Chapter 16

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. Positive control in an inducible operon

inactive activator

What is attenuation? What are the mechanisms by which the attenuator forms when tryptophan levels are high and the antiterminator forms when tryptophan levels are low?

Attenuation is the termination of transcription before the structural genes of an operon are transcribed. Attenuation results from the formation of a termination hairpin, or attenuator, in the RNA. Two types of secondary structures can be formed by the mRNA 5′ UTR of the trp operon. One of them allows transcription to proceed; the other one terminates transcription. When tryptophan levels are high, region 3 pairs with region 4 to form the attenuator hairpin structure, stopping transcription. When tryptophan levels are low, region 2 pairs with region 3 to form the antiterminator hairpin, allowing transcription to proceed through the structural genes.

Name six different levels at which gene expression might be controlled

DNA and chromatin structure, transcription, mRNA processing, mRNA stability, translation, and protein modifi cation.

A mutation at the operator site prevents the regulator protein from binding. What effect will this mutation have in the following types of operons? Regulator protein is a repressor in an inducible operon.

The result will be constitutive expression, and transcription will take place all the time.

What is antisense RNA?

Antisense RNA molecules are small RNA molecules that are complementary to other DNA or RNA sequences.

How does antisense RNA control gene expression?

In bacterial cells, antisense RNA can bind to a complementary region in the 5′ UTR of an mRNA molecule, blocking the attachment of the ribosome to the mRNA and thus stopping translation.

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator Negative control in a repressible operon

Inactive repressor

Briefly describe the lac operon and how it controls the metabolism of lactose.

The lac operon consists of three structural genes—lacZ, lacY, and lacA. The lacZ gene encodes the enzyme β-galactosidase, which cleaves the disaccharide lactose into galactose and glucose and converts lactose into allolactose. The lacY gene encodes lactose permease, an enzyme necessary for the passage of lactose through the E. coli cell membrane. The lacA gene encodes the enzyme thiogalactoside transacetylase, whose function in lactose metabolism has not yet been determined. All three genes have an overlapping promoter and operator region in common. Upstream of the lactose operon, the lacI gene encodes the lac operon repressor, which binds at the operator region and inhibits the transcription of the lac operon by preventing RNA polymerase from successfully initiating transcription. When lactose is present in the cell, the enzyme β-galactosidase converts some of it into allolactose, which binds to the lac repressor, altering its shape and reducing the repressor's affi nity for the operator. Because the allolactose-bound repressor does not bind to the operator, RNA polymerase can initiate transcription of the lac structural genes from the lac promoter.

Explain why mutations in the lacI gene are trans in their effects, but mutations in the lacO gene are cis in their effects.

The lacI gene encodes the lac repressor protein, which can diffuse within the cell and attach to any operator. It can therefore affect the expression of genes on the same molecule or on different molecules of DNA. The lacO gene encodes the operator. It affects the binding of RNA polymerase to DNA and therefore affects the expression of genes only on the same molecule of DNA.

A mutation at the operator site prevents the regulator protein from binding. What effect will this mutation have in the following types of operons? Regulator protein is a repressor in a repressible operon

The operon will never be turned off, and transcription will take place all the time.

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. Positive control in a repressible operon

active activator

For each of the following types of transcriptional control, indicate whether the protein produced by the regulator gene will be synthesized initially as an active repressor, inactive repressor, active activator, or inactive activator. Negative control in an inducible operon

active repressor