Regulation of Gene Expression in Bacteria

Genetics BIOL 2004 Kamran Virginia Tech

control point for regulation

transcription

structural genes

genes that code for proteins

regulator genes

genes that code for proteins that control other genes

promoter

tells RNA polymerase II where to bind

operon

• on/off switch

• multiple genes controlled together -> become one mRNA

• inducible or repressible

activator site

turns genes on

negative control

repressor protein binds to operon and block transcription

positive control

activator protein helps RNA polymerase II bind and induces transcription

constitutive expression

gene is constantly on

inducible operon

• e.g. lac

• normally off, turned on when needed

repressible operon

• e.g. trp

• usually on, turned off when product is abundant

lac operon

• inducible

• lacZ = B-galactosidase (breaks lactose)

• lacY = permease (brings lactose in)

• lacl = repressor protein

CAP-cAMP system

• low glucose = high cAMP

• cAMP binds CAP

• cAMP/CAP complex binds DNA

• complex helps RNA polymerase II bind to DNA

• high glucose = low cAMP

• RNA polymerase II struggles to bind

lac system

• no lactose = lac operon is off

• lactose, no glucose = lac operon on

• lactose, high glucose = lac operon on but muted

lac operon mutations

• cis acting

• trans acting

• Lacl-

• Lacl5

• LacOo

• LacP-

cis acting lac operon mutation

affects only same DNA molecules

trans acting lac operon mutation

affects other DNA molecules too

Lacl- mutation

• repressor is broken and can’t find operator

• operon always on

Lacl5 superrepressor

• repressor is always bound to operator

• operon always off

LacOo Operator mutation

• repressor can’t bind

• operon always on

LacP- Promoter Mutation

• RNA polymerase can’t bind

• operon off

trp operon

• makes tryptophan

• low tryptophan = repressor inactive

• high tryptophan = tryptophan binds repressor

• repressor binds operator

• transcription is shut down

attenuation

stops transcription early

• high tryptophan = ribosomes quickly make terminator hairpin → transcription ends

• low tryptophan —> ribosome stalls, forms antiterminator so transcription continues

antisense RNA

binds mRNA and blocks translation

riboswitches

• stop transcription/translation

• mRNA changes shape when riboswitch binds

ribozymes

• RNA that cuts like an enzyme

• cuts our mRNA and degrades it