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Lecture12 Based on Chapter 18 Regulation of Gene
Expression in Eukaryotes
Copyright © 2010 Pearson Education Inc.
1 -Levels of Control of Gene Expression in
Eukaryotes
11 – The Role of Chromatin in Regulating Gene
Transcription
12 – Gene Silencing and Genomic Imprinting
13 – Gene Silencing by DNA Methylation
14 – Genomic Imprinting
15 – RNA Processing Control
1 -Levels of Control of Gene Expression in
Eukaryotes
17 – RNA Interference
1 -Levels of Control of Gene Expression in
Eukaryotes
16 – mRNA Translation Control
1. Ribosomal translational control, selecting mRNAs for
translation, also has an impact on gene expression.
a. Unfertilized eggs are an example, in which mRNAs show
significant increases in translation after fertilization without new
mRNA synthesis.
b. Stored mRNAs are associated with proteins that both protect
them and inhibit their translation.
c. Poly(A) tails promote translation initiation, and stored mRNAs
generally have shorter tails.
i. In some mRNAs of mouse and frog oocytes, a normal-length
poly(A) tail is added and then trimmed enzymatically.
ii. Particular mRNAs are marked for deadenylation by a region
in the 39 untranslated region, called the adenylate/uridylate
(AU)-rich element (ARE), with the consensus sequence
UUUUUAU.
iii. Activation of the stored mRNA occurs when a cytoplasmic
polyadenylation enzyme recognizes the ARE and adds about
150 A residues, making a full-length poly(A) tail.
18 – Regulation of Gene Expression – Control
of mRNA Degradation
19 - Regulation of Gene Expression – Control
of Protein Degradation
1. Protein regulation occurs in many ways. Examples:
a. A constitutively produced mRNA may be translated continuously, and so
the protein degradation rate determines its level.
b. A short-lived mRNA may make a very stable protein, so that it persists for
long periods in the cell.
2. Protein stability varies
3. Proteolysis (protein degradation) in eukaryotes requires ubiquitin, a protein
cofactor.
4. Protein stability is directly related to the amino acid at the N terminus of the
protein (the N-end rule). In yeast, stability of the same protein was measured
with different N-terminal amino acids:
5. The N-terminal amino acid directs the rate of ubiquitin binding, which in turn
determines the half-life of the protein.
6. To summarize, prokaryotes control gene expression mainly at the
transcriptional level, while eukaryotes regulate at transcriptional,
posttranscriptional, and posttranslational levels. Eukaryotic systems control:
Transcription, Precursor-RNA processing, Transport from the nucleus,
Degradation of mature RNAs, Translation of mRNAs, and Degradation of
protein.