Chapter29Notes 11.45.23 PM

Chapter29Notes 11.45.23 PM - Page 1 of 21 Biochemistry 461...

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Page 1 of 21 KEY CONCEPTS : ! E.coli is model study system for transcription of DNA into RNA. ! Promoter sequences signal the start of transcription. ! RNA transcripts are post-transcriptionally modified in many ways. ! In eucaryotes, promoter recognition and transcription involves many proteins (transcription factors) in addition to RNA polymerases. ! Eucaryotic enhancer sequences can stimulate transcription initiation from distant sites. ! Eucaryotic transcripts for mRNAs are 5'-capped , have 3'-poly(A) tails, and introns that are removed by splicing. Occasionallly, mRNA codons are edited. ! Ribonucleoprotein complexes ( spliceosomes, snRNPs ) catalyze splicing of many eucaryotic nuclear transcripts. ! Ribozymes : RNA molecules that self-splice - NO proteins needed! They are RNA enzymes . RNA molecules with active ribozyme sequences can be made by recombinant DNA methods. Biochemistry 461 Fall 20 10 Chapter 2 9 Notes (some additional material will be found in Powerpoint slides) CHAPTER 2 9 : RNA SYNTHESIS (TRANSCRIPTION) AND SPLICING LECTURE TOPICS ! PROCARYOTIC TRANSCRIPTION Post-transcriptional modifications ! EUCARYOTIC TRANSCRIPTION Transcription factors Post-transcriptional modifications ! RNA SPLICING MECHANISMS (also are post-transcriptional modifications)
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Page 2 of 21 Transcription is : E. coli RNA POLYMERASE AND TRANSCRIPTION INITIATION: No primer is required. The DNA template, RNA polymerase, and new RNA chain all interact during initiation , elongation , and termination of transcription. ! RNA polymerase holoenzyme is a 450kd, multisubunit protein with 4-subunits [ ± , ² , ² ’, and ³ (sigma)] ( Table 28.1). ! The sigma ( ³ ) subunit is required by the holoennzyme during transcription initiation for promoter recognition. After initiation, it falls off leaving core enzyme ( ± 2 , ² , ² ' subunits) that contains the catalytic site. Sigma ( ³ ) subunit decreases (by 10 4 -fold) the enzyme's affinity for non-promoter sequences. Promoter is located by sliding diffusion (one-dimensional) on intact DNA without unwinding the helix.
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Page 3 of 21 ! RNA polymerase functions in transcription : ! searches for initiation site among thousands of E. coli promoters. ! unwinds a small stretch of template DNA. ! selects correct NTP for base-pairing to initiate RNA synthesis. ! detection of termination signals ! interacts with modulators (activator or repressor proteins) which control levels of gene expression. ! Promoters are specific initiation alignment sequences that are recognized by and to which RNA polymerase binds. These sequences are found within sequences of about 60 bases that are protected from deoxyribonuclease digestion when RNA polymerase holoenzyme is bound to the “promoter”. This promoter DNA can be isolated and sequenced directly or sequenced indirectly by footprinting experiments (Fig.28.3).
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Page 4 of 21 ! Promoters are START SIGNALS : Promoters have two commonly conserved sequences of about 6 bases which are centered at -10 (Pribnow box) and -35
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Chapter29Notes 11.45.23 PM - Page 1 of 21 Biochemistry 461...

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