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Line 10: − + Line 16:
Line 16: − '''RNA Synthesis'''+ − − '''Termination'''+ −
→Prokaryotic Transcription
===Prokaryotic Transcription===
===Prokaryotic Transcription===
[[Image:Bacterial_rna_polymerase.png|left|thumb|Prokaryotic RNA polymerase]]
[[Image:Bacterial_rna_polymerase.png|left|thumb|Prokaryotic RNA polymerase]]
====Initiation====
Bacterial RNA polymerases are composed of multiple subunits. The core enzyme consists of 2 α, β and β′ subunits. This enzyme contains the catalytic activity for RNA synthesis. Association of the core enzyme with the σ subunit is required for initiation of transcription. This complex is referred to as the holoenzyme. The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence. In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35. The σ subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site). This complex is referred to as the closed complex.
Bacterial RNA polymerases are composed of multiple subunits. The core enzyme consists of 2 α, β and β′ subunits. This enzyme contains the catalytic activity for RNA synthesis. Association of the core enzyme with the σ subunit is required for initiation of transcription. This complex is referred to as the holoenzyme. The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence. In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35. The σ subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site). This complex is referred to as the closed complex.
<br />The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription. This is referred to as the open complex.
<br />The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription. This is referred to as the open complex.
[[Image:Image010.gif|right|thumb|Termination of Prokaryotic Transcription]]
[[Image:Image010.gif|right|thumb|Termination of Prokaryotic Transcription]]
====Elongation====
The first two rNTPs complementary to the DNA bind to the active site of RNA polymerase and the first phosphodiester bond is formed. Synthesis is always in the 5’ to 3’ direction and involves the nucleophic attack by the 3’OH on the 5’ phosphate with the release of pyrophosphate. After approximately 8-10 ribonucleotides are added, σ factor is released and RNA polymerase core enzyme moves away from the promoter, synthesizing RNA as it goes.
The first two rNTPs complementary to the DNA bind to the active site of RNA polymerase and the first phosphodiester bond is formed. Synthesis is always in the 5’ to 3’ direction and involves the nucleophic attack by the 3’OH on the 5’ phosphate with the release of pyrophosphate. After approximately 8-10 ribonucleotides are added, σ factor is released and RNA polymerase core enzyme moves away from the promoter, synthesizing RNA as it goes.
====Termination====
Transcription continues until a termination signal is reached. The current model is that the RNA dissociates from the DNA – RNA hybrid to form a stem-loop structure. The remaining AU base pairs are not strong enough to keep RNA hybridized with the DNA. Dissociation of the RNA destabilizes RNA polymerases and transcription terminates.
Transcription continues until a termination signal is reached. The current model is that the RNA dissociates from the DNA – RNA hybrid to form a stem-loop structure. The remaining AU base pairs are not strong enough to keep RNA hybridized with the DNA. Dissociation of the RNA destabilizes RNA polymerases and transcription terminates.