L19 & Tutorial 8 - Trp & Lac genes

Question 1

What is a promoter? What is an operator?

Answer 1

Promoter - DNA sequences that define where transcription of a gene by RNA polymerase begins.

Operator- segment of DNA to which a transcription factor binds to regulate gene expression. The transcription factor is typically a repressor, which can bind to the operator to prevent transcription.

Question 2

What is the function (overall) of the genes in the trp operon?

Answer 2

Trp synthesis

Question 3

What binds to the promoter of the trp operon? What binds to the operator?

Answer 3

RNA polymerase binds to the promoter

Trp repressor binds to the operator

Question 4

What molecule binds to the TrpR protein? Is the TrpR molecule functional with or without this molecule?
(ie. does its binding activate or inactivate the TrpR protein?)

Answer 4

Trp, functional with this molecule

Question 5

If a mutation in the promoter completely disrupts the RNA polymerase binding site, are the genes in the trp operon transcribed, translated, both or neither?

Answer 5

Neither

Question 6

If a mutation disrupts a critical codon in the trpE gene, does transcription occur, translation occur, and is tryptophan produced? Is trpE transcribed or translated?

Answer 6

Both occurs but Trp is not produced nor is it transcribed or translated.

Question 7

If a mutation disrupts a critical codon in the trpR gene, are the genes in the trp operon expressed?

Answer 7

Yes, always expressed.

Question 8

If a mutation critically disrupts the operator sequence of the trp promoter, but RNA polymerase can still bind, are the genes in the trp operon expressed in the presence and absence of tryptophan?

Answer 8

Always expressed as repressor cannot bind.

Question 9

If a base pair insertion occurs in the promoter, but this does not affect the binding of RNA polymerase or the repressor, is there a frameshift in the coding sequence?

Answer 9

No

Question 10

If a base pair insertion occurs in the trpE coding sequence, are the genes transcribed, or translated? Is there a frameshift only in trpE, or a frameshift mutation in all proteins?

Answer 10

Genes still transcribed and translated except for frameshift mutation in trp E only.

Question 11

What is the function of the genes in the lac operon? (in general) What is the operator for?

Answer 11

To breakdown lactose. Operator (lacl) is innately active and is inactivated in the presence of lactose.

Question 12

What molecule binds to the lac repressor protein? Does the binding of this molecule make the protein more active or less active?

Answer 12

Allolactose depresses the operon by inactivating it so that enzymes for lactose utilisation are induced.

Question 13

Is the lac operon expressed in the presence or absence of lactose?

Answer 13

Presence

Question 14

Which of the trp operon and lac operon is repressible and which is inducible? Why are they called this? Do either of the operons show negative regulation and/or positive regulation?

Answer 14

Trp - the operon is repressible in the presence of Trp
Lac - the operon is inducible in the presence of Lac
Both negative regulation

Question 15

What is combinatorial control?

Answer 15

Several important regulators of a single gene (messages are integrated for a single response)

Question 16

What is cAMP? What is CAP? Where does CAP bind? What does it achieve when it binds?

Answer 16

cAMP is produced in the presence of low glucose levels. CAP is a protein that cAMP binds to and activates. It binds to a different part of the promoter and activates it so mRNA is produced that code for enzymes in the lactose pathway.

Question 17

Is the lac operon expressed when there is low glucose and high lactose? Is the lac operon expressed when there is high glucose and high lactose? When there is high glucose, is there high or low cAMP?

Answer 17

Yes, no, low cAMP

Question 18

Is gene activation synergistic?

Answer 18

Yes

Question 19

What does binding of the lacI repressor do to the DNA of the lac promoter region?

Answer 19

Allows RNA polymerase to transcribe mRNA that translates to Galactoside permease so that lactose can enter the cell. B-Galactoside breaks lactose down into galactose and glucose.

Question 20

What are the characteristics of the DNA sequence of the lac operator that make it likely to be bound by dimer? (note: these dimers then bind together in a tetramer)

Answer 20

Reverse repeats

Question 21

How do we know which DNA sequence is most likely to be recognized by a typical transcriptional activator?

Answer 21

It will have reverse repeats