Indole Can Act as an Extracellular Signal in Escherichia coli

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Abstract
Previous work has shown that lacZ fusions to thecysK, astD, tnaB, and gabT genes inEscherichia coli are activated by self-produced extracellular signals. Using a combination of ethyl acetate extraction, reversed-phase C18 chromatography, and thin-layer chromatography, we have purified an extracellular activating signal from E. coli supernatants. Mass spectrometry revealed a molecule with an m/z peak of 117, consistent with indole. Nuclear magnetic resonance analysis of the purified E. colifactor and synthetic indole revealed identical profiles. Using synthetic indole, a dose-dependent activation was observed withlacZ fusions to the gabT, astD, andtnaB genes. However,cysK::lacZ and several control fusions were not significantly activated by indole. Conditioned medium prepared from a tnaA (tryptophanase) mutant, deficient in indole production, supported 26 to 41% lower activation of thegabT and astD fusions. The residual level of activation may be due to a second activating signal. Activation of thetnaB::lacZ fusion was reduced by greater than 70% in conditioned medium from a tnaAmutant.