Download Regulating Protein Synthesis

REGULATION OF PROTEIN SYNTHESIS
II. Eukaryotes
Complexities of eukaryotic gene expression
! Several steps needed for synthesis of mRNA
! Separation in space of transcription and
translation
! Compartmentation of proteins
Why is eukaryotic gene expression
complex?
! Archaea provided genes for DNA metabolism,
transcription, translation, DNA repair
! Bacteria provided genes for carbohydrate,
amino acid, lipid metabolism
! Some bacterial genes persist in mitochondria,
chloroplasts
Complexities of eukaryotic gene expression
! Epigenetics: which
genes are expressed?
DNA methylation
Histone modification
siRNA gene silencing
! Several steps needed
for synthesis of mRNA
Uncoiling of chromatin
Remodeling of
chromatin
Transcription
Transcript processing
DNA must uncoil to be
used as a template
•Interphase vs mitosis
•Differential expression
as part of development
Evidence for the uncoiling of chromosomes
•“Lampbrush” chromosomes from Axilotl ova
•Chromosome “puffs” in Drosophila salivary glands
Remodeling
• Removal of histones
• Initiation of transcription
Transcription factors promote the binding of
RNA polymerase to template
! In eukaryotes, transcription is
generally under positive control
(proteins promote, rather than inhibit,
RNA polymerase binding to DNA
template).
! Transcription factors bind to sequences upstream
from gene (up to1000 base pairs or more before
promoter).
! DNA bends to form transcription complex.
Control of genes requires specific
combination of transcription factors
! One gene, multiple factors
! One factor, multiple genes
(SRE: “stress response element”)
Eukaryotic transcript RNA must be
processed before use
! Remove introns
! Cap
! Attach poly-A sequences.
Exons are the sequences
preserved in the mRNA!
Eukaryotic transcript RNA must be processed before
use
! Remove introns (and splice exons together): alternative
splicing can produce different mRNAs from one transcript
Removal of eukaryotic introns involves RNA
enzymes
! Some introns of pre-mRNAs (called Group I
introns) are self-removing
! Some introns are removed (and the adjoining
exons “spliced” together) by spliceosomes, made
of protein plus RNA, with the RNA identifying the
intron-exon boundaries
Eukaryotic transcript RNA must be
processed before use
! Remove introns
! Cap
! Attach poly-A sequences.
Eukaryotic transcript RNA may be
broken down before it can be used
! siRNAs and miRNAs (smallinterfering RNAs and micro RNAs)
can regulate translation
! siRNAs and miRNAs are cut from
double-stranded RNA; one strand
joins a protein complex
! The protein-siRNA complex breaks
down mRNAs that contain
complementary sequences
! The protein-miRNA complex breaks
down mRNAs, or it binds to them,
preventing translation
(Petunia plants expressing chalcone synthase
antisense RNA)
Some proteins must be guided to their
destination and processed.
! Eukaryotic cells have many compartments.
! Leader sequences signal import into E.R.
! Transit sequences direct transport into
mitochondria, plastids, nuclei.
! Leader, transit sequences generally removed.
Turnover (breakdown) of proteins also controls
their concentration in the cell
A lack of proper breakdown of proteins causes a “conformational disease”--A new compound (CPZ) can help
Sifers, Science, 9 July 2010, 154-155
Summary
Regulation of protein synthesis is necessary in all
cells, but much more complex in eukaryotes, because
both the cells and the organism they form are more
complex.
Uncoiling of chromatin: DNA, histone
modification
Remodeling of chromatin: removing histones
Transcription: binding of transcription factors
Transcript processing: intron removal,
capping, poly-A addition
Transcript turnover: siRNAs, miRNAs
Translation: compartmentation
Protein turnover