FIG. 1.

Ampicillin-mediated induction factor of the lexA gene in S. aureus RN450 or its recA-mutant strain RN981. The induction factor was measured by quantitative reverse transciption-PCR, and in all cases, it is the ratio of the relative lexA mRNA concentration in ampicillin-treated cells to that in untreated cells. The relative lexA mRNA concentration was calculated as described previously (3). Values were calculated 5 h after ampicillin addition. In each case, the mean value from three independent experiments (each in triplicate) is shown. Amp+, addition of ampicillin at 0.2 μg/ml; Amp−, no ampicillin added.