Regulation of Transcription

C483 Spring 2013

1. Which of the following is true about transcription regulation? A) A repressor protein activates transcription of a negatively regulated gene. B) A positively regulated gene can be transcribed only in the absence of active repressor. C) In the absence of an activator, a negatively regulated gene is poorly transcribed. D) The initiation of transcription of regulated genes is controlled by regulatory proteins which bind to specific DNA sequences. E) All of the above.

2. Protein X is found to bind to a certain gene's promoter only when substance A is not present. The gene can be transcribed in the presence of both X and A. It is not transcribed in the presence of X alone. Protein X is a(n) ________. Substance A is a(n) ________. A) repressor; inducer B) repressor; corepressor C) activator; inducer D) activator; corepressor

3. An operon is ________. A) a set of genes whose transcription is controlled by a single promoter B) the site of repressor binding C) a protein that activates the transcription of a whole class of genes D) a multisubunit complex responsible for the splicing of exons

4. Which of these is not part of the lac operon?A) An enzyme that hydrolyzes b-galactosidesB) Galactoside permeaseC) Lac repressorD) An enzyme that marks unusable galactosides

Regulation

• Housekeeping genes—constitutive• Regulation of gene expression can occur at

any point of information flow, but occurs most often at level of transcription

• Positive regulation requires Activator• Negative regulation controlled by Repressor• Regulation proteins are often allosteric– Inducers and corepressors

Lac Operon

• Example of “simple” regulation at the level of transcription

• Both positive and negative regulation– DON’T NEED TO KNOW REGULATION DETAILS

• Goal: understand an example of why regulation is necessary and useful– First three paragraphs of 21.7

E. Coli and Fuel Sources

• Prefer glucose; can use b-galactosides (lactose)

• Only synthesize metabolic enzymes for lactose if lactose is present and glucose low!

• Three necessary proteins– Transporter– Cut useful galactosides– Mark nonuseful galactosides

Operon

• Three genes for three proteins controlled by single promoter region (operon)

Lac repressor (not part of operon)

B-galactosidase (hydrolyse lactose)

Lactose permease

Thiogalactosidetransacetylase

Regulation• Very tight repressor

binding• Rare spontaneous

release allows for one transcription by promoter that is poise

• “escape synthesis”• Low level of permease

available to allow lactose to enter cell O = “operator”

Answers

1. D2. A3. A4. C