15.5: Prokaryotic Transcription - Elongation and Termination in Prokaryotes
- Explain the process of elongation and termination in prokaryotes
Elongation in Prokaryotes
The transcription elongation phase begins with the release of the σ subunit from the polymerase. The dissociation of σ allows the core RNA polymerase enzyme to proceed along the DNA template, synthesizing mRNA in the 5′ to 3′ direction at a rate of approximately 40 nucleotides per second. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it. Since the base pairing between DNA and RNA is not stable enough to maintain the stability of the mRNA synthesis components, RNA polymerase acts as a stable linker between the DNA template and the nascent RNA strands to ensure that elongation is not interrupted prematurely.
Termination in Prokaryotes
Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly-made mRNA. Depending on the gene being transcribed, there are two kinds of termination signals: one is protein-based and the other is RNA-based.
Rho-dependent termination is controlled by the rho protein, which tracks along behind the polymerase on the growing mRNA chain. Near the end of the gene, the polymerase encounters a run of G nucleotides on the DNA template and it stalls. As a result, the rho protein collides with the polymerase. The interaction with rho releases the mRNA from the transcription bubble.
Rho-independent termination is controlled by specific sequences in the DNA template strand. As the polymerase nears the end of the gene being transcribed, it encounters a region rich in C–G nucleotides. The mRNA folds back on itself, and the complementary C–G nucleotides bind together. The result is a stable hairpin that causes the polymerase to stall as soon as it begins to transcribe a region rich in A–T nucleotides. The complementary U–A region of the mRNA transcript forms only a weak interaction with the template DNA. This, coupled with the stalled polymerase, induces enough instability for the core enzyme to break away and liberate the new mRNA transcript.
Upon termination, the process of transcription is complete. By the time termination occurs, the prokaryotic transcript would already have been used to begin synthesis of numerous copies of the encoded protein because these processes can occur concurrently in the cytoplasm. The unification of transcription, translation, and even mRNA degradation is possible because all of these processes occur in the same 5′ to 3′ direction and because there is no membranous compartmentalization in the prokaryotic cell. In contrast, the presence of a nucleus in eukaryotic cells prevents simultaneous transcription and translation.
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Key Points
- The transcription elongation phase begins with the dissociation of the σ subunit, which allows the core RNA polymerase enzyme to proceed along the DNA template.
- Rho-dependent termination is caused by the rho protein colliding with the stalled polymerase at a stretch of G nucleotides on the DNA template near the end of the gene.
- Rho-independent termination is caused the polymerase stalling at a stable hairpin formed by a region of complementary C–G nucleotides at the end of the mRNA.
Key Terms
- elongation : the addition of nucleotides to the 3′-end of a growing RNA chain during transcription