Negative and Positive regulation Another important concept is the distinction between negative and positive regulation by regulatory proteins. If a regulatory protein in its active state turns off the expression of the operon, the operon is said to be negatively regulated by the regulatory protein. If the regulatory protein in its active state turns on the operon, the operon is positively regulated by the regulatory protein. An operon regulated by a repressor is therefore negatively regulated, because the presence of the active repressor prevents transcription of that operon. In contrast an operon regulated by an activator is positively regulated because in its active state the activator protein turns on transcription of the operon under its control Activator and repressor proteins usually bind to different regions of the dna
• Negative and Positive Regulation • Another important concept is the distinction between negative and positive regulation by regulatory proteins. If a regulatory protein in its active state turns off the expression of the operon, the operon is said to be negatively regulated by the regulatory protein. If the regulatory protein in its active state turns on the operon, the operon is positively regulated by the regulatory protein. An operon regulated by a repressor is therefore negatively regulated, because the presence of the active repressor prevents transcription of that operon. In contrast, an operon regulated by an activator is positively regulated, because in its active state the activator protein turns on transcription of the operon under its control. • Activator and repressor proteins usually bind to different regions of the DNA
Activators usually bind upstream of the-35 sequence of the promoter, where they can make contact with the rna polymerase bound to the promoter. repressors often bind to the promoter region itself, or at least very close to it, and thereby block access by rna polymerase to the promoter Some regulatory proteins can be both repressors and activators depending upon the situation. The n repressor is an example. It represses transcription from the the pi and pr promoters but activates the prM promoter. The binding site on the dna for the regulatory protein often changes when the protein shifts from being an activator to a repressor
• Activators usually bind upstream of the -35 sequence of the promoter, where they can make contact with the RNA polymerase bound to the promoter. Repressors often bind to the promoter region itself, or at least very close to it, and thereby block access by RNA polymerase to the promoter. • Some regulatory proteins can be both repressors and activators, depending upon the situation. The λ repressor is an example. It represses transcription from the the pL and pR promoters but activates the pRM promoter. The binding site on the DNA for the regulatory protein often changes when the protein shifts from being an activator to a repressor
Inducers and corepressors Whether a regulatory protein is active sometimes depends on whether it is bound to a small molecule. Small molecules that bind to proteins and change their properties are called effectors An effector that binds to a repressor or activator and thereby initiates transcription of an operon is called the inducer of the operon. In contrast, an effector that binds to a repressor and causes it to block transcription is called a corepressor The activity of regulatory proteins is not necessarily altered only by binding to small molecule effectors. Some repressors and activators are covalently altered under some conditions for example, by methylation or phosphorylation
• Inducers and Corepressors • Whether a regulatory protein is active sometimes depends on whether it is bound to a small molecule. Small molecules that bind to proteins and change their properties are called effectors. An effector that binds to a repressor or activator and thereby initiates transcription of an operon is called the inducer of the operon. In contrast, an effector that binds to a repressor and causes it to block transcription is called a corepressor. • The activity of regulatory proteins is not necessarily altered only by binding to small molecule effectors. Some repressors and activators are covalently altered under some conditions, for example, by methylation or phosphorylation
Genetic Evidence for negative and positive Regulation Negatively and positively regulated operons behave very differently in genetic tests. One difference is in the effect of mutations that inactivate the regulatory gene for the operon. If an operon is negatively regulated a mutation that inactivates the regulatory gene will allow transcription of the operon genes, even in the absence of inducer. If the regulation is positive mutations that inactivate the regulatory gene will prevent transcription of the genes of the operon, even in the presence of the inducer. a mutant in which the genes of an operon are al ways transcribed even in the absence of inducer, is called a constitutive mutant. Constitutive mutations are much more common with negatively than with positively regulated operons because any mutation that inactivates the repressor will result in the constitutive phenotype. With positively regulated operons, a constitutive phenotype can be caused only by changes that do not inactivate the activator protein but alter it so that it can activate transcription without binding to the inducer. Such changes tend to be rare
• Genetic Evidence for Negative and Positive Regulation • Negatively and positively regulated operons behave very differently in genetic tests. One difference is in the effect of mutations that inactivate the regulatory gene for the operon. If an operon is negatively regulated, a mutation that inactivates the regulatory gene will allow transcription of the operon genes, even in the absence of inducer. If the regulation is positive, mutations that inactivate the regulatory gene will prevent transcription of the genes of the operon, even in the presence of the inducer. A mutant in which the genes of an operon are always transcribed, even in the absence of inducer, is called a constitutive mutant. Constitutive mutations are much more common with negatively than with positively regulated operons because any mutation that inactivates the repressor will result in the constitutive phenotype. With positively regulated operons, a constitutive phenotype can be caused only by changes that do not inactivate the activator protein but alter it so that it can activate transcription without binding to the inducer. Such changes tend to be rare
Complementation tests reveal another difference between negatively and postively regulated operons. Constitutive mutations of a negatively regulated operon are often recessive to the wild type This is because any normal repressor protein in the cell encoded by a wild-type copy of the gene will bind to the operator and block transcription, even if the repressor encoded by the mutant copy of the gene in the same cell is inactive. In contrast. constitutive mutations in a solely positively regulated operon should be dominant to the wild type. A mutant activator protein that is active without inducer bound might activate transcription even in the presence of a wild-type activator protein
• Complementation tests reveal another difference between negatively and postively regulated operons. Constitutive mutations of a negatively regulated operon are often recessive to the wild type. This is because any normal repressor protein in the cell encoded by a wild-type copy of the gene will bind to the operator and block transcription, even if the repressor encoded by the mutant copy of the gene in the same cell is inactive. In contrast, constitutive mutations in a solely positively regulated operon should be dominant to the wild type. A mutant activator protein that is active without inducer bound might activate transcription even in the presence of a wild-type activator protein