Closed complex aThe initial binding of polymerase to a promoter DNA remains double stranded a The enzyme is bound to one face of the helix 21
21 ◼The initial binding of polymerase to a promoter ◼DNA remains double stranded ◼The enzyme is bound to one face of the helix Closed complex
Open complex a the DNA strand separate over a distance of N 14 bp (11 to +3)around the start site (+1 site) a transcription bubble forms
22 Open complex ◼ the DNA strand separate over a distance of ~14 bp (-11 to +3 ) around the start site (+1 site) ◼ transcription bubble forms
Stable ternary complex ■ The enzyme escapes from the promoter ■ The transition to the elongation phase a Stable ternary complex EDNA +RNA enzyme 23
23 Stable ternary complex ◼ The enzyme escapes from the promoter ◼ The transition to the elongation phase ◼ Stable ternary complex =DNA +RNA + enzyme
RNA polymerase Fig 12-3-initiation upstream downstream DNA DNA promoter binding Binding(closed (closed complex complex) promoter maltin Promoter“ melting” (ope complex (open complex) initial Initial RNA transcription 24
24 Fig 12-3-initiation Binding (closed complex) Promoter “melting” (open complex) Initial transcription
Elongation a Once the RNa polymerase has synthesized a short stretch of RNA ( nt), transcription shifts into the elongation phase. a This transition requires further conformational change in polymerase that leads it to grip the template more firmly. a Functions: synthesis RNA unwinds the dna in front, re-anneals it behind, dissociates the growing RNA chain
25 Elongation ◼ Once the RNA polymerase has synthesized a short stretch of RNA (~ 10 nt), transcription shifts into the elongation phase. ◼ This transition requires further conformational change in polymerase that leads it to grip the template more firmly. ◼ Functions: synthesis RNA, unwinds the DNA in front, re-anneals it behind, dissociates the growing RNA chain