6.3
Transcription in Prokaryotes
Transcription is a highly regulated process that converts genetic information into RNA molecules. The transcription cycle is divided into three key st…
Transcription includes initiation, elongation, and termination.
The bacterial core RNA polymerase binds a sigma factor, forming a holoenzyme for initiation.
This sigma factor interacts with promoter regions, like the -35 and -10 sequences, positioning the RNA polymerase near the transcription start site.
After binding, the holoenzyme unwinds the DNA at the -10 site, exposing the template strand.
The sigma factor dissociates after adding a few ribonucleotides, and the core RNA polymerase continues elongating the RNA until it encounters a terminator sequence.
Bacterial transcription termination occurs through intrinsic signals in the RNA transcript, which form stem-loop structures, or through proteins like Rho factors, which dissociate RNA polymerase and RNA.
In archaeal promotors, the TATA-binding protein binds to the TATA box, while transcription factor B recognizes the B recognition element sequence. These factors recruit the RNA polymerase to the initiation site.
Termination in archaea occurs via intrinsic signals in the RNA transcript, like the stem-loop structures, and is independent of protein factors.
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Q1: How does the bacterial RNA polymerase holoenzyme initiate transcription?
The bacterial core RNA polymerase binds a sigma factor to form a holoenzyme, which recognizes promoter sequences like the -35 and -10 regions. This positions the holoenzyme near the transcription start site. The holoenzyme unwinds DNA at the -10 site to expose the template strand, allowing transcription to begin.
Q2: What role does the sigma factor play during bacterial transcription initiation?
The sigma factor interacts with promoter regions, specifically the -35 and -10 sequences, to position the RNA polymerase correctly for transcription initiation. After a few ribonucleotides are added to the nascent RNA, the sigma factor dissociates, leaving the core RNA polymerase to continue elongation independently.
Q3: How do intrinsic and Rho-dependent termination differ in bacterial transcription?
Intrinsic termination occurs when the RNA transcript forms stem-loop structures followed by uracil residues, destabilizing the RNA-DNA hybrid and causing RNA polymerase to dissociate. Rho-dependent termination involves the Rho protein binding to specific RNA sequences and moving along the transcript to displace the RNA polymerase.
Q4: How does archaeal transcription initiation differ from bacterial initiation?
In archaea, the TATA-binding protein binds to the TATA box, while transcription factor B recognizes the B recognition element sequence. These factors recruit RNA polymerase to the initiation site, resembling eukaryotic transcription initiation rather than the sigma factor mechanism used in bacteria.
Q5: What happens to the DNA during bacterial transcription elongation?
During elongation, RNA polymerase synthesizes RNA in the 5' to 3' direction by adding ribonucleotides complementary to the DNA template strand. The polymerase unwinds DNA ahead of the transcription bubble and rewinds it behind, ensuring continuous RNA synthesis as it moves along the template.
Q6: How does archaeal transcription termination compare to bacterial termination?
Archaeal termination is generally intrinsic, relying on stem-loop structures in the RNA transcript to release RNA polymerase. Unlike bacterial transcription, archaeal termination does not involve additional protein factors like Rho, making it a simpler mechanism dependent solely on RNA secondary structure.
Q7: Why is transcription considered a key step in bacterial gene expression?
Transcription converts genetic information into RNA molecules, serving as the first major step in coordination of gene expression processes in bacteria. The three stages—initiation, elongation, and termination—are each driven by specific molecular mechanisms that regulate when and how genes are expressed.