The Trg protein is one of a family of chemosensory receptors that gain or lose methyl esters on specific glutamyl residues during sensory adaptations in Escherichia coli. In this study, we have characterized some of the mutations of Trg created in vitro by site-directed mutagenesis. The mutations have changes in methylation sites such that all possible methyl-accepting residues, either glutamate or glutamine, are replaced by glutamate, glutamine, or alanine only. Behavioral analysis for the cells containing mutated Trg using computerized motion analysis package revealed that those mutations exhibited altered signalling properties compared to the wild type. All glutamate-replaced mutant Trg showed exclusively counterclockwise (CCW)-bias of flagellar rotation in modification-deficient ($cheR^-$ $cheB^-$) host while glutamine or alanine ones exhibited significant clockwise (CW)-bias. It was also demonstrated that Gin or Ala mutations were excitable upon stimulation with attractant ribose. Moreover, the excited state persisted after prolonged stimulation. This results imply that the methylation set the level of signalling output of the transducer. In addition, the complete lack of the methylation shown in Ala mutation did not abolish the excitatory signalling of the transducer. Therefore, it is likely that the methylation operates as a fine-tuning device for the transducer to adjust the level of signalling output while the adaptational feedback requires an additional modulator, possibly a Che protein.