In eukaryotes, three RNA polymerases transcribe different sets ofgenes.RNA polymeraseI(a)Tandemrepeatsof rRNAgenes aretranscribedbyRNApolymerasel.transcribesrRNARNApolrRNAgenesPromoterTranscriptionrRNAsaremade-?oftandem repeatsononeormorePrimaryrRNAtranscriptschromosomes.RNA polymeraseISubsequentprosessing28SrRNA5.8SrRNA18SrRNAtranscribes oneinnucleolusprimary rRNAtranscript which isfurther processedSmallLargeto 28S, 5.8S, andribosomalribosomalsubunitsubunit18S rRNA.Ribosome18-6
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-6 RNA polymerase I RNA polymerase I transcribes transcribes rRNA. rRNAs are made are made of tandem repeats of tandem repeats on one or more on one or more chromosomes. chromosomes. RNA polymerase I RNA polymerase I transcribes one transcribes one primary primary rRNA transcript which is transcript which is further processed further processed to 28S, 5.8S, and to 28S, 5.8S, and 18S rRNA. In eukaryotes, three In eukaryotes, three RNA polymerases RNA polymerases transcribe different sets of transcribe different sets of genes
RNA polymeraseIItranscribes all protein-and micro-RNA-encodinggenes.The cis-acting regulatoryregions of RNA pol.Il-transcribed genescontain promoters and enhancers.(c)Protein-encodingandmicro-RNA-encodinggenesaretranscribedbyRNApolllRNApol IITranscriptionfactorsExonIntronExonIntronExonLPromoterEnhancer-PrimaryRNAtranscriptProcessedmRNATAAAAA5'capPolypeptide productD18-7
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-7 RNA polymerase II RNA polymerase II transcribes all protein transcribes all protein- and micro and micro-RNAencoding genes. encoding genes. The cis-acting regulatory regions of RNA acting regulatory regions of RNA pol. II-transcribed genes transcribed genes contain contain promoters promoters and enhancers enhancers
RNA polymerase III transcribes tRNAs and other small RNAs(5SrRNA,SnRNAs)(b)SmallRNAgenesaretranscribedbyRNApol IlltRNAgenesRNApol II5SRNAgeneTranscriptionunitPromoterPromoter18-8
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-8 RNA polymerase III RNA polymerase III transcribes transcribes tRNAs and other small and other small RNAs (5S rRNA, snRNAs)
16.2 Control of transcription initiation throughpromoters and enhancersProteins act in trans, but DNA sites act only in cis. Cis-acting elements:Short DNA sequences that constitute thecontrol elements adjacent to genes.They can onlyinfluencethe expression of adjacent genes on the same DNA moleculeTrans-acting elements: Genes that code for DNA-bindingproteins. Their protein products can diffuse throughcytoplasm and act at target DNA sites on any DNA moleculeinthecell.18-9
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-9 Proteins act in Proteins act in trans, but DNA sites act only in , but DNA sites act only in cis. Cis-acting elements acting elements: Short DNA sequences that constitute the : Short DNA sequences that constitute the control elements adjacent to genes. They can only influence control elements adjacent to genes. They can only influence the expression of adjacent genes on the same DNA molecule. the expression of adjacent genes on the same DNA molecule. Trans-acting elements acting elements: Genes that code for DNA : Genes that code for DNA-binding binding proteins. Their protein products can diffuse through proteins. Their protein products can diffuse through cytoplasm and act at target DNA sites on any DNA molecule in cytoplasm and act at target DNA sites on any DNA molecule in the cell. the cell. 16.2 Control of transcription initiation through 16.2 Control of transcription initiation through promoters and enhancers promoters and enhancers
Cis-acting elements of a genePromoter（启动子）：DNAseguencesnearthebeginningofgenesthat signal RNA polymerase where to begin transcription Promoters contain initiation site andTATA box.Enhancer（增强子）:DNA sequences thataugmentorrepressbasal levels of transcriptionfrom nearby genes DNA sequences that bind transcription factorsCan liefarwayfromgeneCan bereversedTranscriptioncis-actingelements ofagenestart siteRTranscribedEnhancerPromoterregionAPromoterTranscribedEnhancerregion18-10Fig.16.2
Copyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or display 18-10 Cis-acting elements of a gene acting elements of a gene Promoter ( Promoter (启动子): DNA sequences near the beginning of genes : DNA sequences near the beginning of genes that signal RNA polymerase where to begin transcription. that signal RNA polymerase where to begin transcription. Promoters contain initiation site and TATA box. Promoters contain initiation site and TATA box. Enhancer ( Enhancer (增强子): DNA sequences that augment or repress DNA sequences that augment or repress basal levels of transcription from nearby genes. basal levels of transcription from nearby genes. DNA sequences that bind transcription factors. DNA sequences that bind transcription factors. Can lie far way from gene. Can lie far way from gene. Can be reversed. Can be reversed. Fig. 16.2