Overexpression and purification of the sigma subunit of Escherichia coli RNA polymerase.

We have constructed a plasmid that overexpresses 100-fold the sigma subunit of Escherichia coli RNA polymerase. The plasmid was constructed by placing the pLoL promoter-operator of bacteriophage lambda upstream from rpoD, the gene encoding the sigma subunit. A simple procedure for purification of the overexpressed protein has been developed based on guanidine hydrochloride denaturation/renaturation, DEAE cellulose chromatography, and Sephacryl S-200 chromatography. The purified product has been characterized and found to be indistinguishable from normally expressed sigma protein purified by previous protocols as judged by enzymatic activity, heat inactivation, and partial proteolysis.