Jacob and Monod: Identifying Mutants Defective in Regulation of lac Operon

negative control/repressor

repressing transcription by binding to specific site

positive control/activator

triggers transcription by binding to enhancer or promoter

Galactoside Permease

symporter to import lactose

Lactose

inducer

made of galactose and glucose

lac locus

entire region

promoter | lacI | terminator | promoter | lacO | lacZ | lacY | lacA | terminator

lac operon

region with structural genes

promoter | lacO | lacZ | lacY | lacA | terminator

lacI

regulatory, monocistronic gene

mRNA codes for repressor

lacO

operator regulatory site

repressor binds lacO site and stops transcription before the structural genes

lacZ

structural gene that codes for BGalactosidase

only transcribed if lactose if present

lacY

structural gene that codes for galactoside permease

only transcribed if lactose is present

lacA

structural gene that codes for transacetylase

only transcribed if lactose is present

polycistronic

mRNA codes for multiple genes

ex. mRNA from lac operon section consists of lacZ, lacY, and lacA genes which code for different proteins

monocistronic

mRNA codes for one gene

ex. lacI mRNA only codes for the repressor protein

process with lactose (inducer)

lactose changes into allolactose —> allolactose binds to repressor coded by lacI —> repressor does NOT bind to lacO site —> holoenzyme allowed to transcribe lacZ, lacY, and lacA —> Bgalactosidase, galactoside permease, and transacetylase are synthesized

without lactose

lacI codes for repressor protein —> repressor binds to lacO site —> holoenzyme is blocked from transcribing lacZ, lacY, and lacA

constitutive expression

lacI is mutated but lacO is still on

Jacob and Monod

identified mutants defective in regulation of lac operon

generated mutants and grew colonies on minimal medium+glucose and grew a replica on minimal medium+lactose

lacO mutations

lacO^c

constitutive expression of lac operon

repressor cannot bind due to mutated lacO site

lacI mutations

lacI^-

constitutive expression of lac operon

mutated lacI codes for inactive repressor —> cannot bind to lacO