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IUB BIOL-L 211 - Transcription in Prokaryotes I

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BIOL-L211 Lecture 16 Outline of Last Lecture I. ArticleII. RNAIII. TranscriptionOutline of Current Lecture I. Transcription InitiationII. Binding of RNA PolymeraseIII. IsomerizationIV. Initial Transcription/Promoter EscapeCurrent LectureTranscription in Prokaryotes II. Transcription InitiationA. RNA polymerase binds to double stranded promoter (aka closed complex)B. Double stranded DNA separates, yielding an open complexC. RNA polymerase escapes promoter (after initially transcribing small promoter segments), and begins transcribing the rest of the DNAII. Binding of RNA PolymeraseA. RNA Polymerase (RNAP)1. αI, αII, B', B, and Ω are the five main subunits that make up the RNA polymerase core2. Core structured like a crab claw- RNA synthesis catalyzed at base of pincers3. σ factor: additional subunit that chooses a specific promoter sequence and guides RNA polymerase in commencing transcriptionThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.B. Promoters1. DNA sequences that determine where transcription begins2. Defined by sigma factors3. σ70: housekeeping sigmaa. Directs transcription in genes with essential/routine functionb. Finds and binds to promoters with -35 and -10 regions, which are binding sites for σ70 relative to +14. Consensus sequence: average of all promoter sequences, which is thepreferred sequence for a σ70 to bind to5. The strongest promoters have sequences very similar/identical to consensus6. The variability in σ70 promoter sequences allows variability in gene expression levelIII. IsomerizationA. Formation of an open complex (melting of double stranded DNA)B. Process1. Occurs between -11 and +2, recognized by σ22. Two bases in the non-template strand (A-11 and T-7) are flipped out by σ23. -10 region is then single stranded; promoter DNA opens up furtherIV. Initial Transcription/Promoter EscapeA. InefficientB. Abortive Synthesis: RNAP initially produces and releases lots of short transcripts fromthe promoterC. Suggested Mechanisms for Abortive Synthesis:1. Transient Excursions: RNAP moves forward, makes transcript, releases transcript, and retreats2. Inchworming: Only the front component of RNAP can move downstream, and is then forced to retract, releasing mini transcripts in the process*Unofficially: the hungry-hungry hippo model*3. Scrunching: RNAP remains in place, bound to the promoter while pulling DNA into itselfa. Thought to be the correct modelb. Still being researchedD. Promoter Escape1. RNAP escapes promoter after achieving a transcript greater than ten nucleotides long2. Promoter escape:a. Polymerase-promoter and core-sigma factor interactions brokenb. Scrunching mechanism provides energy for above breakagec. Sigma factor leaves, and elongation can


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