Tuning the responsiveness of a sensory receptor via covalent modification.

Down-regulation or adaptation of receptors is an essential part of the chemotaxis mechanism to sense gradients. Using localized mutagenesis it is shown that the covalent modification of the receptors makes a slight change in the binding constant (factor of 2) which is far too small to explain the adaptation. The modification does, however, alter the signaling dramatically, an increasing tumbling signal being correlated with increased covalent modification. Responses in the two extreme cases, namely, completely unmodified and completely modified receptor, occur at attractant concentrations separated by 2 orders of magnitude. Amidation of the regulatory glutamate residues causes essentially the same signaling change as methylation. Thus, adaptation in chemotaxis is due to modulation of the receptor's signaling properties, not its affinity for the chemoeffector.