text: RNA polymerase binding in bacteria involves the sigma factor recognizing the core promoter region containing the −35 and −10 elements (located before the beginning of sequence to be transcribed) and also, at some promoters, the α subunit C-terminal domain recognizing promoter upstream elements. There are multiple interchangeable sigma factors, each of which recognizes a distinct set of promoters.  For example, in E. coli, σ is expressed under normal conditions and recognizes promoters for genes required under normal conditions ("housekeeping genes"), while σ recognizes promoters for genes required at high temperatures ("heat-shock genes"). In archaea and eukaryotes, the functions of the bacterial general transcription factor sigma are performed by multiple general transcription factors that work together. The RNA polymerase-promoter closed complex is usually referred to as the "transcription preinitiation complex."
  After binding to the DNA, the RNA polymerase switches from a closed complex to an open complex.  This change involves the separation of the DNA strands to form an unwound section of DNA of approximately 13 bp, referred to as the "transcription bubble".  Supercoiling plays an important part in polymerase activity because of the unwinding and rewinding of DNA.  Because regions of DNA in front of RNAP are unwound, there are compensatory positive supercoils.  Regions behind RNAP are rewound and negative supercoils are present.