Regulation of expression of the adhE gene, encoding ethanol oxidoreductasein Escherichia coli: Transcription from a downstream promoter and regulation by Fnr and RpoS

The adhE gene of Escherichia call, located at min 27 on the chromosome, enc odes the bifunctional NAD-linked oxidoreductase responsible for the convers ion of acetyl-coenzyme A to ethanol during fermentative growth. The express ion of adhE is dependent on both transcriptional and posttranscriptional co ntrols and is about 10-fold higher during anaerobic than during aerobic gro wth, Two putative transcriptional start sites have been reported: one at po sition -292 and the other at -188 from the translational start codon ATG, I n this study we show, by using several different transcriptional and transl ational fusions to the lacZ gene, that both putative transcriptional start sites can be functional and each site can be redox regulated. Although both start sites are NarL repressible in the presence of nitrate, Fnr activates only the -188 start site and Fis is required for the transcription of only the -292 start site. In addition, it was discovered that RpoS activates ad hE transcription at both start sites. Under all experimental conditions tes ted, however, only the upstream start site is active. Available evidence in dicates that under those conditions, the upstream promoter region acts as a silencer of the downstream transcriptional start site. Translation of the mRNA starting at -292, but not the one starting at -188, requires RNase III . The results support the previously postulated ribosomal binding site (RBS ) occlusion model, according to which RNase III cleavage is required to rel ease the RES from a stem-loop structure in the long transcript.