An extracellular stress-sensing protein is activated by heat and u.v. irradiation as well as by mild acidity, the activation producing an acid tolerance-inducing protein

During growth of Escherichia coli roll in broth at pH 5.0, an extracellular protein termed an extracellular induction component (EIC) appears in the m edium, this component being essential for acid tolerance induction. The pre sent study establishes that the EIC arises from an extracellular precursor which is formed during growth at pH 7.0, and that conversion of the precurs or to the EIC occurs at pH 5.0 (and other mildly acidic pH values) in the a bsence of organisms. On the basis that this precursor is produced by non-st ressed cells as well as by stressed ones, and that it is converted to the E IC (which in turn induces the tolerance response) by the stress, the precur sor can be considered to be a stress sensor, the first extracellular stimul us sensor to be reported. The EIC formed at pH 5.0 was inactivated at pH 9. 0. This inactivation probably involved conversion back to the precursor as EIC was reformed if the alkali-inactivated component was incubated at pH 5. 0. Both mild heat treatments (exposure to 40-55 degrees C) and u.v. irradia tion also activated the precursor; the active induction component formed by the mild heat treatments was reversibly inactivated at pH 9.0 and so it se ems likely that the component formed by heat treatment is similar or identi cal to the EIC produced at acidic pH. In contrast, the EIC produced by u.v. irradiation was not inactivated at pH 9.0, suggesting that it is different in some way to the EICs produced from the precursor by acidity or by heat treatment. It is likely that many responses affecting stress tolerance invo lve the functioning of such extracellular sensors, as similar components we re shown to be involved in the acid tolerance responses induced at pH 7.0 b y glucose, L-aspartate and L-glutamate. Extracellular stimulus sensors may also be needed for other inducible responses.