An even larger mystery is posed by the newly ascribed energy-sensing role of the chemoreceptor Tsr. This role adds yet another category of stimuli to the responses mediated by Tsr, a list that already includes serine, external pH, weak acid repellents (cytoplasmic pH), temperature, hydrophobic amino acids, and indole (23). Tsr contains no redox-sensitive cofactor, thus it is possible that proton motive force rather than direct interaction with the electron transport chain is used by Tsr to monitor cellular energy levels. As Tsr contains no specialized energy-sensing domain it seems that energy sensing has been acquired through a minor adaptation of some region present in other chemoreceptors, which despite a high degree of sequence similarity, apparently do not have this capability.
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