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Transcript
How to bioengineer a novel system? Obtain a sequence by PCR, then clone it into a suitable plasmid •We’re adding DNA, but want E. coli to make a protein! 1) In bacteria transcription and translation are initially coupled 1) In Bacteria transcription and translation are initially coupled • RNA polymerase quits if ribosomes lag too much 1) In Bacteria transcription and translation are initially coupled • RNA polymerase quits if ribosomes lag too much • Recent studies show that ribosomes continue translating once mRNA is complete; i.e after transcription is done Bacteria have > 1 protein/mRNA (polycistronic) http://bmb-itservices.bmb.psu.edu/bryant/lab/Project/Hydrogen/index.html#secti on1 •euk have 1 protein/mRNA Bacteria have > 1 protein/mRNA (polycistronic) • Mutations can have polar effects: mutations in upstream genes may affect expression of perfectly good downstream genes! Regulating transcription Telling RNA pol to copy a DNA sequence Regulating transcription Telling RNA pol to copy a DNA sequence Transcription factors bind promoters & control initiation of transcription Regulating transcription Telling RNA pol to copy a DNA sequence Transcription factors bind promoters & control initiation of transcription 1/signal gene senses Regulating transcription Telling RNA pol to copy a DNA sequence Transcription factors bind promoters & control initiation of transcription 1/signal gene senses 1 binding site/signal gene senses Transcription factors Bind surface -> base-pairs form unique patterns in major & minor grooves Transcription factors Bind surface -> base-pairs form unique patterns in major & minor grooves Scan DNA for correct pattern Transcription factors Bind surface -> base-pairs form unique patterns in major & minor grooves Scan DNA for correct pattern need 15 - 20 H-bonds = 5-8 base-pairs Transcription Prokaryotes have one RNA polymerase makes all RNA core polymerase = complex of 5 subunits (a1aIIbb’w) Transcription Prokaryotes have one RNA polymerase makes all RNA core polymerase = complex of 5 subunits (a1aIIbb’w) w not absolutely needed, but cells lacking w are very sick Initiating transcription in Prokaryotes 1) Core RNA polymerase is promiscuous Initiating transcription in Prokaryotes 1) Core RNA polymerase is promiscuous 2) sigma factors provide specificity Initiating transcription in Prokaryotes 1) Core RNA polymerase is promiscuous 2) sigma factors provide specificity • Bind promoters Initiating transcription in Prokaryotes 1) Core RNA polymerase is promiscuous 2) sigma factors provide specificity • Bind promoters • Different sigmas bind different promoters Initiating transcription in Prokaryotes 1) Core RNA polymerase is promiscuous 2) sigma factors provide specificity • Bind promoters 3) Once bound, RNA polymerase “melts” the DNA Initiating transcription in Prokaryotes 3) Once bound, RNA polymerase “melts” the DNA 4) rNTPs bind template Initiating transcription in Prokaryotes 3) Once bound, RNA polymerase “melts” the DNA 4) rNTPs bind template 5) RNA polymerase catalyzes phosphodiester bonds, melts and unwinds template Initiating transcription in Prokaryotes 3) Once bound, RNA polymerase “melts” the DNA 4) rNTPs bind template 5) RNA polymerase catalyzes phosphodiester bonds, melts and unwinds template 6) sigma falls off after ~10 bases are added Structure of Prokaryotic promoters Three DNA sequences (core regions) 1) Pribnow box at -10 (10 bp 5’ to transcription start) 5’-TATAAT-3’ determines exact start site: bound by s factor Structure of Prokaryotic promoters Three DNA sequences (core regions) 1) Pribnow box at -10 (10 bp 5’ to transcription start) 5’-TATAAT-3’ determines exact start site: bound by s factor 2)” -35 region” : 5’-TTGACA-3’ : bound by s factor Structure of Prokaryotic promoters Three DNA sequences (core regions) 1) Pribnow box at -10 (10 bp 5’ to transcription start) 5’-TATAAT-3’ determines exact start site: bound by s factor 2)” -35 region” : 5’-TTGACA-3’ : bound by s factor 3) UP element : -57: bound by a factor Structure of Prokaryotic promoters Three DNA sequences (core regions) 1) Pribnow box at -10 (10 bp 5’ to transcription start) 5’-TATAAT-3’ determines exact start site: bound by s factor 2)” -35 region” : 5’-TTGACA-3’ : bound by s factor 3) UP element : -57: bound by a factor Structure of Prokaryotic promoters Three DNA sequences (core regions) 1) Pribnow box at -10 (10 bp 5’ to transcription start) 5’-TATAAT-3’ determines exact start site: bound by s factor 2)” -35 region” : 5’-TTGACA-3’ : bound by s factor 3) UP element : -57: bound by a factor Other sequences also often influence transcription! Eg Trp operator Prok gene regulation 5 genes (trp operon) encode trp enzymes Prok gene regulation Copy genes when no trp Repressor stops operon if [trp] Prok gene regulation Repressor stops operon if [trp] trp allosterically regulates repressor can't bind operator until 2 trp bind lac operon Some operons use combined “on” & “off” switches E.g. E. coli lac operon Encodes enzymes to use lactose lac Z = b-galactosidase lac Y= lactose permease lac A = transacetylase lac operon Make these enzymes only if: 1) - glucose lac operon Make these enzymes only if: 1) - glucose 2) + lactose lac operon Regulated by 2 proteins 1) CAP protein : senses [glucose] lac operon Regulated by 2 proteins 1) CAP protein : senses [glucose] 2) lac repressor: senses [lactose] lac operon Regulated by 2 proteins 1) CAP protein : senses [glucose] 2) lac repressor: senses [lactose] encoded by lac i gene Always on lac operon 2 proteins = 2 binding sites 1) CAP site: promoter isn’t active until CAP binds lac operon 2 proteins = 2 binding sites 1) CAP site: promoter isn’t active until CAP binds 2) Operator: repressor blocks transcription lac operon Regulated by 2 proteins 1) CAP only binds if no glucose -> no activation lac operon Regulated by 2 proteins 1) CAP only binds if no glucose -> no activation 2) Repressor blocks transcription if no lactose lac operon Regulated by 2 proteins 1) CAP only binds if no glucose 2) Repressor blocks transcription if no lactose 3) Result: only make enzymes for using lactose if lactose is present and glucose is not Result [b-galactosidase] rapidly rises if no glucose & lactose is present W/in 10 minutes is 6% of total protein!
