ssrB activates the promoter of sifA. The expression of the luxCDABE operon under the control of the PsifA promoter was analyzed in the wild type and the isogenic ssrA mutant and ssrB mutant with or without complementation. A, the strains were grown in MgM or in LB for six hours, and the light emitted by bacterial suspensions of similar cell density were captured using the Xenogen IVIS imaging system (Alameda, CA) and displayed as pseudocolor images with blue representing the lowest and red the highest light intensity. 1, wild type + PsifA-lux; 2, wild type + pSB384 vector control; 3, ssrA + PsifA-lux; 4, ssrB + PsifA-lux; 5, ssrB + PsifA-lux + pssrB; 6, ssrB + PsifA-lux + pWSK29 control. B, quantification of the activation of the PsifA promoter. The light emitted by 100 microliters of culture was measured using a luminometer. The values represent the mean ± SEM of three independent experiments.