Addition of L-malate as free acid to the culture (end concentration of 25 mM), thereby lowering CP673451 the pH to 5.6-6.2 (depending on the growth stage in BM medium), resulted in an immediate induction of activity (Figure 3). To determine if this effect was caused by the low pH or by L-malate, we further studied the influence of both parameters separately. After inoculation, cells were allowed to adapt for two hours to the medium.

After addition of neutralized L-malate (25 mM final concentration) the pH of the cultures was adjusted with HCl to the desired values and samples for luciferase measurements were withdrawn in intervals of 30 min for two hours. Figure 4 summarizes the fold change values of promoter activity after two hours of measurement. Lowering the pH, without addition of malate, resulted in an increased activity of both promoters in the wildtype as well as in the ΔmleR background. These data clearly demonstrate that both promoters are acid inducible and see more that this behaviour was not caused by post-exponential phenomena. Furthermore, it shows that the influence of MleR is weak at neutral pH conditions. By contrast, the presence

of L-malate at low pH significantly enhanced the activity of both promoters, but only in the presence of a functional copy of mleR. This allows four conclusions: (a) L-malate is the coinducer of MleR; (b) enhanced transcription in the presence of L-malate requires an acidic pH; (c) MleR positively regulates its target Vitamin B12 genes and furthermore (d) its own transcription. A positive auto-regulation would be a special feature, since most LTTR repress their own transcription. However, exceptions exist e.g. LrhA [19]. However, no significant induction of mleR after two hours exposure to 25 mM free malic acid was observed using quantitative real time PCR (See below). Figure 3 Promoter activity of mleR in the presence of malate. Influence of L-malate (25 mM, not neutralized) on the promoter activity

of wildtype S. mutans carrying mleR p-luc in BMS medium under anaerobic conditions. Open diamond, growth without malate; Grey diamond, RLU, no addition of L-malate; Triangle, RLU, addition of L-malate after 30 min; Circle, RLU, addition after 2.5 hours; Square, RLU, addition after 4.5 hours. Figure 4 Influence of pH and L-malate on promoter activity of mleR and mleS. Cells of wildtype and ΔmleR were cultivated in BMS under anaerobic conditions. Neutralized L-malate was added to the respective samples and the pH was adjusted to the desired values. A: Fold change of RLU after two hours of strains carrying mleS p-luc. Left, wildtype. Right, ΔmleR Epacadostat mutant. B: Fold change of RLU after two hours of strains carrying mleR p-luc. Left, wildtype. Right, ΔmleR mutant. White bars, no addition of L-malate; Red bars, addition of 25 mM L-malate.