Exam 16: Regulation of Gene Expression in Prokaryotes

The ara operon is controlled by a regulator protein that exerts ________.

A constitutive mutation in the lac operon may be of several types. Name two types of constitutive mutations.

The enzyme permease cleaves the linkage between glucose and galactose residues in lactose.

(a) Describe by labeled diagram the structural components of the lac operon in E. coli. (b) State the function of the lac regulator gene. (c) State the function of β-galactosidase in the lac system. (d) Show by diagram the manner in which lactose brings about transcription of the three structural genes of the lac operon. (e) Explain why certain mutations in the regulator gene (I^-) of the lac system result in maximal synthesis of β-galactosidase, permease, and transacetylase even in the absence of the inducer (lactose).

Under a system of negative control, genetic expression occurs unless such expression is shut off by some form of regulator.

State whether the following statement is true or false, then give your reasoning. The terminating "hairpin" loop occurs in the trp operon when sufficient tryptophan is present.

Explain why lacO^c mutations are cis-acting while lacI mutations can be trans-acting.

The trp operon is typically characterized as being both under negative control and repressible.

Describe what is meant by a gratuitous inducer. Give an example.

Attenuation is known to occur in the lac operon.

Compare and contrast positive and negative control of gene expression in bacteria.

Present an overview of prokaryotic regulation in terms of growth efficiency.

Present a detailed description of the actions of the regulatory proteins in inducible and repressible enzyme systems.

Which of the following terms best characterizes catabolite repression associated with the lac operon in E. coli?

The trp and lac operons are both subject to forms of control that are typically called negative.

The cAMP-CAP complex and RNA polymerase bind more efficiently to the lac operon together than either does alone. What term is applied to this increased efficiency of binding?

Genetic regulation in prokaryotes can involve alterations in RNA secondary structure. What phenomenon occurs in the trp operon that involves such alterations?

Regarding the trp operon, trpR^- maps to a considerable distance from the structural genes. The mutation either inhibits the interaction with tryptophan or inhibits repressor formation entirely. In the presence of tryptophan in the medium, would you expect the trp operon to be transcriptionally active? Explain.

In the accompanying diagram, what type of control, positive or negative, is operating?

The accompanying table lists several genotypes associated with the lac operon in E. coli. For each, indicate with a "+" or a "-" whether β-galactosidase would be expected to be produced at induced levels. Genotype                         $\beta$ -galactosidase production                                         No Lactose         With Lactose (a) $\quad I + O + Z + / F ^ { \prime } I + O + Z +$ (b) $\quad I - O c Z - / F ^ { \prime } I - O c Z -$ (c) $\quad I - O c Z + / F ^ { \prime } I - O + Z +$ (d) Is Oc $\mathrm { Z } - \mathrm { F } ^ { \prime }$ Is $\mathrm { O } + \mathrm { Z } ^ { + }$ $I ^ { + } =$ wild-type repressor $I ^ { - } =$ mutant repressor (unable to bind to the operator) Is $=$ mutant repressor (insensitive to lactose) $\mathrm { O } ^ { + } =$ wild-type operator $\mathrm { OC } =$ constitutive operator (insensitive to repressor)