['Two component signalling involves interaction between sensor kinase (SK) and response regulator (RR) proteins which depends on their phosphorylation status.', 'In this study we report the development of an in vitro FRET assay for studying interaction between fluorescently tagged SK and RR proteins.', 'Using TCS proteins of Mycobacterium tuberculosis, we demonstrate that phosphorylation status of SK affects the SK-RR interaction, which varies from one TCS to another. The observation was strengthened by recordings from mutant SK and RR proteins. The assay retained the specificity/crosstalk potential of the participating proteins and reflected the inherent phosphotransfer potentials.'] Does fRET reveal multiple interaction states between two component signalling system proteins of M. tuberculosis?