<Li> Region 4.2 recognizes and binds to the promoter - 35 element . </Li> <P> One exception to this organization is in σ - type sigma factors . Proteins homologous to σ / RpoN are functional sigma factors, but they have significantly different primary amino acid sequences . </P> <P> The core RNA polymerase (consisting of 2 alpha (α), 1 beta (β), 1 beta - prime (β'), and 1 omega (ω) subunits) binds a sigma factor to form a complex called the RNA polymerase holoenzyme . It was previously believed that the RNA polymerase holoenzyme initiates transcription, while the core RNA polymerase alone synthesizes RNA . Thus, the accepted view was that sigma factor must dissociate upon transition from transcription initiation to transcription elongation (this transition is called "promoter escape"). This view was based on analysis of purified complexes of RNA polymerase stalled at initiation and at elongation . Finally, structural models of RNA polymerase complexes predict that, as the growing RNA product becomes longer than ~ 15 nucleotides, sigma must be "pushed out" of the holoenzyme, since there is a steric clash between RNA and a sigma domain . However, a recent study has shown that σ can remain attached in complex with the core RNA polymerase, at least during early elongation . Indeed, the phenomenon of promoter - proximal pausing indicates that sigma plays roles during early elongation . All studies are consistent with the assumption that promoter escape reduces the lifetime of the sigma - core interaction from very long at initiation (too long to be measured in a typical biochemical experiment) to a shorter, measurable lifetime upon transition to elongation . </P> <P> It long has been thought that the σ factor obligatorily leaves the core enzyme once it has initiated transcription, allowing the free σ to link to another core enzyme and initiate transcription at another site . Thus, the σ cycles from one core to another . However, Richard Ebright and coworkers, using fluorescence resonance energy transfer, later showed that the σ does not obligatorily leave the core . Instead, the σ changes its binding with the core during initiation and elongation . Therefore, the σ cycles between a strongly bound state during initiation and a weakly bound state during elongation . </P>

When does sigma factor normally dissociate from rna polymerase
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