Regulation of transcription in bacteria II

Question 1

what is an inducible gene ?

Answer 1

A gene system, often encoding a coordinated group of enzymes involved in a catabolic pathway, is inducible if an early metabolite in the pathway causes activation, usually by interaction with and inactivation of a repressor, of transcription of the genes encoding the enzymes

Question 2

The gene encoding for biosynthetic enzyme (anabolic) are usually

Answer 2

Repressible genes

like trp operons

Question 3

Why is there an inducible gene

Answer 3

If the function of the gene’s product is to break a molecule down, and this molecule is present in abundance, that molecule will serve as an inducer of this inducible gene, it will switch the gene on. As is the case with lactose and the lac operon.

Question 4

Why is there an repressor gene

Answer 4

If the function of the gene’s product is to synthesize a molecule, and this molecule is present in abundance, that molecule will serve as a co-repressor of that repressible gene, it will switch the gene off. As is the case with the trp operon.

Question 5

Tryptophan Biosynthetic Pathway in E. coli

Answer 5

Multi-step pathway, all enzymes are encode by genes that belong to a single operon.
The final product is amino acid tryptophan

Question 6

What is feedback inhibition?

Answer 6

Feedback inhibition
Where the product of a pathway inhibits the
activity of an enzyme earlier in the pathway
Often the end product of the pathway will bind
to an enzyme for the first step reducing its
activity

Question 7

Feedback inhibition in Trp operon

Answer 7

The end product (tryptophan) binds to the allosteric site of the first enzyme, TrpE, and inhibits activity.
All the genes are all still being expressed to make each of these enzymes, but without the first intermediate the enzymes ‘sit idle (not active)

Question 8

What is Feedback repression?

Answer 8

– Where the product of a pathway interacts with
a regulatory protein (e.g. a repressor) to stop
transcription of genes encoding enzymes in
the pathway

Question 9

Action of an allosteric repressor protein in feedback repression

Answer 9

Repressor normally can not bind to the operator site on
the DNA
No repression of transcription
• Repressor can bind the molecule that is the end
product of the pathway
Causes a change in conformation
Can bind to the operator site on DNA
Represses transcription

Question 10

Regulation of transcription initiation

Answer 10

involve altering promoter activity
These mechanisms alter transcription initiation
– Repressors turn promoters off (or down)
– Activators turn promoters on (or up)

Question 11

Regulation after transcription initiation

Answer 11

Some regulatory mechanisms function after initiation of transcription
– E.g transcriptional attenuation of the tryptophan biosynthesis operon

Question 12

transcriptional attenuation

Answer 12

The trp operon is transcribed as a single polycistronic mRNA
• Has a long leader sequence
– This sequence contains two Trp codons
• When tryptophan is absent from the
medium the whole mRNA is transcribed
• When tryptophan is present only the leader sequence is transcribed
– Then transcription stops

Question 13

The trp operon is regulated by the

Answer 13

TrpR repressor
and a second level of control involving transcriptional attenuation
involves an upstream leader peptide (L, TrpL) that is
tryptophan rich

Question 14

Transcriptional attenuation is used to

Answer 14

Transcriptional attenuation is used to
regulate other amino acid biosynthesis
operons
– All operons have leader sequences with runs
of the specific amino acid in the leader