8.20
Bacterial Transcription
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Q1: How does RNA polymerase find the promoter sequence on bacterial DNA?
RNA polymerase holoenzyme initially binds non-specifically to DNA with low affinity, then slides along the DNA strand to locate the promoter. When it encounters the promoter regions at -10 and -35 positions upstream of the initiation site, the sigma subunit binds tightly and unwinds the DNA to form the open-promoter complex, positioning the enzyme for transcription initiation.
Q2: What role does the sigma factor play during bacterial transcription?
The sigma subunit of RNA polymerase recognizes and binds tightly to the promoter sequences, enabling DNA unwinding to form the open-promoter complex. After approximately 10 nucleotides of RNA synthesis, the sigma factor dissociates from the core enzyme, allowing the polymerase to continue elongation independently along the DNA template.
Q3: How does RNA polymerase know when to stop transcribing a gene?
Transcription terminates when the polymerase encounters a termination signal consisting of a symmetrical, inverted GC-rich sequence followed by a poly-A tail. When transcribed into RNA, this self-complementary sequence forms a stable hairpin loop structure that destabilizes the mRNA-DNA association, causing the polymerase to stall and dissociate from the template.
Q4: What is the difference between a closed complex and an open complex in bacterial transcription?
A closed complex forms when RNA polymerase initially binds the promoter sequences with template DNA strands still base-paired. An open complex forms when the polymerase breaks hydrogen bonds between base pairs and binds tightly to single-stranded DNA at the start site, enabling mRNA synthesis to begin.
Q5: How does bacterial transcription differ from eukaryotic transcription in terms of timing?
Bacteria lack a membrane-bound nucleus, allowing transcription and translation to occur simultaneously on the same DNA template. In contrast, eukaryotes compartmentalize these processes, with transcription occurring in the nucleus and translation in the cytoplasm, enabling separate regulation of gene expression.
Q6: What happens to DNA structure as RNA polymerase moves along the template during elongation?
As RNA polymerase proceeds along the DNA template during elongation, the DNA ahead of the enzyme continuously unwinds while the DNA behind it rewinds back into a double helix. This dynamic unwinding and rewinding allows the polymerase to access the template strand while maintaining overall DNA structure integrity.
Q7: Why is the hairpin loop structure important for transcription termination?
The hairpin loop structure forms from the self-complementary termination sequence and destabilizes the association between the newly synthesized mRNA and the DNA template. This destabilization causes the polymerase to stall and dissociate, allowing the transcription bubble to collapse and the pre-mRNA transcript to be released from the DNA.