Intrinsic zinc ion is essential for proper conformation of active Escherichia coli RNA polymerase. 
The DNA-dependent RNA polymerase (RPase) from Escherichia coli contained 2 mol of Zn/mol of holoenzyme (alpha 2 beta beta' sigma).  An in vitro protocol involving sequential denaturation of RPase in 8 M urea and low pH (2.2), in the presence of 10 mM ethylenediaminetetraacetic acid (EDTA), was developed to completely remove the two intrinsic Zn ions.  Subsequent reconstitution of the denatured, Zn-free RPase in the absence and presence of 10(-5) approximately 10(-4) M ZnCl2 yielded respectively the inactive apoenzyme and active (50 +/- 10%) RPase containing one Zn ion (rec-Zn1-RPase).  Active rec-Cd1-RPase was similarly obtained when CdCl2 instead of ZnCl2 was used in the reconstitution.  The use of 65Zn as a tracer in the two-step reconstitution procedure showed that the metal was incorporated into renatured enzyme only in the last step of reconstitution.  The subunit location of the incorporated metal was identified to be in the beta subunit by the use of Affi-Gel Blue column chromatography of rec-Cd1-RPase.  The analysis of apo- and rec-Zn1-RPases by sucrose density gradient sedimentation showed that the inactive apo-RPase appeared to be consisted of randomly folded protein species with S20,w values ranging from 5 to 18 S, while rec-Zn1-RPase contained a major, active 13S RPase species and a minor, inactive 7.9S species that could be separated by DNA-cellulose column chromatography.  Both 13S and 7.9S RPase contained 1 mol of Zn and the five subunits.(ABSTRACT TRUNCATED AT 250 WORDS)