Gene expression in prokaryotes

Operon system One promoter can regulate many genes ?What about eukaryotes

Why the operone system Quick response to deal with environmental changes This happens only in prokaryotes like bacteria

Examples for operon system Lactose operone Tryptophan operone Those are present in E. Coli bacteria

Lac operon One promoter regulates 3 genes The gene expression depends on presence/absence of lactose in the medium

Lactose versus glucose Both are used as a carbon source for energy production Which is more preferred by the bacteria?

Lac operon system Components

Control Circuit for the lac Operon I P O || Z | Y | A | I repressor protein P promoter O operator Lac Z for ß-galactosidase Lac Y: ß-galactosidase permease Lac A: ß-galactosidase transacetylase

How it works

Lac I gene expresses a repressor protein It is expressed all the time no matter if lactose is present or absent This repressor protein binds to the operator This binding will result into halting the expression of lac operon structural genes

Lactose is present in the medium: Lactose will result in the formation of allolactose It is membrane permeable Can bind to the repressor protein Result in conformational changes Can the repressor protein bind operator?

In the presence of lactose : Lac structural genes are expressed

What if the medium contain both glucose and lactose? Lac operon will be on Lac operon will be off Lac operon will be facultative

If glucose and glucose are present in the medium The bacteria will consume : 1- Glucose first 2- Lactose first 3- Both at the same time

In the presence of lactose : Lac structural genes are expressed What if Lac and glucose are both present in the medium. Catabolite repression

In the presence of glucose Glucose transport will affect another signaling molecule cAMP. cAMP is an enhancer ( inducer) for the lac operon Glucose and cAMP are inversely proportional Allolactose and cAMP are absent Thus the lac operon is off

Mutations in lac operon Mutant Phenotype I- :constitutive expression because the operator is never closed

Mutations in lac operon Mutant Phenotype O- : constitutive expression because the repressor protein can not bind

Mutations in lac operon Mutant Phenotype P- :no expression of the operon because RNA polymerase cannot bind

Mutations in lac operon Mutant Phenotype lac Z-: no glucose or galactose production from lactose lac Y- : lactose will not be taken into the cell

Mutations in the lac operon

Tryptophan operon Biosynthesis pathway What was the kind of the lac operon?

Trp operon

Gene expression in eukaryotes There are two major mechanisms: Methylation Histone modification

DNA Methylation Occurs within the Cytosine in CPG regions Addition of methyl group to the Cytosine via methyltransferase enzyme to form 5- methylcytosin In mammals, up to 70% of CpG are methylated CpG are mostly present within the promoter regions So, what is the effect of this on gene expression?

DNA methylation Results in gene silencing

DNA methylation and disease regulation If methylation happens within the promoters of disease suppressor genes……. If hypomethylation occurs within the promoters of cancer genes …..

Open chromatin structure versus closed one

Histone protein modification I- Histone acetylation /deacetylation II- Histone phosphrylation/dephosphrylation Internet assignement