Association of several of these generates the RNA polymerase holoenzyme . The sigma subunit is only transiently associated with the holoenzyme. This subunit is required for accurate initiation of transcription by providing polymerase with the proper cues that a start site has been encountered. In both prokaryotic and eukaryotic transcription the first incorporated ribonucleotide is a purine and it is incorporated as a triphosphate. In E. coli several additional nucleotides are added before the sigma subunit dissociates.
Nucleotide addition continues until specific termination signals are encountered.
Following termination the core polymerase dissociates from the template. The core and sigma subunit can then reassociate forming the holoenzyme again ready to initiate another round of transcription. Two structural features of all E. coli factor-independently terminating genes have been identified. The formation of the stem-loop in the RNA destabilizes the association between polymerase and the DNA template. This is further destabilized by the weaker nature of the AU base pairs that are formed, between the template and the RNA, following the stem-loop.