Transcription Control inEukaryotesTranscription control in eukaryotes is more complexthan in prokaryotes, with more gene-geneinteractions, presumably required to produce moredifferent cell types in more complex organisms.We will consider some examples and models toillustrate some general principles.Example: Activation of GAL Genes in YeastGAL7 GAL10 GAL1 GAL4 GAL80ActivatorRepressorTightly linked genes GAL1, GAL7, and GAL10 are coordinatelyactivated or repressed by the activity of two genes on differentchromosomes.Their protein products are enzymes required for conversion ofgalactose to glucose-1-phosphate which enters the glycolysispathway.The activator binds to upstream enhancer sequences; the ?indicates that the position of these sites is unknown (to me).ActivatorRepressor?? ?galactoseActivatorRepressorGeneral Model of Transcription Control byActivator Proteins and Enhancer Sequences1. A transcriptional activatorprotein binds to an enhancersequence (maybe very farupstream or downstream fromgene).2. Activator protein bindsTranscription Factor IID.3. TATA-binding proteincomponent of TFIID isrecruited to the promoter andbinds RNA polymeraseholoenzyme (polymerase plusother proteins) .4. RNA polymerase holoenzymebinds to promoter and beginstranscription.Text figure 11.25More Detailed Model of Transcription InitiationText figure 11.26Multiple Distant Transcription Control SitesLocus Control Region of the globin genes in the β-globincluster lies upstream from β-globin gene, consisting of fourdifferent sites. There are two more sites downstream whichaffect gene expression.Gene Regulation by Chromosome StructureDNA in chromosomes ishighly folded and compacted,partly by being wouldaround structures callednucleosomes made of histoneproteins.Chromatin = DNA plusassociated proteinsText figured 7.11Gene Regulation by Chromosome StructureGenes in nucleosomes areinaccessible to transcriptioncomplexes.Genes must be moved off ofnucleosomes before they can betranscribed.Text figure 11.27Gene Regulation by Chromosome StructureChromatin model. Sticks are DNA, colored spheres are nucleosomes.Red spheres labelled A and B are transcription complexes.Blue chromatin near B is too dense to allow transcription complexaccess to genes.Yellow chromatin near A is partially unfolded to allow access.Text figure 7.10Coordination• Many genes are involved in the transcription control of one gene.• These genes are controlled by other genes (and sometimes byeach other).• Transcription control involves extremely complex networks ofgene interactions that must be coordinated.• Identifying these interactions requires ingenious geneticexperiments.• Understanding the interactions and their coordinations requireshigh-quality mathematical and computer analyses, part of thegrowing field of bioinformatics, looking for large-scaleproperties of networks and general
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