Interaction of adenylate cyclase CyaC with regulators Smc01817 and Clr of Sinorhizobium meliloti: cAMP as an ON/OFF trigger in sensor-regulator complexes?

The symbiotic bacterium Sinorhizobium meliloti contains a large number of adenylate cyclases (AC) for the control of different life styles. ACs produce cyclic AMP (cAMP) as a secondary messenger. Earlier, the redox responsive membrane-intrinsic AC CyaC has been shown to produce a trimeric signalling complex CyaCxCycRxcAMP-Clr. Here, use of the bacterial two-hybrid system BACTH showed that the LysR-type transcriptional regulator Smc01817 interacts in vivo with CyaC and the general cAMP-regulator Clr, suggesting the formation of a sensor-regulator complex CyaCxSmc01817xcAMP-Clr. Therefore, formation of sensor-regulator complexes from ACs and transcriptional regulators seems to be a means in S. meliloti to setup specific signalling routes in a background with a large number of signalling routes applying the same signalling molecule (cAMP). Use of caged cAMP allows to address a specific signalling route and set of regulators, which is at variance with the classical role of cAMP as a diffusing secondary messenger.