In prokaryotes, genes are regulated at the transcriptional level to control protein synthesis. There are two main types of gene regulation: negative and positive. Negative regulation occurs via a repressor protein that binds to an operator region to block transcription. Positive regulation happens when an inducer molecule activates the promoter, promoting transcription. The lac operon is an example of this, containing genes for lactose metabolism that are induced by the presence of lactose via the lac repressor protein. The operon model of gene regulation explained by Jacob and Monod helped describe this process.

1. GENE REGULATION IN PROKARYOTES
Presented by Faisal khan
Roll NO 19
BS-GENETICS-19

2. Introduction
 In prokaryotic organisms, genes synthesis different types of proteins which needed in
different stages.
 All theseproteinsare not needed atonetime. Most of protein are synthesis in different
times.
 However, some genes are constantly transcribed and expressed inall cellsatall time,because
theirproduct are constantly needed in allcells. Such genesarecalled constitutivegenesor
house keeping genes.
 somegenesexpressed only in some typeof cellsor on specialoccasion. Such genes are called
non constitutive or luxury genes.

3.  The control of gene expression or protein synthesis is called generegulation.
 The regulation of gene expression in prokaryotic cells occurs at the transcriptional
level.
 Theregulation may be negativeorpositive.
 i)Negativeregulation
 ii)Positiveregulation:

4. i)Negative regulation:
 In Negative regulation an inhibitor or repressor is present in thecell.
 . A repressor is a DNA or RNA-binding protein that bind to an operator
region to block the action of RNA polymerase.
 Activators bind to the promoter to enhance the binding of RNA
polymerase. that increases transcriptionof ageneorsetof genes.
 Mostactivatorsare DNA binding proteins that bind to promoter. The DNA
site bound by theactivator is referred toas an "activator site.
 Eg:The catabolite activator protein(CAP; also known as cAMP repressor
protein, CRP) activates transcription at the lac operon of the bacteriumE.coli.

5. ii)Positive regulation:
In positive regulation, aninducer molecule activates the promoter,
which promotes transcription of mRNA.
 In inducible gene regulation the structural gene or genesare
induced or ‘switch on’ toproduce mRNA
 The substancewhich induces thegene forprotein synthesis is
known asinducer.
 Thegenecomplex responsible for thesynthesisof inducible
enzymes is called inducible system.
 Eg:lacoperon

6. OPERON MODEL
 Francis Jacob and Jacques Monad (1961),explained that gene regulation isby operon
model.

7.  Theoperon model for lactosecatabolism iscalled lac operon.
 An operon isagroupof closely linked structuregenes and associated controlgenes.
 Theoperonconsistof regulatorygene, structural gene, operator and promotersite.

9. STRUCTURE OF LAC OPERON
1.Regulatorygene:
 The regulatorygene is the i gene thatcode for the
repressorprotein
 This i gene is expressed in all the time hence it is also
known as a constitutivegene.
2.Structural genes:
 Three structural genes lac z,lacy,laca involved in the
synthesisof enzymes for the lactosecatabolism.

10.  The 3 genesas transcribed asapolycistronicmRNA.
 Lac z code forBeta galactosidase
 Lacycode for Lac permease
 Lac acode for Transacetylase
3.Promoter
A promoter is a sequence of DNA needed to turn a gene on or off. The process of
transcription is initiated at the promoter.
4.Operator
Operator – a segment of DNA to which a repressor binds.

11. Negative control of lac operon
 The lac repressor is synthesized through the activity of lac i gene called the
regulatorgene.
 This repressor is an allosteric protein(A protein whose shape is changed when it
bindsa particular molecule).
 i)it bind the operatorsite ii)it bind the lactosesugar(inducer)
 In theabsenceof inducer, DNA binding siteof repressor is functional.
 The repressorprotein binds to the DNA at theoperator site of the lac locus and
blocks the transcription of lac genes by RNApolymerase.
 The lac enzyme synthesis is inhibited.

13. Positive control of lac operon
 It is an additional regulatory mechanism which allows the lac operon
to sense the presence of glucose,an alternativeand preferred energy
source to lactose.
 If glucoseand lactoseare both present,cellswill use up the glucose
first.
 The presence of glucose in the cell switch off the lac operon by a
mechanism called catabolitereperssion, which involves a protein
called catabolite activator protein(CAP)