pv. glycinea PG4180 synthesizes high levels of the phytotoxin coronatine at the virulence-promoting temperature of 18 degrees , but not at 28 degrees , the optimal growth temperature. Temperature-dependent coronatine biosynthesis is regulated by a modified two-component system, consisting of the response regulator, CorR, the histidine protein kinase CorS, and a third component, CorP. To elucidate the mechanism by which CorRSP functions, genetic, transcriptional, and biochemical analyses were applied, including in vitro and in planta reporter gene analysis, mRNA quantification, protein expression, mutagenesis, and membrane topology analysis. A combination of these techniques helped to elucidate, to a considerable extent, the temperature-sensing activity of CorS, which seems to act as a membrane-bound molecular thermometer.