뒤로Transcription and RNA Processing in Genetics
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Transcription is the synthesis of a single-stranded RNA molecule by RNA polymerase using the DNA template strand.
1. Promoter recognition
2. Transcription initiation
3. Chain elongation
4. Chain termination
The promoter is a DNA sequence upstream of the transcription start site (+1) that controls RNA polymerase access to the gene.
It consists of a pentameric core enzyme plus a sigma (σ) subunit that directs the enzyme to specific promoters.
Sigma subunits alter RNA polymerase specificity, allowing binding to different promoter consensus sequences for gene regulation.
At −10: 5′-TATAAT-3′ and at −35: 5′-TTGACA-3′ on the coding strand.
RNA polymerase holoenzyme first forms a closed promoter complex, then unwinds DNA at −10 to form an open complex, and begins RNA synthesis at +1.
After the first 8 to 10 nucleotides of RNA are synthesized, sigma dissociates and the core enzyme continues elongation.
Termination sequences in DNA cause RNA polymerase to release the RNA transcript and dissociate from DNA.
Intrinsic termination involves an inverted repeat forming a hairpin in mRNA followed by a string of uracils, causing RNA polymerase to pause and release the transcript.
Rho protein binds to rut sites on mRNA, moves toward RNA polymerase using ATP, and causes dissociation of the RNA transcript.
Eukaryotes have three RNA polymerases: RNA pol I (rRNA genes), RNA pol II (protein-coding and most snRNA genes), RNA pol III (tRNA and some snRNA).
The TATA box at approximately −25 with consensus 5′-TATAAA-3′; if absent, initiator and downstream core-promoter elements may be present.
General transcription factors (GTFs), including TFIID (with TBP), TFIIA, TFIIB, TFIIF, TFIIE, and TFIIH, assemble the initiation complex.
Enhancers increase transcription by binding activators; silencers repress transcription by binding repressors; both can act at a distance.
After 20-30 nucleotides are synthesized, a guanine is added to the 5′ end by guanylyl transferase via a 5′ to 5′ triphosphate linkage and methylated.
Protects mRNA from degradation, facilitates splicing, aids nuclear export, and enhances translation initiation.
Pre-mRNA is cleaved downstream of the AAUAAA signal and a poly-A tail of 20-200 adenines is added by polyadenylate polymerase.
Enhances mRNA stability, facilitates nuclear export, and promotes translation efficiency.
Introns are removed and exons joined by the spliceosome, which recognizes conserved 5′ GU and 3′ AG splice sites and a branch point adenine.
A pyrimidine-rich sequence 20-40 nucleotides upstream of the 3′ splice site containing an invariant adenine essential for lariat formation.
A complex of snRNAs and proteins that catalyzes intron removal and exon ligation during pre-mRNA splicing.
SR proteins bind ESEs in exons to facilitate recognition of splice sites and ensure accurate splicing.
Processing of the same pre-mRNA in different ways to produce multiple mature mRNAs and protein variants from one gene.