RNA Synthesis

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Lecture 32 Modak
RNA Synthesis
Mukund Modak, Ph.D.
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U1-U6 lt100 nt 1
Splicing RNAs
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PROKARYOTIC RNA POLYMERASE
Single enzyme with 5 subunits a, a, b, b s
Holoenzyme (Sigma subunit finds start point) a,
a, b, b Core enzyme (elongation of RNA
chain)
DNA template and RNA polymerase
4 rNTPS
RNA PPi
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3
3
5
5
3
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Figure 30.8 Local unwinding of DNA caused by RNA
polymerase
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ENZYMATIC SYNTHESIS OF RNA
Differences/similarities with DNA polymerases i.
Substrate ii. Template conservation (Sense
strand mRNA) iii. Primer need iv. Proof
reading Template recognition and copying by base
pairing direction of synthesis 5 -
3 Initiation with A or G (Sigma factor also
needed) Termination U sequence hairpin r
factor Prokaryotic system (ATP dependent)
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Figure 30.7 Structure of the prokaryotic
promoter region
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EUKARYOTIC RNA POLYMERASES
RNA polymerase I (rRNA) in Nucleoli RNA
polymerase II (mRNA) RNA polymerase III (tRNA)
Nucleoplasmic
EUOKARYOTES PROMOTERS -110 -60 -25 CAAT GCGC
TATA (box)
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Figure 30.13 A. Eukaryotic general transcription
factors bound to the promoter. CTF, SP1, and
TFIID are general transcription factors. B.
Enhancer stimulation of RNA polymerase II
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Figure 30.17 Posttranscriptional modification of
mRNA showing the 7-methylguanosine cap and poly-A
tail
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RNA Synthesis
Small RNAs and their functions (all require to
form protein complexes)

• Functions in  Silencing of mRNA
• 7SL RNA RNA required in transport (to Golgi) of
  secretory proteins. 
• SN-RNA (small Nuclear RNAs) form RNPs required
  in processing of mRNA.
• Oligo A (20-50 nucleotide long) stretch of A
  nucleotides linked from 2 to 5. Aids in
  interferon action.
• tRNAs Amino acid specific tRNAs required in
  protein synthesis.

• miRNAs (Micro-RNAs) typically 21 to 25 base long
  and is produced from stem loop structured
  precursors RNAs.
• Involved in destruction of specific gene
  transcripts and suppression of translation of
  specific mRNAs.
• SiRNA or RNAi small interfering RNAs similar to
  miRNAS, produced by RNAase III enzyme.

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TRANSFER RIBOSOMAL RNA SYNTHESIS
Synthesized as long transcripts
16 S
tRNA
23S
5S
At arrows - spacer sequences Cleaves at spacer
(marked as arrows) sequences with specific
RNase CCA ends are added to all.
tRNAs Modifications of some bases, e. g.
methylation in both tRNA and rRNA
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INHIBITORS/DRUGS OF RNA SYNTHESIS
1. Enzyme specific Rifamycin for E. coli
enzyme a-amanitin for pol II 2. DNA
intercalating agents Actinomycin
D Quinacrine 3. Substrate analogs Ara -
C Ara - A
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