Overview
This section explains the process of transcription in prokaryotes, covering the roles of RNA polymerase, promoters, and how transcription is initiated, elongated, and terminated.
Prokaryotic Genome Organization
- Prokaryotic cells lack membrane-bound nuclei and have a single, circular chromosome located in the nucleoid region.
- Prokaryotes may also possess plasmids, which are small, circular DNA molecules carrying additional genes like antibiotic resistance.
Transcription Basics in Prokaryotes
- Transcription requires partial unwinding of the DNA double helix to form a transcription bubble.
- The template strand is used for mRNA synthesis and is complementary to the mRNA product.
- The coding (nontemplate) strand has a sequence almost identical to mRNA, except T is replaced by U in RNA.
Transcription Initiation
- The +1 site is the DNA nucleotide where transcription begins; nucleotides before it are upstream (−), after are downstream (+).
- Prokaryotic RNA polymerase has five subunits forming the holoenzyme for accurate transcription initiation.
- The sigma (σ) subunit enables polymerase to recognize initiation sites, ensuring correct mRNA synthesis.
- Promoters are DNA sequences upstream of genes where RNA polymerase binds to start transcription.
- Promoter consensus sequences at −10 (TATAAT) and −35 (TTGACA) are recognized by the sigma subunit.
Elongation and Termination of Transcription
- After initiation, the sigma subunit dissociates, and the core enzyme synthesizes mRNA in the 5' to 3' direction.
- RNA polymerase acts as a stable link between the DNA template and growing RNA to prevent premature termination.
Termination Mechanisms
- Rho-dependent termination uses the rho protein to release the mRNA when the polymerase stalls at a run of G nucleotides.
- Rho-independent termination involves a CG-rich mRNA region forming a hairpin, followed by an AT-rich region that destabilizes the complex and releases mRNA.
Coupled Transcription and Translation
- In prokaryotes, transcription, translation, and mRNA degradation can occur simultaneously since there is no nuclear envelope.
- Multiple RNA polymerases can transcribe a gene simultaneously, and ribosomes can translate mRNA as it is being made.
Key Terms & Definitions
- Transcription bubble — the unwound region of DNA where mRNA synthesis occurs.
- Template strand — DNA strand used as a template for mRNA synthesis.
- Coding strand (nontemplate strand) — DNA strand with sequence similar to mRNA.
- Promoter — DNA sequence where RNA polymerase binds to initiate transcription.
- Sigma (σ) subunit — part of RNA polymerase holoenzyme that recognizes the promoter.
- Holoenzyme — complete RNA polymerase with all five subunits.
- Rho protein — protein involved in terminating transcription at certain sites.
Action Items / Next Steps
- Review the steps of transcription and the roles of each polymerase subunit.
- Familiarize yourself with the structure and function of prokaryotic promoters.
- Study differences between rho-dependent and rho-independent termination.
- Watch recommended animations on prokaryotic transcription for visual understanding.