In this study, we investigated

the oxygen-sensitive regulator FNR in V. fischeri. Vibrio fischeri fnr complemented Palbociclib concentration an E. coli fnr mutant, and like fnr in E. coli, it is required for fumarate- and nitrate-dependent anaerobic respiration. Moreover, our data and another recent bioinformatic analysis (Ravcheev et al., 2007) suggest that the FNR-box recognition site is conserved in V. fischeri. For example, we observed fnr-mediated regulation of reporters for arcA (Fig. 3), dmsA (Dunn & Stabb, 2008), torE (Dunn & Stabb, 2008), and yfiD (data not show), which have predicted FNR boxes upstream. Taken together, FNR’s function in V. fischeri appears to be similar to that in its fellow gammaproteobacterium E. coli. As the first experimental examination of FNR in the Vibrionaceae, this study should underpin future efforts to understand FNR-mediated regulation in this important bacterial family. We initiated this study largely learn more because FNR is cited as an activator of luminescence in V. fischeri (e.g. see Meighen, 1994; Spiro, 1994; Sitnikov et al., 1995; Ulitzur & Dunlap, 1995; Stevens & Greenberg, 1999). However, that paradigm was based on a preliminary study that used the MJ1 lux genes cloned in E. coli (Muller-Breikreutz & Winkler, 1993). Our results appear to contradict that report, showing instead that FNR mediates repression of the luminescence-generating lux system in

V. fischeri under anaerobic conditions (Fig. 2). It is perhaps not surprising that lux regulation should be different in transgenic E. coli than in V. fischeri. For example, LitR, which activates luxR transcription, is absent in E. coli (Fidopiastis et al., 2002). It is also possible that FNR does activate luminescence in V. fischeri under conditions

different from those tested here, and that the discrepancy between our study and previous work simply reflects methodological differences. Repression of the lux genes anaerobically may minimize the production of luciferase when its O2 substrate is unavailable. This is consistent with the finding that luminescence is repressed by the ArcAB two-component regulatory system, which is more active under relatively reduced conditions (Bose et al., 2007). The observation that arcA∷lacZ reporters showed a lower expression in the absence of fnr (Fig. 3) suggests that the effect of FNR on bioluminescence C-X-C chemokine receptor type 7 (CXCR-7) may at least in part be indirect and mediated by FNR’s stimulation of arcA. Consistent with this idea, fnr did not exert much influence on luminescence in arcA mutant backgrounds, although arcA fnr double mutants were noticeably attenuated in anaerobic growth (data not shown). We speculate that FNR may amplify the repressive effect of ArcA on luminescence under reduced conditions. Although we cannot rule out the possibility that FNR exerts a direct effect by binding the lux region, as described above, we believe this model is unlikely.