Lac operon

Lac operon

1. THE lacOPERON

2. The control of gene expression
Eachcellinthehumancontainsallthegenetic
materialforthegrowthanddevelopmentofa
human
Someofthesegeneswillbeneedtobe
expressedallthetime
Thesearethegenesthatareinvolvedinofvital
biochemicalprocessessuchasrespiration
Othergenesarenotexpressedallthetime
Theyareswitchedonanoffatneed

3. Operons
An operon is a group
of genes that are
transcribed at the
same time.
They usually control
an important
biochemical process.
They are only found
in prokaryotes.
(Find out the Few
Exceptions)
© NobelPrize.org
Jacob, Monod & Lwoff

4. Lactose Operon
Structural genes
lac z, lac y, & lac a
Polycistronic mRNA
Regulatory gene
Repressor
Operator
Promoter
Inducer –(lactose)
i
Operon
Regulatory
Gene
po z y a DNA
m-RNA
-Galactosidase
Permease
Transacetylase
Protein
Difference between Polycistronic and Monocistronic mRNA ?

5. The lacOperon
Thelacoperonconsistsofthreegenes
eachinvolvedinprocessingthesugar
lactose
Oneofthemisthegenefortheenzymeβ-
galactosidase
Thisenzymehydrolyseslactoseinto
glucoseandgalactose

6. Structure of the lac Operon
Thelacoperonconsistsof3protein-codinggenesplus
associatedcontrolregions.
The3genesarecalledz,y,anda.
lacZcodesfortheenzymebeta-galactosidase,which
splitslactoseintoglucoseplusgalactose.
lacYcodesfora“permease”proteinthatallowslactose
toenterthecell
lacAcodesforanenzymethatacetylateslactose.
Togetherthesethreegenesarecalledthe“structural
genes”.
ipo z y a

7. Control Regions
Nearthelacoperonisanothergene,calledlacI,
orjust“i”.Itcodesforthelacrepressorprotein,
Playsanessentialroleinlacoperoncontrol.
Thelacrepressorgeneisexpressed
“constitutively”,meaningthatitisalwayson(but
atalowlevel).
Itisacompletelyseparategene,producinga
differentmRNAthanthelacoperon.
ipo z y a

8. Justupstreamfromthetranscriptionstartpoint
inthelacoperonaretworegionscalledthe
operator(o)andthepromoter(p).
Neitherregioncodesforprotein:theyactas
bindingsitesontheDNAforimportantproteins.
ThepromoteristhesitewhereRNApolymerase
bindstostarttranscription.Promotersarefound
upstreamfromallprotein-codinggenes.
ipo z y a

9. Visual

10. Adapting to the environment
E.colicanuseeitherglucose,whichisa
monosaccharide,orlactose,whichisa
disaccharide
However,lactoseneedstobehydrolysed
(digested)first
Sothebacteriumpreferstouseglucosewhenit
can

11. Four situations are possible
1.Whenglucoseispresentandlactoseisabsentthe
E.colidoesnotproduceβ-galactosidase.
2.Whenglucoseispresentandlactoseispresentthe
E.colidoesnotproduceβ-galactosidase.
3.Whenglucoseisabsentandlactoseisabsentthe
E.colidoesnotproduceβ-galactosidase.
4.Whenglucoseisabsentandlactoseispresentthe
E.colidoesproduceβ-galactosidase

12. The control of the lacoperon

13. 1. When lactose is absent
A repressor protein is continuously synthesised. It sits on
a sequence of DNA just in front of the lacoperon, the
Operatorsite
The repressor proteinblocks the Promotersitewhere
the RNA polymerase settles before it starts transcribing
Regulator
gene
lacoperon
Operator
site
z y a
DNA
I
O
Repressor
protein
RNA
polymeraseBlocked

14. 2. When lactose is present
A small amount of a sugar allolactose is formed within
the bacterial cell. This fits onto the repressor protein at
another active site (allosteric site)
This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the
operator site. RNA polymerase can now reach its
promoter site
z y a
DNA
I O

15. 2. When lactose is present
A small amount of a sugar allolactose is formed within
the bacterial cell. This fits onto the repressor protein at
another active site (allosteric site)
This causes the repressor protein to change its shape (a
conformational change). It can no longer sit on the
operator site. RNA polymerase can now reach its
promoter site
Promotor site
z y a
DNA
I O

16. 3. When both glucose and lactose are
present
Thisexplainshowthelacoperonistranscribed
onlywhenlactoseispresent.
BUT…..thisdoesnotexplainwhytheoperonis
nottranscribedwhenbothglucoseandlactose
arepresent.

17. When glucose and lactose are present RNA
polymerase can sit on the promoter site but it is
unstable and it keeps falling off
Promotor site
z y a
DNA
I O
Repressor protein
removed
RNA polymerase

18. 4. When glucose is absent and
lactose is present
Anotherproteinisneeded,anactivatorprotein.This
stabilisesRNApolymerase.
Theactivatorproteinonlyworkswhenglucoseisabsent
InthiswayE.colionlymakesenzymestometabolise
othersugarsintheabsenceofglucose
Promotor site
z y a
DNA
I O
Transcription
Activator
protein steadies
the RNA
polymerase

19. (Activator protein) Catabolite
Activator Protein (CAP)-also k/a
cAMP receptor protein, CRP)

20. Summary
CarbohydratesActivator
protein
Repressor
protein
RNA
polymerase
lacOperon
+ GLUCOSE
+ LACTOSE
Not bound
to DNA
Lifted off
operator site
Keeps falling
off promoter
site
Basal level
transcription
+ GLUCOSE
-LACTOSE
Not bound
to DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
-GLUCOSE
-LACTOSE
Bound to
DNA
Bound to
operator site
Blocked by
the repressor
No
transcription
-GLUCOSE
+ LACTOSE
Bound to
DNA
Lifted off
operator site
Sits on the
promoter site
Activated level
Transcription

22. Negative and Positive Regulation
Asdescribedabove,thelacoperonisnegatively
regulated:i.e.theregulatoryprotein(repressor)
causestranscriptiontostop.
Positiveregulation,wheretheregulatoryprotein
causestranscriptiontostart,ismorecommon.
Thelacoperonalsocontainsanexampleofpositive
regulation,called“cataboliterepression”.E.coli
wouldprefertouseglucoseasitsfoodsource.
Inthepresenceofglucose,thelacoperon(and
othersimilargenes)areturnedoff,eveniflactoseis
presentinthemedium.

23. ►An Activator and a Repressor Together
Control the lacGenes
TheactivatoriscalledCAP(Catabolite
ActivatorProtein).CAPcanbindDNAand
activatethelacgenesonlyintheabsenceof
glucose.
ThelacrepressorcanbindDNAandrepress
transcritiononlyintheabsenceoflactose.
BothCAPandlacrepressorareDNA-binding
proteinsandeachbindstoaspecificsiten
DNAatornearthelacpromoter.