We tried another construct pCJK96 (rhamnose induction [30]), but faced the same issues (data not

shown). Thus, although we did not determine the threshold necessary for the ebpA expression, the presence of ebpR was confirmed to be critical for ebpA expression. One difference between ebpR and ebpA expression profiles see more in the presence of bicarbonate (vs. absence of bicarbonate) occurred after entry into stationary phase. ebpR and ebpA expression without bicarbonate begins to decrease, while it remained constant in the presence of bicarbonate. This difference may be explained either by an induction pathway that remains active (in the presence of HCO3 -) in stationary phase or by inhibition early in stationary phase of a repression pathway (e.g., quorum sensing or phase dependent regulator). The first mechanism would also explain the slight difference observed in the presence of HCO3 – during log

growth phase. A potential candidate is a RegA homologue, an AraC/XylS-like regulator from C. rodentium [19]. Among the E. JAK inhibitor faecalis AraC/XylS-like regulators, none shares additional significant similarity with RegA. A second possibility would be a quorum sensing MAPK Inhibitor Library order mechanism. A likely candidate would be the Fsr system [6]. However, the Fsr system, although a weak repressor of ebpR, does not appear to mediate the bicarbonate effect, since a similar ebpA expression pattern compared to OG1RF was observed in an fsrB mutant in the presence or absence of bicarbonate. Finally, we looked at the stress response pathway including ers and its regulon [26, 27]. Interestingly, several members of the ers regulon were affected by a 15 min bicarbonate exposure, including EF0082-3 and EF0104-6. However, although both operons are activated by ers, EF0082-3 were strongly repressed (-8 fold), while EF0104-6 were activated C1GALT1 (3 fold) by bicarbonate exposure. In addition, ers was not affected. In conclusion, the regulation pathways in E. faecalis resemble a network with several targets genes being under the control of independent regulation pathways illustrated by ebpR-ebpABC being independently a member of the bicarbonate

and the fsr regulon, and EF0082 a member of the bicarbonate and ers regulon. We also showed using microarray profiling that expression of many other genes (mostly PTS systems and ABC transporters) was altered in response to HCO3 -. Among those genes are EF2641 and EF2642, which encode a putative glycine betaine/L-proline ABC transporter and permease protein, respectively. Interestingly, this ABC transporter shares some homology with the bicarbonate transporter described in B. anthracis (Tau family of ABC transporters) [25]. However, we did not find a TauA motif, that has been proposed as the bicarbonate binding motif, associated with the EF2641-2 locus or in available E. faecalis genomes including OG1RF. Interestingly, expression of ebpR-ebpABC was not affected by the 15 minutes bicarbonate exposure.