The interaction between the core form of bacterial RNA polymerases and σ factors is essential for specific promoter recognition, and for coordinating the expression of different sets of genes in response to varying cellular needs. The interaction between Escherichia coli core RNA polymerase and σ70 has been investigated by surface plasmon resonance. The His-tagged form of σ70 factor was immobilised on a Ni2+–NTA chip for monitoring its interaction with core polymerase. The binding constant for the interaction was found to be 1.9×10−7 M, and the dissociation rate constant for release of σ from core, in the absence of DNA or transcription, was 4×10−3 s−1, corresponding to a half-life of about 200 s.
