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Unformatted text preview: Transcription in Eukaryotes
http://www.mun.ca/biochem/courses/3107/images/yeast_RNAPII.gif 1 Last Time
✤ Gene regulation overview Example of transcriptional control of gene expression: The lac operon
✤ ✤ How (and when) the lac operon is repressed How (and when) the lac operon is induced in transcriptional gene regulation ✤ ✤ Themes 2 Transcription in Eukaryotes
✤ Overview of eukaryotic transcription Features of an eukaryotic gene: regulatory elements and transcription factors processing ✤ ✤ RNA ✤ Splicing CAP and PolyA tail modiﬁcations ✤ 3 Eukaryotic organisms are more complex than prokaryotes www.lakeshorebulldogs.com http://www.harlem-school.com/10TH/sci_pdf/graphics/animal_cell.gif 4 Three RNA polymerases in euks
✤ RNA polymerase I rRNA genes (except 5S RNA) RNA polymerase II mRNA genes (structural genes) RNA polymerase III tRNA genes and 5S RNA ✤ ✤ 5 Important sequence motifs in structural genes (RNAP II)
core promoter regulatory sequence basal level of transcription TATA box speciﬁes where RNAP should start
6 Important sequence motifs in structural genes (RNAP II)
Upstream promoter element core promoter enhancers* or silencers* increases or decreases transcription rate * can be close to gene or really far away!
7 Transcription factors: proteins bind speciﬁc sequences to control gene expression
General transcription factors and RNAP core promoter Speciﬁc transcription factors Upstream promoter element enhancers* or bound by activators & silencers* repressors
8 General transcription factors and the initiation complex
help RNAP get on (and stay on!) promoter 5 general TFs: TFIID, TFIIB, TFIIF, TFIIE, TFIIH 9 Mediator - huge complex of proteins
mediates interactions between RNAP II and Speciﬁc TFs http://www-dsv.cea.fr/var/plain/storage/original/media/File/IBITECS/Actusscientiﬁques/VignWerner20071.jpg 10 Chromatin remodeling
RNAP accesses the DNA Blocking or allowing RNAP access to DNA is one way to control gene expression histones become methylated Histone acetylation: a switch between repressive and permissive chromatin Anton Eberharter & Peter B. Becker EMBO reports 3, 3, 224–229 (2002)
11 RNA processing
✤ pre-RNA needs to be processed
✤ spliced, modiﬁed, trimmed... 12 rRNA processing 13 tRNA
endonuclease endonuclease (RNaseP) mature tRNA
14 RNaseP is an example of a ribozyme (RNA + protein)
protein RNA RNA wikimedia.org 15 “Self-splicing” introns of nonstructural genes
✤ Two types of self-splicing introns:
✤Group ✤Group I II (shown) ✤Differ in the mechanism of splicing 16 Modiﬁcations of structural genes (mRNA)
Introns (“intervening sequences”) are spliced out of transcript Exons (“expressed sequences”) are joined pre-mRNA relies on a protein-RNA complex to remove introns
17 pre-mRNA is spliced by snRNPs “snurps”
recognizes sequences within introns snips out intron and ligates exons http://images.usatoday.com/life/_photos/2005/10/06/inside-smurfs.jpg 18 Features important for splicing 1. recognize boundary 2. cut mRNA 3. reanneal exons 19 Why all the fuss?
Presence of exons and introns allows gene to encode more than one protein - “alternative splicing” alter which parts are considered exons Different protein products are produced using the same gene (Note that not all genes do this) 20 Final modiﬁcation of mRNAs - getting dressed to go out (of the nucleus)!
5’ CAP - 3’ poly A tail- http://www.antiquemapsandprints.com/scansj/j-20704.jpg 21 The 5’ CAP - a modiﬁed Guanine (GTP)
trim hair to ﬁt 1st base of RNA strand /images.encarta.msn.com CAP (GMP) on backwards 1st base of RNA strand
22 The 3’ poly A tail 23 Now, we’re ready for translation! 24 Why all the fuss of mRNA modiﬁcation? 5’-CAP intron splicing poly-A tail 1. Helps the export of mRNA from nucleus 2. Protects mRNA from degradation 3. Helps ribosomes attach for translation
25 Special features of transcription
Promoters upstream sequence that speciﬁes which strand is template (in both prok/euk) positions RNAP on DNA Eukaryotic promoters have a ‘TATA’ box motif along with other regulatory sequences Euk RNAP requires many transcription factors (proteins helpers) to position it and keep it on the DNA
26 Special features (con’t)
Chromatin remodeling - adjusts access of RNAP to promoter RNA is processed from pre-RNA to mature RNA removal of extra bits of RNA (endo and exonucleases) in tRNA self splicing introns (Group I and Group II) in rRNA spliceosome (snRNPs) - removes introns from pre-mRNA, addition of 5’ CAP and 3’ Poly A tail
27 Ungraded problems
✤ Chapter 12: C20, C23, C25, C27, C30 28 Next Time
Gene regulation in Eukaryotes http://www.cs.stedwards.edu/chem/Chemistry/CHEM43/CHEM43/hypoxia/1d7g_molmol_1.gif 29 ...
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