Topic 2 Molecular Genetics: Regulation of Prokaryotic Gene Expression

Question 1

• Region of DNA that controls gene transcription and consists of:

Answer 1

1. Operon

Question 2

a. Promoter
b. Operator
c. Structural Genes
d. Regulatory Genes

Answer 2

Types of Operon

Question 3

• Sequence of DNA where RNA polymerase attaches to begin transcription

Answer 3

a. Promoter

Question 4

• Region that can block action of RNA polymerase if occupied by repressor proteins

Answer 4

b. Operator

Question 5

• DNA sequences that code for related proteins

Answer 5

c. Structural Genes

Question 6

• Located outside of operon region, and produce repressor proteins. Others produce activator proteins that assist the attachment of RNA polymerase to the promoter region

Answer 6

d. Regulatory Genes

Question 7

• Controls the breakdown of lactose; the regulatory gene produces an active repressor that binds to the operator and blocks RNA polymerase

Answer 7

2. Lac Operon

Question 8

• The lac operon consists of three lac genes (Z, Y, A), which code for the following:
  1. B-galactosidase that converts lactose → glucose and galactose
  2. Lactose permease that transports lactose into the cell
  3. Thiogalactoside transacetylase

Answer 8

Note on Lac Operon

Question 9

• When lactose is available, lactose binds to the repressor and inactivates it, therefore allowing RNA polymerase to transcribe the genes. Moreover, lactose induces the operon, and enzymes that the operon produces as a result are termed “inducible enzymes”

Answer 9

Note on Lac Operon

Question 10

• The lac operon isn’t only controlled by lactose, however. The important signaling molecule cAMP plays a regulatory role as well:
• When glucose is low, cAMP is high. This cAMP binds to a CAP binding site of the promoter, which enhances the binding and transcription via RNA polymerase, allowing for lactose to be broken down.

Answer 10

Note on Lac Operon

Question 11

• If lactose AND glucose are high, the operon is shut off

- This is because cAMP is low, and doesn’t bind to CAP. Bacteria uses one sugar at a time, and prefers glucose.

Answer 11

Note on Lac Operon

Question 12

• Produces enzymes for tryptophan synthesis; regulatory genes produce an inactive repressor, which allows RNA polymerase to produce enzymes.
• When tryptophan is available, we no longer need to synthesize it internally: it binds to an inactive repressor and activates the repressor, which binds to the operator and blocks RNA polymerase.
  a. Tryptophan is a co-repressor here

Answer 12

3. Trp Operon (E. coli)

Question 13

• As above, are when structural genes stop producing enzymes only in the presence of an active repressor. Unlike repressive enzymes, some genes are constitutive (constantly expressed) either naturally or due to mutation

Answer 13

4. Repressible Enzymes