Promoter recognition and discrimination by Eσ<sup>S</sup> RNA polymerase

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<jats:p>Although more than 30 <jats:italic>Escherichia coli</jats:italic> promoters utilize the RNA polymerase holoenzyme containing σ<jats:sup>S</jats:sup> (Eσ<jats:sup>S</jats:sup>), and it is known that there is some overlap between the promoters recognized by Eσ<jats:sup>S</jats:sup> and by the major <jats:italic>E. coli</jats:italic> holoenzyme (Eσ<jats:sup>70</jats:sup>), the sequence elements responsible for promoter recognition by Eσ<jats:sup>S</jats:sup> are not well understood. To define the DNA sequences recognized best by Eσ<jats:sup>S</jats:sup><jats:italic>in vitro</jats:italic>, we started with random DNA and enriched for Eσ<jats:sup>S</jats:sup> promoter sequences by multiple cycles of binding and selection. Surprisingly, the sequences selected by Eσ<jats:sup>S</jats:sup> contained the known consensus elements (−10 and −35 hexamers) for recognition by Eσ<jats:sup>70</jats:sup>. Using genetic and biochemical approaches, we show that Eσ<jats:sup>S</jats:sup> and Eσ<jats:sup>70</jats:sup> do not achieve specificity through ‘best fit’ to different consensus promoter hexamers, the way that other forms of holoenzyme limit transcription to discrete sets of promoters. Rather, we suggest that Eσ<jats:sup>S</jats:sup>‐specific promoters have sequences that differ significantly from the consensus in at least one of the recognition hexamers, and that promoter discrimination against Eσ<jats:sup>70</jats:sup> is achieved, at least in part, by the two enzymes tolerating different deviations from consensus. DNA recognition by Eσ<jats:sup>S</jats:sup> versus Eσ<jats:sup>70</jats:sup> thus presents an alternative solution to the problem of promoter selectivity.</jats:p>

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