《生物化学》课程PPT教学课件（英文版）Chapter 12 RNA的合成和加工 Synthesis and Processing of RNA

Chapter 12. Synthesis and Processing of RNa Bacterial rNas Eukaryotic RNAS

Chapter 12. Synthesis and Processing of RNA Bacterial RNAs Eukaryotic RNAs ……

Overview of Differences Between Prokaryotic and Eukaryotic Gene Expression and Regulation Prokaryotes Eukaryotes Structure of Single, generally circular Genome found in genome genome sometimes chromosomes nucleosome st accompanied by smaller pieces ructure limits DNA of accessory DNA, like plasmids accessibility Size of genome Relatively small Relatively large Location of Coupled; no nucleoid envelope Nuclear transcription and gene barrier because of cytoplasmic translation transcription prokaryotic cell structure and translation Gene clustering Operons where genes with Operons generally not found similar function are grouped in eukaryotes; each gene has together its own promoter element and enhancer element(s) Default state of on Off transcription DNA structure Highly supercoiled DNA with Highly supercoiled chromatin some associated proteins associated with histones in nucleosomes

Overview of Differences Between Prokaryotic and Eukaryotic Gene Expression and Regulation Prokaryotes Eukaryotes Structure of genome Single, generally circular genome sometimes accompanied by smaller pieces of accessory DNA, like plasmids Genome found in chromosomes;nucleosome st ructure limits DNA accessibility Size of genome Relatively small Relatively large Location of gene transcription and translation Coupled; no nucleoid envelope barrier because of prokaryotic cell structure Nuclear transcription and cytoplasmic translation Gene clustering Operons where genes with similar function are grouped together Operons generally not found in eukaryotes; each gene has its own promoter element and enhancer element(s) Default state of transcription On Off DNA structure Highly supercoiled DNA with some associated proteins Highly supercoiled chromatin associated with histones in nucleosomes

Differences between Prokaryotic and Eukaryotic Gene Expression and Regulation RNA polymerase in prokaryotes can access almost any promoter in a dNa strand without the presence of activators or repressors. Thus, the"ground state"of DNA expression in prokaryotes is said to be nonrestrictive,or on "In eukaryotes, however, the ground state of expression is restrictive in that, although strong promoters might be present, they are inactive in the absence of some sort of recruitment to the promoter by transcription factors From Phillips T: Regulation of transcription and gene expression in eukaryotes Nature Education 2008, 1(1)

Differences Between Prokaryotic and Eukaryotic Gene Expression and Regulation • RNA polymerase in prokaryotes can access almost any promoter in a DNA strand without the presence of activators or repressors. Thus, the "ground state" of DNA expression in prokaryotes is said to be nonrestrictive, or "on." In eukaryotes, however, the ground state of expression is restrictive in that, although strong promoters might be present, they are inactive in the absence of some sort of recruitment to the promoter by transcription factors. From Phillips T: Regulation of transcription and gene expression in eukaryotes. Nature Education 2008, 1(1)

12. 1 Synthesis and processing of bacterial rnas 535 RNA 3 DNA RNA polymerase

12.1 Synthesis and processing of bacterial RNAs

12.1.1 Synthesis of bacterial transcripts 200 400 600 800 1000 1200 C 492515 13083239 951y10651230 1243 Lys 1306 Non-template strand upstream DNA 00 downstream DNA 5b000000050d 939Q9999999 Template strand A DNA RNA hybrid (9bp) Region enclosed by the polymerase 29102130 330400466 109511891260 C 200 400 600 800 1000 1200 1400

12.1.1 Synthesis of bacterial transcripts

The polymerase does not synthesize its transcript at a constant rate A pause rarely lasts longer than 6 milliseconds and might be accompanied by the polymerase moving in reverse along the template

• The polymerase does not synthesize its transcript at a constant rate. • A pause rarely lasts longer than 6 milliseconds and might be accompanied by the polymerase moving in reverse along the template

Termination of a bacterial transcript Transcription is a discontinuous process, with the polymerase pausing regularly and making a choice between continuing elongation or terminating Which choice is depends on which alternative is more favorable in the thermodynamic terms Bacteria appear to use two distinct strategies for transcription termination In the template strand, about half the terminating position contains an inverted palindrome followed by a run of deoxyadenosine nucleotides. Intrinsic terminators Rho dependent

Termination of a bacterial transcript • Transcription is a discontinuous process, with the polymerase pausing regularly and making a “choice” between continuing elongation or terminating. • Which choice is depends on which alternative is more favorable in the thermodynamic terms. • Bacteria appear to use two distinct strategies for transcription termination. • In the template strand, about half the terminating position contains an inverted palindrome followed by a run of deoxyadenosine nucleotides. Intrinsic terminators. • Rho dependent

Template strand DNA EITIT GGCCATCCGC TTTTTGCGGATGGCC TAAAAAAAAAA 5 CCGGTAGGCG CGCCTACCGG ETTTTTTTTTT Transcription RNA5′ FUUUUUUUUUU- 3 c·G A-U base pairs RNA G U·A A·U GGcG G DNA LLLLL RNA polymerase

Rho RNA HH DNA LLLL Elongation complex stalls at a hairpin loop Rho breaks the RNA-DNa base pairs TTTTT

12.1.2 Control over the choice between elongation and termination Antitermination site Promoter Termination signals DNA Antiterminator protein attached to the dna Antiterminator protein (1) HIlT attaches to the RNa polymerase and prevents Termination termination at signal signal 1 number 1

12.1.2 Control over the choice between elongation and termination