Mutational analysis of β′260-309, a σ70 binding site located on Escherichia coli core RNA polymerase

eubacteria, the a subunit binds to the core RNA polymerase and directs transcription initiation from any of its cognate set of promoters, Previously, our laboratory defined a region of the beta' subunit that interacts with sigma(70) in vitro. This region of beta' contained heptad repeat motifs indicative of coiled coils. In this work, we used 10 single point mutations of the predicted coiled coils, located within residues 260-309 of beta', to look at disruption of the sigma(70)-core interaction, Several of the mutants mere defective for binding sigma(70) in vitro. Of these mutants, three (R275Q, E295K, and A302D) caused cells to be inviable in an in vivo assay in which the mutant beta' is the sole source of beta' subunit for the cell. All of the mutants were able to assemble into the core enzyme; however, R275Q, E295K, A302D were defective for E sigma(70) holoenzyme formation. Several of the mutants were also defective for holoenzyme assembly with various minor a factors. In the recently published crystal structure of Thermus aquaticus core RNA polymerase (Zhang, G., Campbell, E. k, Minakhin, L,, Richter, C,, Severinov,,, and Darst, S, A. (1999) Cell 98, 811-824), the region homologous to beta'(260-309) of Escherichia coli forms a coiled coil. Modeling of our mutations onto that coiled coil places the most defective mutations on one face of the coiled coil.