Learning Objective: Describe the process of prokaryotic transcription—from promoter recognition to termination—and explain how transcription couples with translation to produce mRNA and proteins efficiently.
Promoter Recognition
Prokaryotic RNA polymerase requires a sigma (σ) factor to identify promoter sequences.
Key Promoter Elements
- –35 region: Consensus sequence recognized by σ factor
- –10 region (Pribnow box / TATA box): AT-rich region where DNA unwinds
Function of the Promoter
- Determines transcription start site (+1)
- Determines which DNA strand is the template
- Orients RNA polymerase for the correct direction of synthesis
Initiation of Transcription
- RNA polymerase binds the promoter with σ factor.
- DNA strands unwind.
- Transcription starts at +1 nucleotide.
- σ factor dissociates once transcription begins.
Elongation
- Core RNA polymerase moves 3′ → 5′ along the template strand.
- RNA is synthesized 5′ → 3′ using NTPs (ATP, GTP, CTP, UTP).
- No primer required
- No proofreading → higher mutation rate than DNA polymerase
Termination of Transcription
Rho-Independent Termination
Driven by RNA structure:
- GC-rich inverted repeats → form stable hairpin loop
- Followed by poly-U tail → weak AU binding → RNA release
Rho-Dependent Termination
- Rho protein binds RNA at the rut site
- Moves toward paused RNA polymerase
- Uses ATP to dissociate RNA–DNA hybrid
Transcription–Translation Coupling
Unique to prokaryotes because they lack a nuclear membrane.
Key Features
- Translation begins before transcription ends
- Ribosomes bind to the Shine–Dalgarno sequence in the 5′ UTR
- Translation begins at the AUG start codon
- Protein synthesized N-terminus → C-terminus
Monocistronic vs Polycistronic mRNA
Monocistronic mRNA
- Derived from one gene
- Encodes one protein
Polycistronic mRNA (Operons)
- Multiple genes are transcribed as a single mRNA
- Each gene:
- Has its own Shine–Dalgarno sequence
- Has its own AUG start and stop codon
- Example: lac operon
