Lecture 5: Regulation of Transcription Initiation in Prokaryotes

Question 1

In what was is transcription initiation regulated?

Answer 1

1. Sigma Factors
2. Strength of the promoter
3. Regulatory proteins (usually DNA binding proteins)
   a. repressors
   b. activators
   c. dual function - do either depending
   on conditions

Question 2

How do repressors ‘repress’ expression?

How do activators ‘activate’ expression?

Answer 2

Bind to the operator region of the promoter blocking RNA polymerase binding

Bind to the activator binding site

Question 3

What is the role of regulatory proteins in transcription intitiation?

Answer 3

To control the level of expression of a given gene.

1. repressors negatively regulate gene expression
2. activators positively regulate gene expression

Question 4

How are regulatory proteins regulated?

Answer 4

Regulatory proteins are often regulate by LIGAND BINDING of inducer and corepressors.

1. Inducers - bind and inactivate repressors
2. Corepressors - bind and activate repressors

Question 5

How are regulatory proteins regulated?

Answer 5

Regulatory proteins are often regulate by LIGAND BINDING of inducer and corepressors.

1. Inducers - bind and inactivate repressors
2. Corepressors - bind and activate repressors

Question 6

What is the lac operon?

Answer 6

A strategy of acquiring nutrients. When glucose is not present but lactose is, then lactose alters gene expression of Lac genes require to use lactose as a source of carbon

Question 7

What are the different structural Lac genes? What enzymes do they code?

Answer 7

1. Lac Z - Permease
2. Lac Y - Beta-galactosidase
3. Lac A - Transacetylase

Question 8

What is LacI? What does it express and what does that do?

Answer 8

LacI is the repressor region responsible for regulation of the Lac operon.

LacI expresses the LacI REPRESSOR - this tetramer binds to the operator preventing RNA polymerase binding to the promoter

Question 9

What happens to the LacI repressor in the presence of lactose?

Answer 9

Lactose allosterically binds and causes conformational changes in each repressor subunit.

This reduces its affinity for the operator causing it to be release

Question 10

What is IPTG?

Answer 10

IPTG (Isopropyl thiogalactoside) - used as a non-physiological inducer