Lac operon

What is the lac operon

• Group of genes under same regulatory mechanism

• Mainly found in prokaryotes

• A way of saving energy and resources as if a product is not needed all genes involved in its production are turned off

• Consists of 3 structural genes, lac Y, lac Z and lac A

What enzymes do the 3 genes of lac operon code for

• Lac Y= β-galactosidase

• Lac Z= Lactose permease

• Lac A= Transacetylase
  
  All transcribed on a single loop of mRNA

What is the action of β-galactosidase

Hydrolyses lactose into glucose and galactose

What is the action of lactose permease

Transports protein that becomes embedded in E.coli membrane to transport lactose into cell

Why is lactose important in the lac operon

Lactose triggers enzymes production (inducer molecule)

What happens in lac operon when lactose is absent (down regulation)

1. Regulator gene is expressed and produces repressor protein

2. Repressor protein binds to operator region, partially covering promoter region

3. RNA polymerase cannot bind so lac Z and Y genes can’t be translated and are switched off

What happens in lac operon when lactose is present (up regulation)

1. Lactose an inducer molecule binds to repressor protein

2. Repressor protein dissociates from operator region and promoter region exposed

3. RNA polymerase binds to promoter region and A,Z and Y transcribed- mRNA produced

4. ß-galactosidase /lactose permease can now be synthesised

Describe the role of cyclic AMP

• cAMP upregulates the rate of transcription when it is bound to cAMP receptor protein (CRP)

• When levels of glucose are high then levels of cAMP decrease so transcription of genes in lac operon slows down, so lactose not broken down and glucose is metabolised instead