In chapter 7 we will explore one strategy bacteria employ to control the expression of their genes: by grouping functionally related genes together so they can be regulated together easily. Such a group of contiguous, coordinately controlled genes is called an operon. | Molecular Biology Fifth Edition Chapter 7 Operons: Fine Control of Bacterial Transcription Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The lac Operon The lac operon was the first operon discovered It contains 3 genes coding for E. coli proteins that permit the bacteria to use the sugar lactose Galactoside permease (lacY) which transports lactose into the cells b-galactosidase (lacZ) cuts the lactose into galactose and glucose Galactoside transacetylase (lacA) whose function is unclear 7- Genes of the lac Operon The genes are grouped together: lacZ = b-galactosidase lacY = galactoside permease lacA = galactoside transacetylase All 3 genes are transcribed together producing 1 mRNA, a polycistronic message that starts from a single promoter Each cistron, or gene, has its own ribosome binding site Each cistron can be translated by separate ribosomes that bind independently of each . | Molecular Biology Fifth Edition Chapter 7 Operons: Fine Control of Bacterial Transcription Lecture PowerPoint to accompany Robert F. Weaver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The lac Operon The lac operon was the first operon discovered It contains 3 genes coding for E. coli proteins that permit the bacteria to use the sugar lactose Galactoside permease (lacY) which transports lactose into the cells b-galactosidase (lacZ) cuts the lactose into galactose and glucose Galactoside transacetylase (lacA) whose function is unclear 7- Genes of the lac Operon The genes are grouped together: lacZ = b-galactosidase lacY = galactoside permease lacA = galactoside transacetylase All 3 genes are transcribed together producing 1 mRNA, a polycistronic message that starts from a single promoter Each cistron, or gene, has its own ribosome binding site Each cistron can be translated by separate ribosomes that bind independently of each other 7- Control of the lac Operon The lac operon is tightly controlled, using 2 types of control Negative control, like the brake of a car, must remove the repressor from the operator - the “brake” is a protein called the lac repressor Positive control, like the accelerator pedal of a car, an activator, additional positive factor responds to low glucose by stimulating transcription of the lac operon 7- Negative Control of the lac Operon Negative control indicates that the operon is turned on unless something intervenes and stops it The off-regulation is done by the lac repressor Product of the lacI gene Tetramer of 4 identical polypeptides Binds the operator just right of promoter 7- lac Repressor When the repressor binds to the operator, the operon is repressed Operator and promoter sequence are contiguous Repressor bound to operator prevents RNA polymerase from binding to the promoter and transcribing the operon As long as no lactose is available, the lac operon is .
