{'fraction_non_alphanumeric': 0.023884103367267033, 'fraction_numerical': 0.003915426781519186, 'mean_word_length': 5.468354430379747, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Transcriptional activation by FNR and CRP: reciprocity of binding‐site recognition

Anaerobic expression of the focA pfl operon is dependent on the transcription factors ArcA and FNR and transcription is directed by multiple, anaerobically regulated promoters. A FNR‐binding site is centred at −41.5 bp relative to the P6 promoter, inactivation of which severely impairs anaerobic expression of the complete operon. Mutations were introduced into this binding site to create a consensus recognition site for the cAMP‐receptor protein, CRP (CC‐site), and one that was recognised by both CRP and FNR (CF‐site). Transcription directed by these mutant binding sites in vivo in different promoter constructs was analysed by primer extension and by constructing lacZ operon fusions. With a derivative including only the P6 promoter and the CF‐binding site, transcription was shown to be independent of oxygen and was activated by CRP or FNR. In agreement with previous findings, FNR only activated transcription anaerobically. In a construct including the CC‐binding site transcription was strong, CRP dependent and initiated at the identical site to the wild‐type promoter. Transcription activation from the CC‐site was exquisitely sensitive to low cAMP concentration. Surprisingly, in a crp mutant, anaerobically inducible, FNR‐dependent transcription directed by the CC‐site was detected, indicating that FNR can recognise a consensus CRP‐binding site in vivo. A strain unable to synthesise CRP or FNR exhibited no transcription from the P6 promoter. Essentially the same results were observed in a series of constructs that also included the promoter P7 and its regulatory sequences. Evidence is also presented which demonstrates that CRP activates transcription from the natural FNR‐binding site of the P6 promoter. In vitro DNA‐binding studies showed that CRP specifically interacted with the FNR‐binding site, protecting exactly the same sequence as that protected by the FNR protein. Interaction of CRP with the natural FNR‐binding site was reduced greater than 50‐fold compared to its interaction with the mutant CC‐binding site. Although we could not demonstrate that FNR interacted with the CC‐binding site in vitro, it did bind to the CF‐site giving the same protection as observed with the wild‐type FNR‐binding site. FNR also activated transcription from the CF‐site in vitro, giving further support to the idea that a single functional DNA half‐site is sufficient to direct binding and transcription activation by a dimeric transcription factor.