Reading Assignment 14

The lac repressor is composed of how many identical subunits?

4; homotetramer.

Cyclic-AMP is a small effector molecule produced by the enzyme

Adenylyl cyclase

Which protein binds to which site? Catabolite activator protein

CAP site

The transport of ______ into a cell causes a _______ in the concentration of cyclic-AMP by inhibiting adenylate cyclase.

Glucose; decrease

When four molecules of allolactose are bound to lac repressor, and repressor can no longer bind to operator, and RNA polymerase is free to transcribe the operon, the operon has been _______

Induced

Which protein binds to which site? Repressor

Operator

The small effector molecule __ prevents the lac repressor from binding to the operator site.

allolactose

When tryptophan levels are low, the transcription of the trp operon will occur at a ______ rate because ______.

high; the trp repressor cannot bind to the operator site.

An operon encodes a ___mRNA, an RNA that contains the sequences of two or more genes.

polycistronic

When four molecules of allolactose are bound to the lac repressor, the lac operon is induced, and RNA polymerase ______.

transcribes the operon; Reason: Allolactose binding inactivates the repressor.

Why is the transcription rate of the lac operon low when both glucose and lactose are absent?

Lac repressor blocks transcription by binding to the operator region.

Which protein binds to which site? RNA polymerase

Promoter

Why is transcription of the lac operon low when both lactose and glucose are present?

The low levels of cAMP result in CAP not binding to the CAP site. ; Reason: High levels of glucose lead to low levels of cAMP.

Why does transcription of the trp operon continue when tryptophan levels are low?

The ribosome pauses in region 1, preventing the 3-4 stem-loop from forming.

An operon is a group of two or more genes under the transcriptional control of ______.

a single promoter

The transport of glucose into a bacterial cell causes the intracellular level of cyclic-AMP to fall because the enzyme ___ ___ is inhibited

adenylyl cyclase

Transcription begins but terminates before the entire mRNA is made during ________

attenuation

When tryptophan levels are high, tryptophan binds to trp repressor protein causing a conformational change that in turn causes the trp repressor to

bind to the operator site.; Reason: Tryptophan is a corepressor, it causes a change in the trp repressor protein that enhances binding to the operator.

The Lac repressor protein binds to the operator and ______.

blocks transcription

During attenuation, transcription begins, ______.

but it is terminated before the entire mRNA is made

The small effector molecule produced by the enzyme adenylyl cyclase is ______.

cAMP

When the lac repressor is bound to the operator, RNA polymerase ______ the lacZ, lacY, or lacA genes.

cannot transcribe

Monod, Jacob, and Pardee studied an E. coli strain with a lacI- mutation in which the lac operon was expressed ______.

even in the absence of lactose; Reason: Expression in the presence of high levels of lactose is associated with the I+ genotype.

The lac repressor functions as a homotetramer, a protein composed of ______ subunits.

four identical

In catabolite repression, transcription is influenced by the presence of _____.

glucose

Jacob, Monod, and Pardee isolated bacteria with mutations that resulted in the constitutive expression of the ______________ operon even in the absence of lactose.

lac

The ______ gene encodes a repressor protein.

lacI

When both lactose and glucose are present, transcription of the lac operon is _____

low

When both lactose and glucose are present, transcription of the lac operon is ______.

low

A polycistronic mRNA contains the sequence(s) ______.

of two or more genes; Reason: Polycistronic mRNA has a single promoter that regulates expression of two or more genes.

When the lac repressor binds to the lac __ site, RNA polymerase cannot transcribe the lacZ, lacY or lacA genes.

operator

Allolactose is a small effector molecule that binds to the lac repressor and ______.

prevents repressor from binding to DNA; Reason: Allolcatose is an inducer, which when present, binds to the repressor and prevents it from binding to the operator.

In an operon, the ______ signals the beginning of transcription.

promoter

The CAP site and the operator site are ______ that function in gene regulation.

short DNA segments

The operon is flanked by a ____ that signals the beginning of transcription and a _____ that signals the end of transcription.

promoter; terminator

The somewhat imprecise term that describes how transcriptional regulation is influenced by glucose is ___________ __________.

catabolite repression

In the trp operon, tryptophan acts as a(n) ______.

corepressor; Reason: Tryptophan binding activates the repressor but blocks transcription.

When the tryptophan level in the cell is ______, the ribosome pauses in region one of the trpL mRNA, causing region 2 to to bind to region 3 and transcription to continue.

low

A(n) ______ is a group of two or more genes under the transcriptional control of a single promoter.

operon