Lac Operon Lactose is a disaccharide used an energy source for bacteria when glucose is not available in environment Catabolism of lactose only takes place when lactose is present in environment but glucose is not (Note: Operon activity responds to 2 signals [lac] and [glu] Inducible Operon – default is off ; turn gene on when needed

Growth of E. Coli on glucose and lactose sugars

Lactose can be broken down into glucose and galactose

Lac Operon uses both negative and positive controls

Genes not expressed Genes expressed Promoter Operator Genes Fig. 18-UN3 Genes not expressed Genes expressed Promoter Operator Genes Active repressor: no inducer present Inactive repressor: inducer bound Default is OFF Inducer turns gene ON

Link to Lac operon animation

(a) Lactose absent, repressor active, operon off Fig. 18-4a Regulatory gene Promoter Operator DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off

(b) Lactose present, repressor inactive, operon on Fig. 18-4b lac operon DNA lacI lacZ lacY lacA RNA polymerase 3 mRNA mRNA 5 5 -Galactosidase Permease Transacetylase Protein Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

(a) Lactose absent, repressor active, operon off Fig. 18-4 Regulatory gene Promoter Operator DNA lacI lacZ No RNA made 3 mRNA RNA polymerase 5 Active repressor Protein (a) Lactose absent, repressor active, operon off lac operon DNA lacI lacZ lacY lacA RNA polymerase 3 mRNA mRNA 5 5 -Galactosidase Permease Protein Transacetylase Allolactose (inducer) Inactive repressor (b) Lactose present, repressor inactive, operon on

Figure 18.22a Positive control: cAMP receptor protein

Figure 18.22b Positive control: cAMP receptor protein

(a) Lactose present, glucose scarce (cAMP level Fig. 18-5 Promoter Operator DNA lacI lacZ CAP-binding site RNA polymerase binds and transcribes Active CAP cAMP Inactive lac repressor Inactive CAP Allolactose (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized Promoter Operator DNA lacI lacZ CAP-binding site RNA polymerase less likely to bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized

Link to Lac operon animation