Transcription in prokaryotes

this ppt talks about the process of transcription in prokaryotes.

1. Transcription in prokaryotes S.Kaayathri Devi I6UTIO III B.Sc., Biotechnology ANJAC

2. Central dogma of biology REPLICATION DNA DNA TRANSCRIPTION POST TRANSCRIPTIONAL MODIFICATION RNA TRANSLATION POST TRANSLATIONAL MODIFICATION PROTEIN

3. INTRODUCTION Synthesis of RNA by copying DNA template is known as transcription. Only selected portion of the genome (exons)is transcribed . RNA polymerase requires no primers it is a DNA dependent RNA polymerase

4. The process of transcription takes place in three major steps. They are : 1. Initiation. 2.Elongation. 3.Termination.

5. INITIATIATION Transcription is initiated by RNA Polymerase holoenzyme from a specific region called promotor sequence in DNA . Principle enzyme – Bacterial RNA Polymerase RNA Polymerase alone is called core Polymerase . Core polymerase consists of α , β , β ’and ω sub units.

6. The binding of core polymerase to promotor sequence is facilated by σ factor. Core polymerase + σ factor = RNA polymerase holoenzyme Promotor sequence E. Coli : 1. 5’-TTGACA-3’ at – 35 element (pribnow box) 2. 5’ – TATAAT – 3’ at – 10 element (TATAA box)

7. Closed complex formation : Binding of RNA polymerase holoenzyme to promotor sequence. Open complex formation : 1. RNA polymerase holoenzyme separates 10-14 DNA basepairs extending from -11 to +3 and this is called melting. 2. It leads to the formation of open complex. 3. closed complex to open complex isomerization

8. Teritiary complex formation : 1. If the enzyme synthesizes RNA <10 basepairs ,it does not elongate it abortive initiation . 2. Because σ factor acts as a mimic of RNA and blocks the RNA exit channel in open complex. 3. When RNA polymerase synthesizes RNA > 10 base pairs , it ejects σ factor and RNA elongates further and moves out through exit channel. This is known as the teritiary complex

9. ELONGATION After the formation of teritiary complex the RNA elongates further by the addition of dNTPs . The synthesized is proof read by hydrolytic editing. Gre factor enhances the proof reading activity. Pyrophospholytic editing is another proof reading mechanism.

10. TERMINATION The process of termination can happen in two different mechanisms. They are : 1. Rho – independent termination. 2. Rho – dependent termination.

11. Rho – independent termination In this mechanism, transcription is terminated by termination sequence in DNA. Terminator sequence contains invert repeat causing complementary pairing as transcript RNA form hairpin structure. Invert repeat is followed by a large number of T (~8 basepairs), in RNA “U” appears. The load of hairpin structure isn’t tolerated by A=U bps so RNA gets separated from RNA –DNA heteroduplex.

12. Rho – dependent termination In this mechanism,transcription is terminated by rho protein ( ρ ). Rho protein is a single strand ATPase protein. Rho protein binds to the single stranded RNA after termination. In bacteria, transcription and translation occurs simultaneously so Rho protein binds to the mRNA after translation i.e.,when translation is completed but transcription is “ON”.

13. THANK YOU…