Transcript Chapter 16 – Control of Gene Expression in

Chapter 16 – Control of Gene
Expression in Prokaryotes
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
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
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
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
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
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
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
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
Lac operator
• Overlaps 3′ end of promotor and 5′ end of first
structural gene (lacZ)
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
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
Riboswitches
• Sequences of
mRNA that serve as
potential binding
sites for regulatory
proteins
• Determines whether
translation can
occur or not
Ribozymes
• RNA molecule that is
capable of acting as a
biological catalyst
(enzyme)
• Induced self-cleavage
prevents translation