TRANSMEMBRANE SIGNALING BY THE CHIMERIC CHEMOSENSORY RECEPTORS OF ESCHERICHIA-COLI TSR AND TAR WITH HETEROLOGOUS MEMBRANE-SPANNING REGIONS

The serine and aspartate chemosensory receptors (isr and Tar) of Esche richia coil have two membrane-spanning regions TM1 and TM2. To investi gate their roles in transmembrane signalling, we constructed two chime ric receptors from Tsr and Tar with heterologous combinations of TM1 a nd TM2: the N-terminus of one receptor, including TM1 and the periplas mic domain, was fused to the C-terminus of the other, beginning with T M2. Both of the chimeric receptor genes rescued the chemotactic defect of a receptorless E. coil strain, indicating that the chimeric recept ors are functional. Their apparent affinities for the specific ligands were the same as those of Tsr or Tar. Therefore, as far as transmembr ane signalling abilities are concerned, the TM2 regions of Tsr and Tar are interchangeable, suggesting that sequence-specific interaction be tween TM1 and TM2 may not be required for the signal transmission acro ss the membrane. The cells expressing either of the chimeric receptors , however, showed 'smooth', biased, basal swimming patterns. Moreover, they adapted quickly after stimulation with the repellent glycerol. T his rapid adaptation was observed even in the methyltransferase-defect ive strain. Therefore, exchange of TM2 might impose structural constra ints on the chimeric receptors that stabilize conformations which elic it smooth swimming.