Prokaryotic transcription involves RNA polymerases catalyzing the synthesis of RNA molecules using a DNA template. There are three main steps: initiation, elongation, and termination. Initiation begins with the binding of RNA polymerase to promoter regions on the DNA. Elongation then occurs as the polymerase unwinds the DNA and adds nucleotides to the growing RNA chain. Termination happens when either Rho-dependent or Rho-independent termination signals are reached, causing the release of the completed RNA molecule.

1. PROKARYOTIC
TRANSCRIPTION

2. Introduction
 RNA molecules are synthesised by using a
portion of one strand of DNA as template in a
polymerization reaction.
 Catalyzed by RNA polymerases
 Process by which RNA molecules are initiated,
elongated and terminated – Transcription

3. The Central Dogma
3
DNA RNA PROTEIN
Transcription Translation
Replication
Central dogma : Intracellular information flow from DNA to
mRNA to Protein.

4. RNA Polymerases
 Principal enzyme in transcription
 Catalyzes polymerization of ribonucleotides as
directed by DNA template
 Catalyses growth of RNA chains in 5'  3'
direction
 Preformed primer is not required
 Transcription initiates denovo at specific sites at
the beginning of gene


5.  RNA POLYEMERASE:
 Complex enzyme – made up of multiple polypeptide chain
 6 subunits :
 2α subunit – binding of enzyme to promoter region
 β – nucleotide addition
 β‘ – part of core enzyme , keeps the RNA bound to DNA template
 ω – part of enzyme, but no specific function
 σ – sigma factor, recognize promoter region binds to it. weakly
bound, which dissociates from enzyme during elongation
[ Core enzyme : σ free unit , ie., α2ββ‘ Holoenzyme : the complete enzyme,
ie., α2ββ‘σ ]
 One of the largest enzyme (mw – 465,000Da) ,contact with many
DNA bases simultaneously

6. Selection of site – The Promoter
 DNA sequence to which the RNA polymerase binds to initiate
transcription of gene – Promoter
 2 types of regions important in promoter function :
 Pribnow – Schaller box : (prokaryotes)
 Located about 10 bp upstream from start of transcription
 Goldberg – Hogness box : (eukayotes)
 Located 20-30 bp upstream from start of transcription
 Also termed as TATA box.
 Both are A-T rich regions.
 Concensus sequences : basic sequence derived from a large set of
observed similar sequence

7. Steps : Transcription
 Initiation of transcription
 RNA chain elongation
 Termination and release of newly synthesised RNA

8. Initiation of Transcription
 σ subunit binds specifically to sequence in both -35 and -10
promoter regions
 σ subunit binds non specifically to DNA low affinity, initial binding ><
polymerase and promoter – closed promoter complex (CPC), DNA is
not unwound
 Polymerase unwinds 12-14 bases of DNA from about -12 to +2 to
form open promoter complex, ssDNA available as template for
transcription
 RNA polymerase contains2 nucleotide binding sites - Iniation site &
elongation site
 Initiation site binds to purine triphosphates , ATP & GTP
 ATP –first nucleotide in chain –first DNA Base transcribed –thymine
 Transcription initiated by addition of NTP’s

9. Chain elongation
 Polymerase + template  continues synthesis of mRNA‘s
 Polymerase unwinds DNA and adds up nucleotides upto 8 to growing
chain
 Polymerase undergo confirmational changes & loses σ subunit
 Along with unwinding of DNA , it rewinds the DNA behind
it,maintaining an unwound region (15bp)
 8-12 bases of growing RNA chain remains bound to complementary
DNA
 β and β‘ subunit – forms crab claw like structure –grips DNA template
 Elongation :
 doesn’t occur at constant rate
 Reduction in rate – Pause
 Pausing follows sequences that forms hairpin in RNA
 Continues till termination signal is reached

10. Termination & release of newly sythesised RNA
 Transcription stops and RNA released from polymerase & enzyme
dissociates from DNA template
 Transcription of GC rich inverted repeat  RNA segment, can form a
stable stemloop structure by base pairing
 Self complementary structure  disrupts association with DNA 
Termination
 H2 boding >< A and U is weaker than >< G and C

12.  TWO TYPES OF TERMINATION :
 Rho(ρ) – dependent
 Rho – independent

13. Rho dependent termination
 Rho - the termination factor protein
 Transcription of some genes is terminated by specific termination protein :
Rho, an ATP dependent helicase
 It moves along RNA transcript (>60 nt) , finds the "bubble", unwinds it and
releases RNA chain
 Specific sequences - termination sites in DNA
 inverted repeat, rich in G:C, which forms a stem-loop in RNA transcript
 6-8 A’s in DNA coding for U’s in transcript

14. Rho dependent termination

15. Rho independent termination
 Termination occurs in absence of Rho protein
 Terminator consist of:
 A short inverted repeat followed by a stretch of A : T basepairs (about 8)
 When polymerase transcribes inverted sequence , a stem and loop structure is
formed
 mRNA moves away from DNA template as a A-U bp. it detaches and so RNA
polymerase cannot move forward by formation of such structre  transcription
ends

16. Rho independent termination

17. Rho dependent termination Rho independent termination