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