REGULATION OF PROTEIN SYNTHESIS. II. Eukaryotes

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1 REGULATION OF PROTEIN SYNTHESIS II. Eukaryotes

2 Complexities of eukaryotic gene expression! Several steps needed for synthesis of mrna! Separation in space of transcription and translation! Compartmentation of proteins

3 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

4 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

5 DNA must uncoil to be used as a template Interphase vs mitosis Differential expression as part of development

6 Evidence for the uncoiling of chromosomes Lampbrush chromosomes from Axilotl ova Chromosome puffs in Drosophila salivary glands

7 Remodeling Removal of histones Initiation of transcription

8 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).

9 ! Transcription factors bind to sequences upstream from gene (up to1000 base pairs or more before promoter).! DNA bends to form transcription complex.

10 Control of genes requires specific combination of transcription factors! One gene, multiple factors! One factor, multiple genes (SRE: stress response element )

11 Eukaryotic transcript RNA must be processed before use! Remove introns! Cap! Attach poly-a sequences. Exons are the sequences preserved in the mrna!

12 Eukaryotic transcript RNA must be processed before use! Remove introns (and splice exons together): alternative splicing can produce different mrnas from one transcript

13 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

14 Eukaryotic transcript RNA must be processed before use! Remove introns! Cap! Attach poly-a sequences.

15 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)

16 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.

17 Turnover (breakdown) of proteins also controls their concentration in the cell

18 A lack of proper breakdown of proteins causes a conformational disease --- A new compound (CPZ) can help Sifers, Science, 9 July 2010,

19 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