METHYLATION SEGMENTS ARE NOT REQUIRED FOR CHEMOTACTIC SIGNALING BY CYTOPLASMIC FRAGMENTS OF TSR, THE METHYL-ACCEPTING SERINE CHEMORECEPTOR OF ESCHERICHIA-COLI

The serine chemoreceptor Tsr and other methyl-accepting chemotaxis pro teins (MCPs) control the swimming behaviour of Escherichia coli by gen erating signals that influence the direction of flagellar rotation. MC Ps produce clockwise (CW) signals by stimulating the autophosphorylati on activity of CheA, a cytoplasmic histidine kinase, and counter-clock wise signals by inhibiting CheA, CheW couples CheA to chemoreceptor co ntrol by promoting formation of MCP/CheW/CheA ternary complexes, To id entify MCP structural determinants essential for CheA stimulation, we inserted fragments of the tsr coding region into an inducible expressi on vector and used a swimming contest called 'pseudotaxis' to select f or transformant cells carrying CW-signalling plasmids. The shortest ac tive fragment we found, Tsr (350-470), stimulated CheA in a CheW-depen dent manner, as full-length Tsr molecules do, It spans a highly conser ved 'core' (370-420) that probably specifies the CheA and CheW contact sites and other determinants needed for stimulatory control of CheA, Tsr (350-470) also carries portions of the left and right arms flankin g the core, which probably play roles in regulating MCP signalling sta le, However, this Tsr fragment lacks all of the methylation sites char acteristic of MCP molecules, indicating that methylation segments are not essential for generating receptor output signals.