Crl facilitates RNA polymerase holoenzyme formation

The Escherichia coli Crl protein has been described as a transcriptional coactivator for the stationary-phase sigma factor σ^S. In a transcription system with highly purified components, we demonstrate that Crl affects transcription not only by the Eσ^S RNA polymerase holoenzyme but also by Eσ⁷⁰ and Eσ³². Crl increased transcription dramatically but only when the σ concentration was low and when Crl was added to σ prior to assembly with the core enzyme. Our results suggest that Crl facilitates holoenzyme formation, the first positive regulator identified with this mechanism of action.