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Unformatted text preview: May 16 Gene Regulation in Eukaryotes chap 18 Exam Monday May 19, 5 to 6:50 PM Exam covers lectures 10 through 17. Nothing from lectures this week. All Weds and Thurs 9 AM sections go to Moore 100; Thurs 11 and 1 PM sections go to PAB 1425. Reviews: Thurs 3-4:50 Young 1054 (disc section rm) to make up any missing Disc section problems Friday 2:30-4:30 LS 2320 - bring your problems Sunday 3-5:00 LS 4127 - bring your questions Monday 9-9:50 LaKretz 110 1. Single cell eukaryotes (eg, yeast) 2. Multicellular (multi-tissue) eukaryotes Regulatory elements that map near or at some distance from a gene are cis-acting DNA sequences. cis-acting elements Promoter very close to gene's initiation site Enhancer Can lie far way from gene 100's of kb Enhancer works in either orientation (ie could work in both directions) Augment or repress basel levels of transcription Fig. 18.1 a Regulatory elements that map some distance from a gene are cis-acting sequences that bind transacting gene products Genes that encode proteins that interact directly or indirectly with target genes cisacting elements Known genetically as transcription factors Identified by: Mapping mutants Biochemical studies to identify proteins that bind invitro to cisacting elements Fig. 18.1 b Polymerase complex: Trans-acting proteins control transcription from class II promoters. Basal factors bind to the promoter.
TBP TATA box binding protein TAF TBP associated factors RNA polymerase II binds to basal factors. Fig. 18.4 a transcription factors Also called Activator proteins Bind to enhancer DNA at specific sites Interact with other proteins to activate and increase transcription as much as 100-fold above basal levels Two structural protein domains mediate these functions:
DNA-binding domain Transcription-activator domain Transcriptional activators bind to specific enhancers at specific times to increase transcriptional levels. Fig. 18.5 a Examples of common transcription factors Zinc-finger proteins and helix-loop-helix proteins bind to the DNA binding domains of enhancer elements. Activation domains bind polymerase complex. Fig. 18.5 b Most eukaryotic activators must form dimers to function. Eukaryotic transcription factor protein structure Homomers multimeric proteins composed of identical subunits Heteromers multimeric proteins composed of nonidentical subunits Fig. 18.7 a Fig. 18.11.a: yeast cells are eukaryotes that respond to their envirnoment by specific features. One is a cluster of genes that respond in concert to a common enhancer, in this case the genes needed to ultilize galactose as a carbon and energy source. Fig. 18.11.b: Gal4 is a transcription factor that binds to the enhancer via a DNA binding domain and activates transciption via a polymerase complex binding activator. Gal80 has a repressor function in preventing the activator domain of Gal4 from binding the polymerase complex. System stays off in this diagram. Fig. 18.11.c: Galactose entering the cell binds Gal1/Gal3 proteins; they change their shape as a result and can now bind Gal80 to liberate the activation domain of Gal4. In this diagram the system is now active and transcription of the Gal complex takes place in response to galactose presence. The Gal4 response enhancer and the Gal4 transcription factor can be engineered into higher eukaryotes, minus the Gal1/3 response to galactose, to regulate genes as well in tissue specific fashion. Large region with tissue-specific enhancers of Drosophila string gene (respond to presence of tissue and stage specific transcription factors) Fourteenth cell cycle of the fruit fly embryo A variety of enhancer sites ensure that string is turned on at the right time in each mitotic domain and tissue type. Note distances. Independent of the outside environment. Fig. 18.3 Other mechanisms of gene regulation Chromatin structure Slows transcription Hypercondensation stops transcription. Silences transcription selectively if inherited from one parent RNA splicing can regulate expression. RNA stability controls amount of gene product. mRNA editing can affect biological properties of protein. Noncoding sequences in mRNA can modulate translation. Protein modification after translation can control gene function. Genomic imprinting Some genes are regulated after transcription. Fig. 13.3 nucleosome structure characterizes eukaryotic DNA-How packaged? Normal chromatin structure variable, influenced by transcription: nucleosomes not fixed at locations Fig. 18.13 a-b Hypercondensation and DNA methylation over chromatin domains causes transcriptional silencing. Fig. 18.14 a One gene product regulates the next in a "cascade" of gene regulation steps leading to biological result. Primary fru mRNA transcript made in both sexes Presence of tra protein in females causes alternative splicing encoding fru-F. Absence of tra protein in males produces fru-M. Fig. 18.24 ...
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- Winter '08