The anaerobic transcriptional regulator ANR induces the arginine deiminase and denitrification pathways in during oxygen limitation. The homologous activator FNR of when introduced into an mutant of could functionally replace ANR for anaerobic growth on nitrate but not for anaerobic induction of arginine deiminase. In an FNR-positive strain, the ANR-dependent promoter of the operon, which encodes the enzymes of the arginine deiminase pathway, was not expressed. To analyse systematically these distinct induction patterns, a promoter-probe, broad-host-range plasmid containing various −40 regions (the ANR/FNR recognition sequences) and −10 promoter sequences was constructed. These constructs were tested in and in expressing either ANR or FNR. In conjunction with the consensus −10 hexamer of σ RNA polymerase (TATAAT), the consensus FNR site (TTGAT….ATCAA) was recognized efficiently by ANR and FNR in both hosts. By contrast, when promoters contained the Arc box (TTGAC…. ATCAG), which is found in the promoter, or a symmetrical mutant FNR site (CTGAT…. ATCAG), ANR was a more effective activator than was FNR. Conversely, an extended 22 bp, fully symmetrical FNR site allowed better activation with FNR than with ANR. Combination of the promoter −10 sequence (CCTAAT) with the Arc box or the consensus FNR site resulted in good ANR-dependent expression in but gave practically no expression in suggesting that RNA polymerase of differs from the enzyme in −10 recognition specificity. In conclusion, ANR and FNR are able to activate the RNA polymerases of and when the −40 and −10 promoter elements are identical or close to the consensus sequences.


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