FIG. 2.

Protease activity is influenced by environmental pH. Protease activity present in culture supernatants was measured at various time points during the growth of the cultures by use of an assay based on the cleavage of fluorescein isothiocyanate-labeled casein. Media included unbuffered C medium, C medium buffered with 0.1 M HEPES (pH 7.5), and C medium buffered with 0.1 M MES (pH 6.0, 6.3, and 6.5). A derivative of the wild-type strain, JWR100 (C192S), which produces no SpeB proteolytic activity, was grown in acidic C medium (pH 6.0) and tested to control for the specificity of the reaction. Activity is presented as a percentage of the maximal protease activity produced by HSC5 at 8 h in unbuffered medium. Data represent the means and standard deviations for two independent experiments for samples analyzed in triplicate.