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Understanding Transcription in Prokaryotes
Aug 4, 2024
Transcription in Prokaryotes
Overview
Transcription: Process of converting genetic information from DNA to mRNA.
Coding region: Contains information about proteins or functional RNAs (e.g., ribosomal RNA).
Transcription Start Site (TSS): Marks the beginning of the coding region.
Key Terms
Plus one position
: First base of the coding region.
Downstream
: Refers to the direction towards the end of the coding region (after TSS).
Upstream
: Refers to the direction before the plus one site (before TSS).
Promoter region
: Located upstream of the TSS, initiates transcription.
Steps of Transcription
Initiation
Elongation
Termination
Organization of Promoter DNA
TSS Reference
:
Downstream: +1, +2, +3...
Upstream: -1, -2, ... (no site 0).
Promoter Elements
:
Upstream Promoter Element (UPE)
: -58 to -37 position.
-35 Element
: -35 to -30 position.
-10 Element
: -12 to -7 position, also known as the Pribnow box (TATA box in eukaryotes).
Spacing Between Elements
Optimal spacing between -35 and -10 elements: ~17 bases.
Extended -10 Element
: Found at positions -17 to -14.
Discriminator Element
: Usually 3 bases long.
Functionality of Promoter Elements
UPE: Frequently contacted by RNA polymerase, stabilizing it.
-35, Extended -10, and -10: Contacted by sigma factors during transcription initiation.
Discriminator Element: Stabilizes RNA polymerase binding.
Consensus Sequences
UPE
: Long consensus sequence (uses IUPAC symbols).
-35 Element
: Consensus sequence is TTGACA.
Extended -10 Element
: Consensus is TRTG.
-10 Element
: Consensus is TATAAT.
Discriminator Element
: Consensus is usually a G triplet.
Important Notes on Consensus
Consensus sequences are averages and may vary across different genes.
Example: -10 element's consensus (TATAAT) is biased toward Sigma Factor D (Sigma 70).
Different sigma factors can bind different consensus sequences in different genes.
Promoter Strength
Strong promoters: Closely match consensus sequences, produce more transcripts.
Weak promoters: Deviate from consensus sequences, produce fewer transcripts.
DNA Structure and Promoters
Sequence composition can cause DNA to bend or curve, impacting protein binding and function.
Conclusion
Understanding bacterial promoters is crucial for comprehending transcription processes.
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