FIG 4.

Effects of l-leucine on Lrp binding to and activation of P_rtxA. (A and B) Each 452-bp DNA probe of the P_rtxA regulatory region (probe 1 for panel A and probe 2 for panel B; 5 nM) was radioactively labeled and then incubated with increasing amounts of Lrp as indicated. Increasing amounts of l-leucine were added to the reaction mixture containing the 5 nM labeled DNA and 400 nM (A) or 100 nM (B) Lrp as indicated. The DNA-protein complexes were separated by electrophoresis. B, bound DNA; F, free DNA. (C) The V. vulnificus strains harboring the reporter plasmid with promoterless lacZ fused to P_rtxA were grown to an A₆₀₀ of 0.5 with or without 10 mM l-leucine. The β-galactosidase activities of the V. vulnificus cells were measured and expressed in Miller units. Error bars represent the SD. Statistical significance was determined by the Student’s t test (*, P < 0.05; ns, not significant). WT, lacZ mutant harboring the reporter plasmid; lrp, lrp lacZ double mutant harboring the reporter plasmid.