1. Chapter 16 – Control of Gene Expression in Prokaryotes

2. Genes Structural genes Regulatory genes Regulatory elements
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

3. 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

4. DNA binding proteins Domain Motifs Helix-turn-helix
Region of regulatory protein that binds to DNA
Approx a.a.
Motifs
Simple structure of regulatory proteins
Helix-turn-helix
Common in prokaryotes
Binds to major groove of DNA

5. DNA binding proteins cont
Zinc fingers
Common in eukaryotes
Binds to major groove of DNA
Leucine zipper
Binds to two adjacent major grooves of DNA

6. 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

8. 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

10. Lac operon in E. coli Three genes for lactose metabolism LacZ LacY
β-Galactosidase
Breaks lactose into glucose and galactose
LacY
Permease
Actively transports lactose across cell membrane
LacA
Transacetylase
Function unknown

11. 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

12. Lac operator
Overlaps 3′ end of promotor and 5′ end of first structural gene (lacZ)

15. 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

16. 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

18. Riboswitches
Sequences of mRNA that serve as potential binding sites for regulatory proteins
Determines whether translation can occur or not

19. Ribozymes
RNA molecule that is capable of acting as a biological catalyst (enzyme)
Induced self-cleavage prevents translation