chapter 16 – control of gene expression in prokaryotes
TRANSCRIPT
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Chapter 16 – Control of Gene Expression in Prokaryotes
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Genes• Structural genes
– Code for proteins involved with general processes (metabolism, catabolism) or structural components of cell
• Regulatory genes– Code for RNA/proteins that affect transcription/translation
of other sequences• Usually by binding to DNA
• Regulatory elements– Sequences of DNA that are not transcribed– Site of binding to regulatory proteins
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Levels of Gene Regulation • Alteration of gene structure
– More common in eukaryotes – hetero – vs euchromatin
• Transcriptional control– Whether RNA is created or not
• mRNA processing– Post-transcriptional modifications
in eukaryotes
• Stability of RNA– Degradation of mRNA
• Translational control – Whether or not translation occurs
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DNA binding proteins• Domain
– Region of regulatory protein that binds to DNA
– Approx 60-90 a.a.
• Motifs– Simple structure of
regulatory proteins
• Helix-turn-helix– Common in prokaryotes– Binds to major groove of
DNA
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DNA binding proteins cont• Zinc fingers
– Common in eukaryotes– Binds to major groove of
DNA
• Leucine zipper– Common in eukaryotes– Binds to two adjacent major
grooves of DNA
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Prokaryotic Operon Structure
• In prokaryotes, genes with similar functions are clustered together and are under the control of the same promotor– Transcribed as a single mRNA
• Operon is promotor, operator, and structural genes– Promotor – site for RNA polymerase binding– Operator – “on/off” switch; determines if transcription will occur
or not
• Regulator– Not part of an operon– Codes for a regulatory protein that binds to the operator
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Gene Control • Classified by regulatory protein function
– Negative control• Repressor function – inhibits transcription
– Positive control • Activator function – stimulates transcription
• Classified by “resting” state of operon– Inducible
• Transcription is usually “off”; needs to be activated• Ex: to make enzymes that are necessary only when
substrate is present
– Repressible • Transcription is usually “on”; needs to be silenced• Ex: gene products are always needed for cell functioning,
unless already in high concentration
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Lac operon in E. coli
• Three genes for lactose metabolism – LacZ
• β-Galactosidase– Breaks lactose into glucose and galactose
– LacY• Permease
– Actively transports lactose across cell membrane
– LacA• Transacetylase
– Function unknown
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Lac operon cont
• When lactose is not present, there is a very low level of transcription of these genes
• When lactose is present, rate of transcription increases 1,000x (in a matter of minutes)
• Negative inducible operon– Negative – regulator gene inhibits transcription – Inducible – normally in “off” position
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Lac operator
• Overlaps 3′ end of promotor and 5′ end of first structural gene (lacZ)
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Trp operon
• Contains 5 structural genes for 3 enzymes required for tryptophan synthesis– 2 enzymes are composed of two polypeptide
chains
• Negative – regulatory protein is a repressor
• Repressible – normally in “on” position
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Trp operon cont
• Repressor is produced in an inactive form – The repressor is unable to bind to the
operator; RNA polymerase can bind to promotor, so transcription occurs
• When levels of tryptophan is high, it binds to the repressor, activating it– Repressor can now bind to operator, blocking
attachment of RNA polymerase
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Riboswitches• Sequences of
mRNA that serve as potential binding sites for regulatory proteins
• Determines whether translation can occur or not
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Ribozymes
• RNA molecule that is capable of acting as a biological catalyst (enzyme)
• Induced self-cleavage prevents translation