| This research project was a study of fermentation metabolism, specifically the transcriptional regulation of adhE in Salmonella enterica serovar Typhimurium (official designation of the former S. typhimurium). No study of this gene has been done in bacteria other than E. coli; this is therefore the first investigation of its kind performed in this bacterial species. Our results suggest that adhE is an anaerobic locus essential for acidic-mixed type fermentation. This gene is preferentially expressed under conditions of nutrient deprivation, not nutritional sufficiency, and is specifically expressed during the stationary phase (RpoS-dependent). In addition, the second promoter (P2), and not P1, is used for the synthesis of adhE mRNA. The results of our β-galactosidase and complementation assays indicate that adhE transcription is regulated negatively by CRA and positively by CRP and is dependent on RpoS, whereas FNR does not appear to regulate this gene. In E. coli, it is known that adhE is also repressed by CRA, though CRP is known to have no transcriptional role in this organism. FNR also functions as an activator in E. coli . The study of adhE has also permitted us to isolate a new mutant. The allyl alcohol resistance phenotype (AAR), which is associated with a mutation in the adhE gene of E. coli, is not a phenotype attributable to this locus in S. enterica. In fact, the isolation of a S. enterica AAR mutant, and the molecular characterization of its corresponding gene by cloning and sequencing, led us to study a new locus, adhP or adhI. The protein encoded by this locus is also present in E. coli. Very few studies have been done on it, except for several studies at the protein level in non-enteric bacteria. Our initial attempts to characterize the phenotype resulting from an adhP gene knockout in S. enterica have not succeeded in identifying any associated metabolic problems. In fact, an adhP mutant can metabolize all classes of carbon source, can adequately perform all types of respiration, and fermentation normally. Using only β-gal assays, we have been able to establish, for the first time, a profile of the expression and transcriptional regulation of adhP. According to our results, this gene would be specific to the RpoS-dependent stationary phase, like adhE, but would be optimally activated during aerobic respiration, induced by gluconeogenic substrates, and activated by CRA. The adhI gene is also repressed by the glycolytic class of energy sources and by cAMP/CRP. Repression in the absence of terminal electron acceptors involves FNR and repression by nitrates is probably mediated by NarL (like adhE). Knowing which transregulators are involved and under which conditions adhP is expressed and repressed, it is clear that this alcohol deshydrogenase plays a role, though minor, in the aerobic respiration type of metabolism. Its metabolic role remains to be defined. |
