Biphasic excitation by leucine in Escherichia coli chemotaxis

Leucine concentration jumps (applied by photolysis of inert derivatives) triggered swim or tumble responses in Escherichia coli mutants lacking Tsr or Tar, respectively. Wild-type E. coli bacteria were attracted in spatial assays when the initial leucine concentration difference was 5 to 120 micro M but were repulsed when it was over 0.5 mM. Their responses to concentration jumps confirmed earlier deductions regarding biphasic excitation.