Regulation of Gene expression in Prokaryotes

Question 1

High levels of tryptophan (Repressible trp operon)

Answer 1

1. Tryptophan binds to allosteric site of trp repressor
2. Trp repressor changes from inactive to active conformation, and recognises and bind to operator. Thus, operon is turned off
3. RNA polymerase is unable to bind to promoter, hence, unable to transcribe structural genes of trp operon. mRNA is not synthesised
4. Enzymes that synthesise tryptophan are not synthesised, hence, tryptophan is not synthesised

Question 2

Low levels of tryptophan (Repressible trp operon)

Answer 2

1. Decrease in tryptophan levels results in existing tryptophan to dissociate from trp repressor
2. Active trp repressor is converted to its inactive conformation, and is unable to recognise and binds to the operator.
3. RNA polymerase is able to recognise and bind to promoter, transcribing the structural genes of the trp operon. mRNA is synthesised
4. Enzymes that synthesise tryptophan are synthesised. Hence, tryptophan is synthesised

Question 3

High levels of lactose, NO glucose (Inducible lac operon)

Answer 3

1. Lactose converted to allolactose by b-galactosidase
2. Allolactose binds to allosteric site of lac repressor
3. Lac repressor changes from active to inactive conformation, and is unable to bind to operator. Thus, operon is turned on
4. RNA polymerase is able to recognise and bind to promoter, transcribing structural genes of the lac operon. mRNA is synthesised
5. b-galactosidase, lac permease and transacetylase is produced

Question 4

Low levels of lactose, NO glucose

Answer 4

1. Lac repressor is synthesised in its active conformation and recognises and binds to operator. Thus, operon is turned off
2. RNA polymerase is unable to recognise and bind to promoter, hence, transcription of structural genes of lac operon does not occur. mRNA is not synthesised
3. b-galactosidase, lac permease and transacetylase is not produced

Question 5

Lactose present, high levels of glucose

Answer 5

1. When amount of glucose is high, cAMP concentration decreases. Without cAMP, CAP assumes its inactive conformation and disengages from CAP binding site
2. CAP is inactive, RNA polymerase recognises and binds to promoter at a very low affinity
3. Transcription of the lac operon proceeds at a very low level

*cell uses glucose first. once glucose is depleted, then lactose will be used

Question 6

Lactose present, low levels of glucose

Answer 6

1. When amount of glucose is scarce, cAMP concentration increases. cAMP binds to allosteric site of CAP. CAP assumes its active conformation and recognises and binds to CAP binding site.
2. Attachment of CAP bends the DNA and recruits RNA polymerase to the promoter.
3. RNA polymerase recognises and binds to the promoter at high affinity
4. Transcription of lac operon proceeds at a high level (increase T/S).

*cell uses lactose as the respiratory susbtrate

Question 7

Role of lacI gene in regulation of the lac operon

Answer 7

1. lacI codes for a repressor
2. Repressor recognises and binds to operator, and physically blocks the binding of RNA polymerase to the promoter
3. preventing transcription of structural genes of lac operon