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  • 1
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Expression from both the Escherichia coli nir and nrf promoters is dependent on anaerobic induction by FNR but is further regulated by NarL and NarP in response to the presence of nitrite and nitrate in the growth medium. The nir promoter is activated by NarL in response to nitrate and nitrite and activated by NarP in response to nitrate but not nitrite. The effects of point mutations suggest that NarL and NarP both bind to the same target, which is a pair of heptamer sequences organized as an inverted repeat, centred 691/2 bp upstream of the transcript startpoint. The nrf promoter can be activated by either NarP or NarL in response to nitrite but is repressed by NarL in response to nitrate. Mutational analysis of the nrf promoter has been exploited to corroborate the location of the -10 hexamer and the FNR-binding site, and to find the sites essential for nitrite-dependent activation and nitrate-dependent repression. Optimal activation by NarP or NarL in response to nitrite requires an inverted pair of heptamer sequences, similar to that found at the nir promoter, but centred 741/2 bp upstream from the transcript start. NarL-dependent repression by nitrate is due to two heptamer sequences that flank the FNR-binding sequence. We conclude that NarL and NarP bind to the same heptamer sequences, but that the affinities for the two factors vary from site to site.
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  • 2
    ISSN: 1365-2958
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology , Medicine
    Notes: Transcription initiation at the Escherichia coli nirB promoter is induced by anaerobic growth and further increased by the presence of nitrite or nitrate in the growth medium. Expression from this promoter is totally dependent on the transcription factor, FNR, which binds between positions −52 and −30 upstream of the transcription startsite. The 20 base pairs from position −79 to −60 contain an inverted repeat of two 10-base sequence elements that are related to sequences at the NarL-binding site at the E. coli narG promoter. Comparison of these, and sequence elements at other promoters regulated by NarL, suggests a consensus NarL-binding sequence. Mutations in the putative NarL-binding site at the nirB promoter decrease FNR-dependent anaerobic induction, suggesting that NarL acts as a helper to FNR during transcription activation. These mutations also suppress induction by nitrite: single mutations at symmetry-related positions have similar effects, whilst double mutations have more severe effects, probably because two NarL subunits bind to the inverted repeat. Disruption of narL decreases nitrite induction of the nirB promoter whilst not suppressing induction by nitrate, suggesting that there may be a second nitrate-responsive factor. Nitrate induction was, however, suppressed by double mutations at symmetry-related positions in the NarL-binding site, suggesting that this putative second factor may bind to sequences similar to those recognized by NarL.
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  • 3
    ISSN: 1617-4623
    Keywords: Key words Disulphide bond formation ; Cytochrome c assembly ; Nitrite reduction ; Nitrate reduction ; Escherichia coli
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  The assembly of anaerobically induced electron transfer chains in Escherichia coli strains defective in periplasmic disulphide bond formation was investigated. Strains deficient in DsbA, DsbB or DipZ (DsbD) were unable to catalyse formate-dependent nitrite reduction (Nrf activity) or synthesize any of the known c-type cytochromes. The Nrf+ activity and cytochrome c content of mutants defective in DsbC, DsbE or DsbF were similar to those of the parental, wild-type strain. Neither DsbC expressed from a multicopy plasmid nor a second mutation in dipZ (dsbD) was able to compensate for a dsbA mutation by restoring nitrite reductase activity and cytochrome c synthesis. In contrast, only the dsbB and dipZ (dsbD) strains were defective in periplasmic nitrate reductase activity, suggesting that DsbB might fulfil an additional role in anaerobic electron transport. Mutants defective in dipZ (dsbD) were only slightly more sensitive to Cu++ ions at concentrations above 5 mM than the parental strain, but strains defective in DsbA, DsbB, DsbC, DsbE or DsbF were unaffected. These results are consistent with our earlier proposals that DsbA, DsbB and DipZ (DsbD) are part of the same pathway for ensuring that haem groups are attached to the correct pairs of cysteine residues of apocytochromes c in the E. coli periplasm. However, neither DsbE nor DsbF are essential for the reduction of DipZ (DsbD).
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