DNA Methylation and Transcriptional Regulation

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DNA Methylation and Transcriptional Regulation
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Stages of Regulation
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Antisense sequence of DNA or RNA that
is complementary to mRNA
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Transcriptional activity
vs.
Post-transcriptional activity
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Bacteria
Eukaryotes

• mRNA is transcribed and
• translated almost simultaneously.
• Why? Bacterial mRNA usually
• is unstable and is translated into
• proteins for only a few minutes.

• Synthesis and maturation
• of mRNA occur in the
• nucleus.
• After maturation, mRNA
• is exported to the cytoplasm
• where it is translated.
• Relatively stable and continues
• to be translated for several
• hours.

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Stability of Bacterial mRNA

• Degradation closely follows translation
• Degradation begins 1min after transcription
• 5 end of mRNA starts to decay before 3 end has
  been
• synthesized
• Stability of mRNA influences the amount of
  protein
• expressed
• Average ½ life of mRNA is 2min.

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Terminology
Nascent RNA Monocistronic mRNA Polycistronic
mRNA
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Types of mRNA Regions

• Coding region
• 5 UTR (leader sequence)
• 3 UTR (trailer sequence)
• Intercistronic region

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Polycistronic mRNA
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Eukaryotic mRNA
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Stability of Eukaryotic mRNA

• Both the 5 and 3 ends of mRNA are modifed
  which increases
• mRNA stability
• Transported out of nucleus after modification
• Leaves for cytoplasm 20 min following
  initiation
• Yeast ½ life 1-60 min
• Higher eukaryotes ½ life 4-24 hr

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• Specific site of termination?
• AAUAAA (higher eukaryotes)
• (located 11-30 nts upstream of
• poly(A) addition)
• Deletion of AAUAAA prevents
• polyadenylation.
• AAUAAA needed for cleavage
• and polyadenylation.
• This process requires endonuclease
• (CFI and CFII), poly(A) polymerase
• (PAP), and a specificity component
• (CPSF) which recognizes AAUAAA.

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Gppp pppApNpNp
GpppApNpNp.. pp p
5-5 triphosphate linkage
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Cap0
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Polyadenylation
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3 Polyadenylation

• 3 terminal stretch of A residues poly(A)
  tail or poly(A)
• Sequence is not coded in DNA, added to RNA in
  the nucleus
• after transcription
• Catalyzed by poly(A) polymerase
• 200 A residues are added to 3-OH end
• Associated with poly(A)-binding protein (PABP)

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Role of poly(A)

• Stability
• Removal of poly(A) precedes degradation
• Removal of poly(A), as well as PABP, inhibits
  translation

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Bacterial mRNA Degradation
1)
2)
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Eukaryotic mRNA Degradation

• Occurs in the cytoplasm
• Catalyzed by a multi-enzyme complex 1)
  ribonuclease E,
• 2) PNPase, 3) helicase

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Destabilizing Elements

• ARE AU-rich element
• IRE iron response element

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ARE
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IRE
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