(B). Wild type or aphB mutant containing a P toxT -GDC-0068 molecular weight luxCDABE reporter

plasmid with or without pBAD-tcpPH AG-881 concentration were grown under the AKI condition. 0.01% arabinose was added to induce P BAD -tcpPH. Lux expression (blue bars) was measured and normalized against toxT expression in wild type. The results are the average of three experiments ± SD. Conclusion The ToxR regulon is the classic virulence gene regulation pathway in V. cholerae. In this pathway, AphA and AphB activate tcpP transcriptional expression directly by binding to different promoter regions of tcpP. ToxR and TcpP cooperate in turn by binding different sites of the toxT promoter to activate transcription, leading to the production of the virulence factors TCP and CT. However, mTOR inhibitor the full ToxR regulon is more complex than previously thought. In this paper, we showed that AphA and AphB are also necessary for full ToxR production at the stationary phase. Furthermore, we demonstrated that AphB is sufficient for toxR transcriptional activation in the heterogenic host E. coli through binding of the toxR promoter region. Thus, the effect of AphB on ToxR levels propagates further in the transcription cascade, increasing the transcription of a key gene in V. cholerae pathogenesis, toxT. We have

therefore identified another factor responsible for altering end product levels in the V. cholerae virulence axis. Since AphB is at the top of a virulence cascade with multiple end pathways, it appears now that AphB is a central factor in switching the cell from an environmental state to a virulent one. Since it activates ToxR in addition to TcpP, and further influences porin expression, AphB is a divergence point at which nonlinearity is introduced into the V. cholerae virulence pathway. Eukaryotic cells have extremely

complex networks of protein and DNA interactions leading to precise control of protein expression levels. Having a more complex network of transcriptional activation and repression in the V. cholerae virulence cascade could enable the bacterial cell to fine-tune its expression levels to optimize its ability to colonize the intestine and spread to other hosts. Methods Bacterial strains, plasmids and media All experiments were performed with El Tor Vibrio cholerae C6706 [30] or Escherichia coli DH5α, which were grown in RG7420 LB with relevant antibiotics at 37°C, except where noted. V. cholerae virulence genes were induced in vitro (the AKI condition) as previously described [22]. Briefly, 3 ml of AKI medium was inoculated with 0.5 μl of overnight culture and incubated for 4 hrs at 37°C without agitation. 1 ml of culture was transferred to a fresh tube and incubated with shaking for a further 4 hrs at 37°C. P toxR -luxCDABE fusion plasmid was constructed by polymerase chain reaction (PCR) amplifying the toxR promoter regions, ranging from 450 bp, 300 bp, to 130 bp, respectively, and cloning them into the pBBRlux vector [20].