Transcription in prokaryotes is the process of copying DNA into RNA. It's a crucial step in gene expression, allowing bacteria to quickly adapt to their environment. This section breaks down the key players and steps involved in prokaryotic transcription.
RNA polymerase, promoters, and sigma factors work together to start transcription. The process then moves through elongation and termination phases. Understanding these steps is vital for grasping how prokaryotes control gene expression and respond to their surroundings.
Transcription Initiation
RNA Polymerase and Promoter Recognition
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- RNA polymerase functions as the primary enzyme responsible for transcribing DNA into RNA in prokaryotes
- Consists of multiple subunits (α2ββ'ω) working together to catalyze RNA synthesis
- Promoter serves as the specific DNA sequence where transcription begins
- Contains conserved regions, including the -10 box (TATAAT) and -35 box (TTGACA), crucial for RNA polymerase binding
- Sigma factor acts as a specialized subunit that helps RNA polymerase recognize and bind to promoter sequences
- Different sigma factors allow bacteria to regulate gene expression in response to various environmental conditions (heat shock, nutrient starvation)
Initiation Complex Formation
- Transcription initiation begins with RNA polymerase binding to the promoter region
- Sigma factor guides RNA polymerase to the correct starting position on the DNA template
- Formation of the closed complex occurs when RNA polymerase initially binds to the promoter
- Transition to the open complex involves unwinding approximately 14 base pairs of DNA
- Creation of a transcription bubble allows access to the template strand for RNA synthesis
- Initiation complex stabilizes as RNA polymerase begins synthesizing short RNA fragments
Transcription Elongation and Termination
Elongation Process
- RNA polymerase moves along the DNA template in the 5' to 3' direction
- Nucleotides are added to the growing RNA chain complementary to the DNA template strand
- Transcription bubble moves with RNA polymerase, continuously unwinding and rewinding DNA
- RNA-DNA hybrid forms temporarily within the transcription bubble
- Elongation continues until a termination signal is encountered
- Rate of elongation in prokaryotes reaches approximately 40-80 nucleotides per second
Termination Mechanisms
- Rho-dependent termination involves the Rho protein
- Rho binds to specific sequences on the nascent RNA
- Moves along the RNA until it reaches the RNA polymerase
- Uses ATP hydrolysis to destabilize the RNA-DNA hybrid and release the transcript
- Rho-independent termination relies on intrinsic terminator sequences
- GC-rich hairpin structure forms in the newly synthesized RNA
- Followed by a series of uracil residues
- Hairpin formation destabilizes the RNA-DNA hybrid
- Weak A-U base pairing facilitates the release of the transcript
Prokaryotic Gene Expression
Operon Structure and Function
- Operon consists of a cluster of functionally related genes under the control of a single promoter
- Includes regulatory elements (operator, promoter) and structural genes
- Allows for coordinated regulation of multiple genes involved in related metabolic pathways
- Lac operon (lactose metabolism) and trp operon (tryptophan biosynthesis) serve as well-studied examples
- Regulatory proteins (repressors, activators) interact with operator sequences to control gene expression
- Enables prokaryotes to rapidly adapt to changing environmental conditions
Polycistronic mRNA and Translational Efficiency
- Polycistronic mRNA contains coding sequences for multiple proteins
- Single mRNA transcript encodes information for several genes within an operon
- Allows for simultaneous translation of multiple proteins from one mRNA molecule
- Enhances translational efficiency by coupling transcription and translation processes
- Ribosomes can begin translating the first gene while transcription of later genes is still occurring
- Facilitates rapid protein production in response to cellular needs