Gene regulation in prokaryotes

1. LAC OPERON
ALITA ROSE JACOB
I MSc. LIFE SCIENCE
REG NO: M19LS02

2. INTRODUCTION
• Operon is defined as a group of structural genes that are transcribed together at the
same time.
• They are found only in prokaryotes.
• Function : Regulate the expression of structural genes by controlling transcription.
• Operon Structure :
Structural genes – one end of operon has set of structural genes namely gene a,
gene b and gene c transcribed into single MiRNA and translated to produce enzyme
a, b and c.
 Promoter – lies upstream of first structural gene and RNA polymerase binds to it
and moves downward transcribing the structural genes.

3.  Operator : Regulatory protein binds to a region of operon called operator and
controls transcription.
 Regulatory gene : controls transcription of structural genes having its own promoter
and transcribed into short RNA and translated into small protein called regulatory
protein.
TYPES OF TRANSCRIPTIONAL CONTROL :
 Positive Control : regulatory protein act as activator stimulating transcription.
 Negative Control : regulatory protein act as repressor inhibiting transcription.

4. TYPES OF OPERON
 Inducible Operons : transcription is normally off and some factor has to induce
transcription.
eg : Lac Operon
 Repressible Operon : transcription is normally on and some factor has to inhibit
transcription and turn it off.
eg : Trp Operon

5. LAC OPERON
• It is a negative inducible repressor .
• Lactose operon is designated as Lac Operon.
• This operon codes for enzymes involved in the catabolism of lactose .
• In 1961, Francois Jacob and Jacques Monod described the lac operon model in E.
coli.

6. LACTOSE METABOLISM
Lactose is a major carbohydrate present in milk. E.coli living in the mammalian gut
and can metabolism lactose where it is broken down into glucose and galactose.
Lactose does not easily diffuse across the E. coli cell membrane and must be
actively transported into the cell by the protein permease
 To utilize lactose as an energy source, E. coli must first break it into glucose and
galactose, a reaction catalyzed by the enzyme β-galactosidase.
This enzyme can also convert lactose into allolactose , a compound that plays an
important role in regulating lactose metabolism.
 A third enzyme, thiogalactoside transacetylase , also is produced by the lac operon,
but its function in lactose metabolism is not yet known.

8. LAC OPERON MODEL
It consists of :
• Structural genes –
a. lac Z codes for enzyme β-galactosidase – breaks down lactose into glucose and
galactose.
b. lac Y codes for galactose permease – actively transports lactose into E.coli cells.
c. lac A codes for thiogalactosidase transacetylase
• Operator ( lac O) – binding site for repressor.
• Promoter ( lac P ) – binding site for RNA polymerase.
• Repressor – gene coding for lac repressor protein that binds to operator and prevents RNA
polymerase from binding to promoter.
• Inhibitor ( lac I ) – controls production of repressor protein.

9. FUNCTIONING OF LAC OPERON
• In the absence of lactose (inducer) , the regulatory gene produce repressor protein
that binds to operator and prevents transcription.

10. • When lactose is introduced into the medium , some of it gets converted into
allolactose and binds to repressor preventing it from binding to operator.
• The repressor has two binding sites – one for allolactose and the other for DNA.
• Therefore the operator is set free and it induces the RNA polymerase to bind to the
initiation site of promoter.
• This stimulates the transcription and produce a polycistronic mRNA
• This miRNA codes for three enzymes that catabolize lactose .

18. REGULATION OF LAC OPERON
 MOLECULAR LEVEL
 METABOLIC LEVEL

27. THANK YOU