Competition among seven Escherichia coli sigma subunits: relative binding affinities to the core RNA polymerase

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Abstract
Seven different species of the RNA polymerase σ subunit exist in Escherichia coli, each binding to a single species of the core enzyme and thereby directing transcription of a specific set of genes. To test the σ competition model in the global regulation of gene transcription, all seven E.coli σ subunits have been purified and compared for their binding affinities to the same core RNA polymerase (E). In the presence of a fixed amount of σ70, the principal σ for growth-related genes, the level of Eσ70 holoenzyme formation increased linearly with the increase in core enzyme level, giving an apparent Kd for the core enzyme of 0.26 nM. Mixed reconstitution experiments in the presence of a fixed amount of core enzyme and increasing amounts of an equimolar mixture of all seven σ subunits indicated that σ70 is strongest in terms of core enzyme binding, followed by σN, σF, σE/σFecI, σH and σS in decreasing order. The orders of core binding affinity between σ70 and σN and between σ70 and σH were confirmed by measuring the replacement of one core-associated σ by another σ subunit. Taken together with the intracellular σ levels, we tried to estimate the number of each holoenzyme form in growing E.coli cells.

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