Minimal machinery of RNA polymerase holoenzyme sufficient for promoter melting.

We determined the minimal portion of Escherichia coli RNA polymerase (RNAP) holoenzyme able to accomplish promoter melting, the crucial step in transcription initiation that provides RNAP access to the template strand. Upon duplex DNA binding, the N terminus of the beta' subunit (amino acids 1 to 314) and amino acids 94 to 507 of the sigma subunit, together comprising less than one-fifth of RNAP holoenzyme, were able to melt an extended -10 promoter in a reaction remarkably similar to that of authentic holoenzyme. Our results support the model that capture of nontemplate bases extruded from the DNA helix underlies the melting process.